Why You Shouldn’t Support Mandatory Vaccinations
Last updated 25/08/2020
Click here for an article on how to drive in sand
On the surface it seems like a reasonable assumption that wide tyres or “fatties” would be better for driving in soft sand. It appears logical that a wide tyre wouldn’t dig in as much because it spreads its weight out more. However, practical experience tells us otherwise. In my experience commercial vehicles with standard steel rims and factory skinny cheese cutter tyres perform just as well in sand as fancy 4WD vehicles with fat tyres. I found when I upgraded to a set of fatties in my Suzuki Sierra that it made no difference in soft sand. At correct tyre pressure, I’ve never seen a car with skinny tyres unable to complete an obstacle in sand that a vehicle with wide tyres was able to complete.
In my experience, in the vehicles I’ve owned and other vehicles I’ve driven and seen driven, I’ve NEVER noticed a significant difference in sand performance due to tyre dimensions. But I have always noticed a MASSIVE difference due to tyre pressure.
How can this be?
A nice bogging of a friend’s car. Reduced tyre pressure, some shovel work and grinding through in first gear low range got this car out without any external assistance.
In sand, contact patch is what allows a vehicle to stay on the surface of the sand. Contact patch is the surface area of the tyre that makes contact with the ground. A larger contact patch distributes the force of the vehicle’s weight so that it does not dig into the sand. Contact patch is governed by vehicle weight, tyre construction and tyre pressure, not tyre dimensions. It’s simple physics. The contact patch is dictated by the pressure in the tyre and the force pushing the tyre down.
It is important to understand that it is the air in the tyre that holds up the weight of the vehicle and not the tyre. The tyre is there to hold the air in place. If the air is doing the work of holding up the weight then the air pressure dictates the characteristics of the interface between the tyre and surface.
The unit for pressure (pascal) is force per unit area. One pascal is one newton per square metre. A kilopascal (kPa) is 1000 newtons per square metre. Pressure can be calculated according to the formula:
Pressure = Force / Area
Manipulating this equation, we can get a formula for area, which we will use to calculate contact patch:
Area = Force / Pressure
So contact patch is dependent on force and pressure and has no regard for dimensions. What this means is at the same pressure, a narrower tyre will deform more than a wider tyre to achieve the same contact patch. Any sized tyre will deform just the right amount to achieve the contact patch described by the formula above. Lets substitute some values to see how it works.
We’ll assume a tyre is inflated to 40 psi. This translates to 275 kPa. Lets assume a weight on that tyre of 400 kg. To calculate force from weight, multiply by acceleration due to gravity (9.8m/s/s):
Force = 9.8 x 400 = 3920 N
Now we can calculate area:
Area = 3920 / 275000 = 0.0142 square meters or 142 square centimeters
So the contact patch of a tyre under these conditions is about 142 square centimeters. This is independent of tyre dimensions. If the tyre is say 20 cm wide then the length of the contact patch is about 7.1 cm (7.1 x 20 = 142). If the tyre is 30 cm wide then the length of the contact patch is 4.73 cm. What if we halve the pressure?
Area = 3920 / 137500 = 285 square centimeters
Halving the pressure doubles the contact patch. This occurs for a tyre of any dimensions.
Tyres aren’t perfectly flexible. These formulas assume a perfectly flexible tyre where the rigidity of the tyre does not contribute to holding the weight of the vehicle. In reality the tyre itself would contribute. For example the sidewalls of a tyre can support some weight and so would contribute to supporting the weight of a vehicle. This means the calculations are not very accurate. However the sidewall of any similarly rated and constructed tyre will contribute in a similar fashion. This means in a comparison between tyres (fat vs thin), the effect of sidewall mostly cancels out and the fact that contact patch is governed by pressure essentially holds true (but the relationship is no longer linear). This yields the following conclusion:
Two tyres of similar construction, of different dimensions, will have about the same contact patch when at equal pressure.
When you measure contact patch of a tyre and then measure it again at a lower pressure, the change in contact patch may not very accurately reflect what is predicted by the formula above. The contact patch will always increase less than what the formula predicts, since, at greater deflections, the tyre’s sidewall will contribute more to supporting the weight. Bending the sidewall more means more force must be applied. That extra force from the sidewalls means the contact patch grows less than what pressure alone would predict. More flexible tyres will conform closer to the ideal equation. More rigid tyres will deviate more, with contact patch growing much less than predicted by the pressure equation. This yields another important conclusion:
At equal tyre pressures, a tyre of greater rigidity will have a smaller contact patch than a more flexible tyre.
The above conclusion has nothing to do with tyre dimensions. It’s related to how flexible the tyre construction is. When analysing the effects of a particular parameter (in this case tyre dimensions) you must keep other parameters constant in order to isolate the effects of the parameter you are trying to analyse. This article is about wide tyre vs thin tyre, not rigid tyre vs flexible tyre (or bias tyre vs radial tyre, etc). The key point of this article is that, for similarly constructed tyres at the same pressure, tyre dimensions do not effect contact patch by much. All other things constant, a wide tyre will have a similar contact patch to a narrow tyre and, at the right pressure, both will perform well in sand.
A wider tyre will need to flex less to achieve the same footprint of a narrower tyre. This means a wider tyre may be able to run at a lower pressure when compared to a narrower tyre, since the narrow tyre will deform more and so increase the risk of pinching the tyre tread between the rim and the ground or damaging the sidewall due to excessive flexing. In this case a wider tyre may offer an advantage since it can be run at lower pressure. However, for a typically weighted car running tyres with reasonable profile and width, extreme minimum tyre pressure is determined by the minimum pressure required to keep the tyre bead seated (about 5psi) rather than the rim impacting the tyre. To recover a bogged vehicle, any tyre will allow pressure to be dropped very low, say around 5psi, unless the vehicle is particularly heavy or the tyre has a low profile. The constraint is usually the tyre bead. So a wide tyre may not offer any benefit. In fact a very low profile wide tyre will be worse than a narrower tyre of large profile, since the wide tyre will have so little room to deform and thus risk rim contact with the tyre tread.
There are some arguments that actually favour a narrower tyre in sand. Consider the shape of the footprint. You could argue that a longer footprint afforded by a narrow tyre is better than a wider footprint in a wider tyre because a long footprint behaves more like a tracked vehicle (for example a bulldozer or tank) with the longest dimension in line with the direction of travel. Long and thin minimises the roundness of the interface between tyre and surface. A wider tyre has a wider front edge and rear edge, which is curved and no longer in the direction of travel. This thin and wide contact patch with wide curved edges also encourages the sand to flow more like a fluid, escaping out the curved edges rather than being held captive by a flatter interface. A narrower tyre provides a flatter interface so helps the sand behave more like a bulk solid. Similarly a wider tyre creates a wider “bow wave” and has to push more sand out the way which makes it more difficult to advance when compared to a narrow tyre. These are all interesting theories that support a narrower tyre being superior. Based on my observations in the field I don’t think it makes much difference. Any tyre at correct pressure will perform well in sand. Contact patch is mainly dictated by pressure and real world experience suggests that any tyres perform well in sand when at appropriate pressure.
There are similar theories regarding best sand tyres in terms of tread pattern, construction type, shape, etc. Some say less aggressive tread patterns are better because they don’t dig as much, providing better flotation. Same story for worn tyres being better than new. Sounds plausible. This is substantiated by specialised sand tyres having very smooth tread patterns (looking like airplane tyres). But in the field I can’t say I’ve noticed much difference. I’ve never seen a tyre that doesn’t perform well in sand when at correct pressure. Practically all observed differences can be put down to differences in tyre pressures and vehicle loading. However one thing that does make a difference is tyre profile. Taller tyres have more room to deform, allowing lower pressures to be run. Low profile tyres don’t have much room to deform so run a greater risk of pinching the tyre between the rim and ground. However a low profile tyre still performs the same as a high profile tyre when both at the same pressure. The taller profile tyre is better only because it may allow lower pressures to be run.
Larger overall diameter is helpful in sand. A larger tyre has a flatter interface with the surface and so may help the sand behave more like a solid. Smaller diameter tyres have a sharper radius and so sand may flow out more easily from under the tyre. Also, as a tyre penetrates the surface of the sand (becomes bogged) some parts of the contact patch are no longer horizontal and the relationship between pressure and contact patch becomes distorted. Pressure dictates contact patch perpendicular to the direction of force. When penetrating the surface the tyre’s contact patch becomes larger than what the pressure dictates since some of the contact patch is no longer perpendicular to the weight of the vehicle. A larger diameter tyre will provide better flotation under these conditions. Also, as a tyre penetrates the surface, a larger diameter tyre has a lower angle with the surface to overcome, which makes it easier to advance. A good example are the huge rear wheels on tractors and their incredible ability to not get bogged. Tractors have very larger diameter tyres. They do not have wide tyres.
So for tyres and sand driving, tyre pressure dominates all other factors to the extent that other factors mostly don’t matter. There may be some other factors at play but they are dwarfed by the effect of pressure. Reduce pressure until you float on top. Any tyre will work.
You don’t need fat tyres to enjoy this.
See also:
Bridgestone Dueler D694LT Review
Touring / Travelling Phone and Internet Setup
outbackjoe 
Hey, I’m enjoying your blog!
However, I’m pretty sure you’ve made an erroneous assumption here.
Namely, you’re assuming that the contact pressure at each tyre is equal to the tyre inflation pressure. I don’t think this assumption holds, meaning that in the equation of A = F/P, the pressure should not be fixed. Only the the force should be fixed (at eg. 3920N).
The page at http://www.performancesimulations.com/fact-or-fiction-tires-1.htm seems to have some reasonable data that shows this assumption to be false and indicates that tyre behaviour is *far* more complex than A = F/P. The data also indicates that wider tyres will generally provide a larger contact area and thus lower contact pressure.
Intuitively, I think this can be understood by imagining two solid, undeformable “tyres”, one 20cm wide and one 30cm wide. If both tyres sit on an undeformable surface, then it is obvious that the line of contact is equal to the width of the tyre.
Extend this to slightly deformable, (still solid) “tyres” on a slightly deformable surface and *then* you can use P = F/A to calculate the contact pressure (not inflation pressure). Eg. P1 = F/A1 (20cm wide tyre) and P2 = F/A2 (30cm wide tyre). In this analysis, it seems obvious (at least to me) that A2 > A1 and therefore P2 < P1.
Of course this is a simplification, and it relies on the material of both solid "tyres" deforming equally, irrespective of their widths… but I think this is a reasonably accurate assumption.
I'd love to hear your thoughts/feedback.
And keep up the good work with your blog!
Hey dopplershift
Thanks for your comment. Why do you think A2 would be greater than A1 in your example? The wider tyre would deflect less. The force is distributed over more stuff. The stuff would deflect less no matter what it’s made of, solid, foam, traditional pneumatic, anything. It’s simple physics.
The source data in that article you link to is spurious. No matter how tyres behave, they must obey basic physics. The data indicates less deflection at lower pressure for some data points. This is impossible. So we know there’s errors in the data. Also, a lot of the data is measured at low weight. The lower the weight, the greater the percentage contribution from the sidewall, and the greater observed deviation from the pressure prediction.
For a perfectly flexible tyre, the contact patch would obey exactly according to the air pressure. Sidewall has some contribution to supporting the load and thus it becomes more complicated. The point of my article isn’t that the equation is exactly right. It just helps explain the fact that vehicles with skinny tyres can negotiate sand as well as vehicles with wide tyres when the pressure is set correctly. The relationship between contact patch and air pressure holds to a certain extent. If you want to drive in sand you don’t need wide tyres. Let down the pressure and go for it.
Ok. I’ll concede the point about a wider solid tyre deforming less than a narrower one, for the same load. That was definitely an oversight on my behalf.
However, I’m still not willing to accept that the contact area can accurately modelled by A = F/P. I think the interaction between the tyre and the surface is far more complex than that, especially for compliant surfaces like sand.
Anyway, I agree with your other comments about driving in sandy/beach conditions.
Hi Joe,
I have read your discussion and all the comments about tire width on sand
with interest as I am deciding wheel and tire combinations for a build.
I see your argument and it is compelling, all things being equal, more tread
on sand in line with direction of travel (narrower + taller) is better than more
tread on sand perpendicular to direction of travel (wider and shorter).
You make a convincing argument against the simple dogmatic truism that
a wider tire is always better on sand.
However, I do see one flaw. Not in the logic, but in the fundemental
assumption that at all wheel diameters, tire diamters, tre materials and design,
and tire pressures, the tread flex in line with the direction of travel
is the same as the sidewall flex perpendicular to the direction of travel.
IOW, that each unit of psi tire pressure equals the same tire deflection
regadless of the proportion of width and tire diameter.
To put it another way, your argument is sound in theory, but has issues in
the real world which is almost always more complicated and holds
unforseen dynamic and consequences.
A not uncommon problem in both science and engineering.
I suggest your argument offers a hypothesis (but should not nee seen as
conclusive) and therefore calls for research comparing air down increased
sand tire effective contact patterns across tire width and diameter (do not forget
wheel diameter, not mentioned in your argument, but that also impacts all this)
by tire model and design tread and sidewal specifications and characteristics.
Still, your argument has definately influenced my thinking in terms of wheel and
tire combos for use on large loose soft wind driven sand dunes.
Best, Mike
Hey Mike yeah it’s more complicated, but I’m not trying to suggest that skinny tyres are better. The point I am making in the article is that there are many different hypothesis, all of which are untested, but it doesn’t change the fact that a vehicle with skinny tyres will go great on sand if aired down appropriately.
great insight on skinny vs fat tires..
Another analogy — would be water skis or surf boards…. a longer / narrower water ski tracks much better than a wider shorter one. For surfing– A long board is much more steady in the water than a “short board” …
However if you want to do tricks and turn on a dime in the sand– then maybe wider tires perhaps is better than narrow ones.
My Vanagon Syncro, with the 255 60 r15, was much better on the sand than with the O.E.M. 205 70 r15 tires.
Is it a matter of a bigger diameter? May be.
The Syncro Heresy
Yeah could be bigger diameter. Could be different pressure, different tyre construction, different environment. If you’re human then psychology would play a role.
A correction for my previous comment: the OEM tires of my Vanagon Syncro where 205 70 r14 not r15 as mentioned. So the great improvement came with 255 60 r15, (Fulda Trump 4×4).
Thank you.
I’m really late to the party, but I felt compelled to mention that there are a lot of bad assumptions in the math. The most basic one is that the force is uniformly distributed over the contact patch. It’s not. Pressure is uniform, but the force in the contact patch will be highest at the center of contact, and fade off to zero at the edge.
On AVERAGE, the forces would be the same, but the PEAK forces experienced by narrower tire are higher vs. peak forces of wider tires. This is exactly why very narrow tires will cut into the sand; high peak forces.
In short, for any given tire, lowering the pressure will increase the contact area and reduce peak forces. So, yes, lowering the pressure makes a difference. Is this an argument for narrow vs. wide tires? No.
And, or course, side wall height and strength have a LOT to do with how much a tire deflects under load.
Hi Brandon
Can you enlighten me on the many other bad assumptions? And why vehicles with skinny tyres go so well in sand when at appropriate pressure, including my vehicle that is frequently used for beach camping?
Apart from the contribution from the tyre because it is not perfectly flexible, the contact patch would be of roughly uniform force because the majority of the load is held up by the air and the air cannot exert a different force at difference points.
Regardless, why would this effect favour wide tyres?
Interesting what you have come to , now let me tell you some FACTS about heavy sand driving you are correct about tyre pressure being or can be a critical point , I read and re read you narrow and wide tyre theory . Coming from the west coast of SA my family were pioneers in the abalone industry , we bulldozed access to beaches up the whole coast 50 years ago , drove early land cruisers and land rovers every where dragging boats in and out of the water , early days ALL narrow tyres , the only thing that saved us 100s of times was big powerful winches as PTOs front and back running of the. Motor motor , then come along wider and higher tyres , same vehicles and boats being towed , the skinny tyres were often bogged as I drove past , then we put on in some places in very soft air rated ( bloody dangerous )sand and surf on the beach bomber aeroplane tyres and stop having to use the winch much , anyone with skinny tyres went home and got rid of them
traction is sheer stress via pressure
It is obvious that if an elephant had points as soles, instead of the wide, circle like, feet it has, the movement will was mission impossible.
Not if the points were inflatable points, inflated at low pressure. They would collapse flat.
Very interesting reading – thank you for the information. I also enjoyed dopplershifts comments.
No probs terry, cheers.
The collapsed flat points are not points any more.
The notion is that of a narrow base vs a wide base, on the sand, which is capable to withstand the pressure without burring.
Yeah collapsed point is not a point and a flattened narrow tyre deforms more to match the contact patch of a wider tyre. Not sure your point.
Someone testing this very point
Good video, nicely done, nice matching of tyres. Results confirm everything in this article.
Nope, don’t believe it for a minute.. otherwise race and drag cars wouldn’t bother running wide tyres..
Your error is in assuming that a narrow tyre will deform more, whereas two identically constructed tyre will deform proportiuonally at the same pressure.
Hey Dave, not sure how race cars are relevant to this article, can you explain? Are you suggesting that race cars have wide tyres so they don’t get bogged on the race track?
A narrow tyre MUST deform more, if the laws of physics are anything to go by. How can less rubber support the same weight without deforming more? How is it possible for the narrower tyre to generate the same reaction force against the weight of the car as a wider tyre without deforming more? Doesn’t matter whether it is filled with air or not. If you have less stuff, then, to produce the same force, that stuff must deform more.
Lay down on a bed and measure what the maximum deflection is to support your weight. Then stand on the bed and again measure the deflection. It must deflect more. The weight is the same but it is concentrated over less stuff, the stuff must deflect more. This is common sense. Can you explain how you have come to your physics defying conclusion?
Μy, not deflated, point is that, with my combo, (three lockers, (the central auto), gelande, v6 174hp, 255 60 r15), I was able to go on the sand without the need to deflate my tires.
In this case, as in the video above, the wider tires are better, even a little bit, which, small or bigger difference, may be The difference in some situations.
Another point I keep from the above video is that different makes may behave differently.
Not an easy equation!
Hey Joe,
It’s a very interesting and mathematic aproach of airing down tires, no talking about ‘i think this or that’… well done!!
I drive my 4wd on skinny 7.50×16 tires. Question: Are there any advantages to upgrade to 255/85×16 in relation to contact patch? Yes these tires are wider but also higher. Can i create the same lenght (or longer)of contact patch than the 750×16 tires? And this in relation with ground clearance?
Maurice from The Netherlands.
Hi Maurice
At a given pressure, the length of the contact patch will be less on the wider tyres.
Larger diameter tyre is better since it gives more ground clearance and more room to run lower pressures, and is better when the tyre starts digging in.
Joe
You my friend are so full of s**t that you need flushing. You have obviously spent next to no time driving on soft sand beaches. Maybe your idiotic idea applies on hot, hard underlying desert sand but on powder soft beach and nothing beats width plus low pressure. I sure hope no one takes your advice.
Good contribution Graham. Well done. Bet it took you hours to prepare that. I guess I’ll just take down this article, what with the perfect explanation you provided, backed by a mastery of science and logic.
Hi Joe
A more accurate statement would be to say that narrow tyres don’t perform as bad as we think they do, and in some cases outperform wider tyres. However, on the road, in sand or mud, wide tyres have a clear advantage.
It is generally agreed that taller narrower tyres do better in all conditions but sand snow and mud. This is where wider tyres do better.
Have a look here : http://www.expeditionswest.com/research/white_papers/tire_selection_rev1.html
Some extracts :
“The benefits of a narrow tire:
The Argument: A tall, narrow tire is a better choice for all off-highway surface conditions with the exception of soft sand, snow and soft mud that’s depth exceeds 110% of the vehicles minimum ground clearance. Here is the explanation.
The Benefits of a Wide Tire
Greater Section width for flotation: A typical 33×12.5 all terrain tire will perform better on soft surfaces like deep mud, snow and sand than its metric equivalent (285/75 R16 or 33×11.2) as the weight of the vehicle is spread out over a larger surface area. The wider tread creates less stress to the surface tension of the strata of sand (as expressed in kN/m2) and the vehicle will not sink as easily. The smoother and wider a tire is, the better it will perform in sand, as the width creates flotation and the smoother tread displaces less sand under (horizontal) acceleration (shearing force). The same influences apply with snow and mud. If the snow and mud are deeper than 110% of the vehicles minimum ground clearance, than it is better to run a wide tire, aired down and have the vehicle “float” on the surface.”
Hi OzzieSurfer, thanks for your feedback. Interesting info in that link. If both tyres are at the same air pressure, why wouldn’t the narrower tyre deform more to yield the same contact patch surface area and pressure as the wider tyre?
Hi Joe – I am not sure (in response to your reply).
You have some really good articles on your site.
Here’s my personal experience. Apart from four wheeling in sand for a number of years, I am also a keen mountain biker. I have a fatbike with 26″ x 4.6″ tyres and a normal mountain bike (tyres much thinner but wheels 3 inches bigger at 29″ x 2″).
Both bikes allow me to ride in the sand if I deflate the tyres as much as possible. However, the fatbike with much wider tyres allow me to easily cycle in soft sand and up dunes with a slight to moderate incline.
Dropping pressures to the limit on both bikes has a huge difference in their abilities compared to fully inflated so I agree tyre pressure is key. However even with tyres dropped as far as possible on the 29×2 it is still not adequate – ever tried cycling with a road bike in sand vs take a bmx in a bit of sand? The difference between the two bikes are similar to a degree but dropping pressures on the 29×2 does make it a lot more manageable.
There is no way I can cycle up dunes in soft sand with even a slight incline on my normal mtb with 29×2″ tyres. I have done a couple of back to back tests with a friend and we both agree the fatbike wins by a mile in sand riding ability.
If anything the two bikes I use should serve as the perfect example according to the theory that thin tyres should do just as well or better than wider tyres. (29″x2.0″ vs 26″x4.6″)
I personally think the reason for the better performance of wider tyres in the sand is becuase of the extra flotation that the width of the tyre brings. True, tyres that are bigger in diameter have a longer contact patch but wider tyres have the flotation benefit that is quite important in sand IMO.
One thing that hasn’t been discussed is what is the optimal width? Too narrow and not enough flotation and too wide and the extra resistance becomes too much.
Thats some good first hand experience you have on a bike. I reckon the reason the fatties on your bike perform better is because they allow lower pressure to be run. The tyre has more space to deform. When you say you deflate as far as possible you probably base this on the appearance of the tyre and risk of pinching between rim and tyre. A great experiment would be to compare the two tyres at the same pressure.
Racing tyres run in excess of 90 psi and cut straight into the sand like a knife. Then at 40 psi they already look flat because they have no space to deform in their profile and, due to being so narrow, they deform lengthways a lot to elongate the contact patch to obtain the necessary area and so the rim moves a lot closer to the tyre than a wider tyre would. They look flat despite still being relatively hard and risk of pinching between rim and tyre is very high.
In my article I describe why, at the same pressure, both the wide tyre on your bike and the narrow tyre on your bike would have about the same contact patch and therefore about the same flotation. But if the fatties can run lower pressure then they have better flotation.
In response to your question : If both tyres are at the same air pressure, why wouldn’t the narrower tyre deform more to yield the same contact patch surface area and pressure as the wider tyre?
A simple test : I take 2 mountain bikes out in the sand, 1 with narrow 29″x2″ tyres and another with 26×4.6″ tyres. All tyres deflated. The narrower, taller tyres will have a slightly longer contact patch, the wider tyres will have a shorter contact patch. But the wider tyres will ultimately have a much bigger contact area : 2″ vs 4.6″
That’s my take on it.
But why will it be bigger, since the extra length of the narrower tyre offsets the extra width of the wider tyre? The area is about the same.
“I reckon the reason the fatties on your bike perform better is because they allow lower pressure to be run. The tyre has more space to deform. When you say you deflate as far as possible you probably base this on the appearance of the tyre and risk of pinching between rim and tyre. A great experiment would be to compare the two tyres at the same pressure.”
I couldn’t measure the tyre pressure as on a bike when you go below a certain pressure, traditional tyre guages don’t measure anymore unlike on car tyres. I went as far as I could without having rim pinches.
I can’t verify if the fatties allow lower pressures to be run. What is the case between narrower 4wd and wider 4wd tyres – I would imagine there won’t be any difference?
My take is that the added flotation from wider tyres are sufficiently beneficial to make a moderate difference in sand.
Yeah in a car a wide tyre may be an advantage if it allows lower pressure to be run. The wideness does not add flotation, but it does add some margin between rim and tyre and so allows lower pressure to be run. However because they are tubeless tyres, the lowest pressure is usually dictated by the pressure required to keep the bead seated. These are pressures so low that most people never encounter them anyway and so it makes no difference.
Some more food for thought :
Since my 29er mountain bike has a diameter than was 3 inches bigger (29″x2″) than my fatbike (26″x4.6″), and is running a tubeless setup vs tubes on fattie, surely the extra diameter should lead to two distinct advantages :
– Larger contact area length wise due to 3 inches diameter advantage
– Larger contact area length wise due to narrower tyre width (2″)
In theory at least the narrower tyre should have matched the fatter tyre’s overall contact area, so why does this not translate into real world performance?
Lastly, just to throw all theories out the window :
http://www.performancesimulations.com/wp/fact-or-fiction-tire-contact-patch-size-is-determined-mostly-by-weight-and-tire-pressure/
I don’t agree with your theoretical advantages. It’s repeating the same argument as saying wider is better. Longer contact patch is offset by narrower contact patch and therefore area is roughly the same and so is flotation. Same for any arguments about tyre diameter. It all comes down to pressure and not tyre dimensions. Fatbike, with more room to deform and tubed tyres, is able to run lower pressure.
That article has been linked to above already and has many problems some of which I mention. There’s bad data, bad assumptions and bad conclusions.
“Fatbike, with more room to deform and tubed tyres, is able to run lower pressure.”
So wider is better?
See my comment starting with “Yeah in a car a wide tyre may be an advantage…”
Wider is better if it affords lower pressure, not because it’s wider. The wideness adds no extra flotation. Simultaneously a taller narrower tyre is better than a low profile wide tyre. The taller narrower tyre has more room to deform and so can run lower pressure. So in that case wider is worse. A bead locked skinny tyre is better than a wide tyre because it can run lower pressure. Again wider is worse. Tubed skinny tyre is better than tubless fat tyre if the lowest pressure is dictated by keeping the bead seated rather than pinching tyre with rim. Again, wider is worse.
The best tyre is the tyre that can run the lowest pressure. But for most people it makes no difference since the lowest pressure is below the pressure that allows them to easily do what they need to do. Air down until you can drive easily, any tyre will do. You can verify this by driving a Toyota Landcruiser with factory cheese cutter tyres and seeing its excellent performance on sand once aired down.
Great blog, Mr. Joe!
While we’re discussing the pros and cons of wide tires, can you suggest what kind of TREAD to look for in a tire if you expect to drive mostly on sand?
I’ve driven using expensive mud tires in the Sahara, but I also see plenty of pickups with hardly any tread on their tires and they seem to be fine.
Finally, perhaps I’m wrong, but it seems that a mud tire might be a bit better for rocky desert terrain and at avoiding punctures in such terrain than a regular road tire.
Thoughts?
Hi Francis
I don’t think tread pattern makes much difference. In theory less aggressive tyres are better because they dig in less and so make it harder to get bogged. In practice I haven’t noticed any difference provided the tyre is deflated adequately. I talk a little bit about it on the how to drive on sand article.
For rocky stuff, I think it’s mainly the tyre construction, rather than the tread pattern, that dictates durability. You want a heavy duty 4WD tyre. Light truck construction is a good option. I don’t like mud terrain in general because it increases fuel consumption and I think the only advantage is when in mud.
I’m just down the road from you. I’m working in Mozambique at the moment, on a coal mine. How did you develop such hardened balls of steel to travel in a vehicle throughout all of Africa? The average westerner barely even has the balls to leave the security of their own home! I’d love to bring my vehicle across and do some overlanding throughout Africa. You’re an inspiration!
Joe
Hi Joe! Thanks for explaining why tire tread isn’t that important, although I have no clue how to judge “tire construction.”
Like you, I have a Toyota Hilux. It’s a 2014 model, so you won’t like it since it’s got more bells and whistles. 😉
Do you recommend any brands of tires for it and sand?
Thanks for the compliment about my balls, but there’s a fine line between bravery and stupidity. I often cross into the stupid zone! 😀
I’d love to right alongside your Hilux or have you drive mine. 🙂
I’ll be driving through northern Mozambique in a few months.
You can see the basic path I’ll be taking on Africa54.com
Where you would totally love is driving from Southern Chad to northern Libya across the Sahara.
That’s scheduled for November-Dec 2017.
If you want to coordinate a meetup, just write email me (through the contact form on my website).
Thx!
Nice car selection. When did you upgrade to a Hilux? Your website says you have some Spanish vehicle that I’ve never heard of.
I use Bridgestone Dueler D697. They’re a strong good performing tyre. Popular in Australia on touring vehicles and on mining vehicles. BFGoodrich are also popular but I’ve never used them. There’s some other brands that are pretty good too but I don’t have any experience with them. Some of the tyre sizes have “LT” in the name, this means light truck which is a stronger construction than normal tyres.
Also, to minimise tyre damage on rocks, let the tyres down a bit and drive slowly.
I’m in Tete, in northern Mozambique. Not sure if I’ll still be here in a few months, I’ll see.
I upgraded to the Hilux in Benin, over a year ago. I love it!
That other thing sucked!
Thanks for the LT tip.
I’ll look for that!
I’ll definitely be near Tete in a few months. I’ll be climbing Mozambique’s tallest peak from the Zim side.
Then a month later, I’ll be entering Mozambique from Malawi.
How long do they let you keep your car in Mozambique with a temporary import license?
I need 6 months!
Thx!
Not sure about temporary import, all the cars on the mine are Mozambique registered.
I reckon a good strong all terrain tyre in light truck construction is the way to go for touring. Nothing too special, not too expensive, not too specialised, good availability, good tread life, good fuel economy, durable, excellent performance on all surfaces except deep mud.
Great read, I agree completely apart from one factor,
Newtons third law tells us that you need to think about the sand also. It needs a minimum contact area for the car to “float” regardless of what the tyre pressure is. A wide tyre will sink in less than a narrow tyre in order to achieve this.
Your argument is that the narrow tyre deforms more and so the contact area is equal (similar), however imagine tyres at 40 psi, a wide and a narrow tyre. If we sit them on concrete the narrow tyre will, as you stated, deform more so the contact patch is equal to that of the wide tyre. The concrete will deform insignificantly due to it’s stiffness. Now if we put both tyres in soft sand, the deformation of the tires will be the same as before (quite minimal), however they both sink into the sand. The wide tyre will sink in a smaller amount than the narrow. This is because they sink until they achieve the required surface area as dictated by the “softness” of the sand. Ie as you mentioned many times, the contact area will be the same, therefore due to the width of the wide tyre it will sink in significantly less than the narrow tyre to achieve this. For an example, if we were to assume that the tyres are holding an almost perfect cylinder shape due to the 40 psi (minimal deformation) we have them at, then the amount each tyre sinks into the sand is a result of the chord length between the sand-tyre contact points and width of the tyre (projected surface area). The relationship is not linear, the further the tyre is required to sink in, the smaller the successive increase in projected surface area. A 76cm diameter tyre that sinks in 5cm has the same projected contact area as a tyre half the width that sinks in 22.75cm. Therefore the narrower tyre must “climb” more and move sand out of the way as opposed to the wider tyre which is sitting higher on the sand.
This being said, at lower pressures, the surface areas of the tyres are increased and the amount they sink into the sand is decreased therefore the sand softness becomes less relevant HOWEVER it will always be a factor as long as the tyres continue to leave an imprint in the sand. In response you might say that the contact area is the same therefore the tyres will sink in the same amount but this is not true, the contact area is a result of the tyre deforming AND the sand “softness”, not one of them alone, therefore the wider tyre will always sink to a lesser degree.. Your argument is only true in situations where ALL contact area is a result of deformation of the tyre alone ie on concrete or hard packed sand. Unfortunately for us the “softness” of the sand is not a simple stiffness property but a combination of internal friction and density (buoyancy) among other things so we can’t simply apply the pressure equation to it.
This is why the two bikes, fat tyre and thin as mentioned by OzzieSurfer provide greatly different results. Bicycle tyres cannot run at very low pressures meaning the sand softness is the far greater contributing factor to the contact area. Running them both at a regular pressure would quickly result in the thin tyres digging into the sand and the fat tyres sinking an unnoticeable amount. Even if you increased the pressure in the fat tyres alone, their floating properties will still be far superior.
Just my 2 cents.
Hey Jacko with two tyres of different dimensions at the same pressure and contact patch and therefore supported by the same area of sand, can you explain why one would sink deeper than the other?
outbackjoe,
It is true that the tyres will have the same contact patch however the contact patch will be a result of both the deformation of the tyre and the tyre sinking into the sand.
In your example you calculated a contact area of 0.0142 square metres and a length of 4.73cm for the 30cm wide tyre. Now imagine this 400kg tyre in soft sand, the length of the patch will be closer to something like 40cm, depending on how soft the sand is. My point being that the contact patch size is actually going to be largely governed by the “softness” of the sand.
If the sand needs a 0.1 square metre contact patch for tyre float, then the tyre will sink into the sand to a point where this contact patch requirement is met. (For this arguments sake lets assume the tyre holds the same shape on concrete as it does in the sand). If your tyre has a contact patch of 0.06 square metres on concrete, it will sink in the sand until it achieves the remaining 0.04 square metres required. To achieve this remaining 0.04 square metres, a narrow tyre must sink into the sand farther than a wide tyre.
We know this is happening because we see our tyres leaving tracks. If you measure the contact patch length in soft sand it will ALWAYS be longer than that measured on concrete meaning extra contact area is coming into play as governed by the sand.
Hi Jacko ah yeah I get you now. Interesting point. So if the tyre pressure isn’t low enough and exceeds the pressure that the sand can support, the narrow tyre might sink in a bit more into soft sand which presents some extra resistance that must be overcome. Is this offset by the fact that the wider tyre, although sinking in less, has a wider frontal section of sand that must be overcome? Which is better, narrow and deep or wide and shallow? Minimising the frontal area crossing the direction of travel might be the best, and actually some people use this as an argument against wide tyres, since wide tyres are always ploughing through a wider section of sand. I don’t think it makes much difference.
If the tyre pressure is low enough, any tyre won’t break the surface and will float on top, even though there will still be tracks in the sand from compaction. If the tyres are too hard then you dig in and start churning through. Maybe a narrower tyre will dig in a bit more which makes a wider tyre better. Maybe a narrower tyre, having to plough through less frontal area of sand, is better. Maybe a narrower tyre, with its flatter and longer contact patch, has more traction since there is more tyre surface and mechanical “keying” into the sand perpendicular to the direction of travel, like a tracked vehicle, and so a narrower tyre is better. We could speculate on various scenarios until the cows come home. Cases can be made for wide tyres. Cases can be made for narrow tyres. I think the effects are minor and / or cancel each other out. There’s so much complexity with the sand interface with stuff like friction of being engulfed in a bulk material vs friction of a flat surface, water and air in the sand, contact angles no longer being horizontal as you break the surface, compaction of sand and the fluid like behavior that bulk materials exhibit under dynamic conditions. Since I observe any tyre performing well at low enough pressure, I don’t think it makes much difference either way, but it does make for interesting discussion.
Hi outback joe. Such a great blog you run! And me trying to set up a second hand hilux for touring- just gold to find it.
I agree with jackosaurus. The contact patch might be similar on wide and thin tyres but to gain the same area, the thin ones need to use a greater length. This extends the contact of the sand further around the circumference of the tyre, inherently requiring the thin tyre to sit deeper.
This creates two problems; firstly, the angle of force on the sand changes in the same way as you describe for the small vs large diameter tyre comparison ( by the way increasing contact patch due to lower downward force per area on average). Secondly it increases sand friction with the sides of the tyres which restrains movement. I find it harder to drive with skinny tyres on my beaches. But mine are all soft, wet and chopped up so every tyre, even very low pressure tyres, sink in and have to battle.
Also, I think the skinny tyres struggle to float out of ruts as quickly as wide ones when you need to move over. But I’m not sure of that yet.
Keep up the good work.
A very interesting discussion,
Sounds the the benefit either way will be minor. Intuition however still tells me that fat is better like atv and beach buggy tyres, which for their diameter, are much wider than 4wd tyres. There can be no argument however that the most important factor is tyre pressure. Have you seen the Russian made sherp? Seems like they have this whole pressure and tyre size thing down pat.
http://odditymall.com/sherp-russian-all-terrain-vehicle
Like I say in the article, if space to deform is the constraint for running super low pressures then a wider tyre can help enable lower pressures to be run. Quad bike tyres are designed to run super low pressure and be able to tolerate impacts and aggressive driving so they are both high profile and wide so there’s plenty of room to deform without pinching the tyre or damaging the rim. That sherp is pretty extreme, do those tyres double as water paddles?
Yeah, they do work as paddles, moves over the top of water with a grace that would make Jesus jealous.
I think the only way to get a definite answer is an experiment with strictly controlled variables, until that happens we will never truly know. In the mean time I will take your word for it and won’t go wider on my vitara, might go a little bigger on the diameter though to get some more clearance.
Yeah a good experiment would be very interesting. And it may well validate that wider has some advantages. But by far the dominating factor is tyre pressure. That is the key point. Don’t let someone tell you that you can’t go four wheel driving or camping somewhere coz you don’t have big fancy tyres. Let down the pressure and you’ll go anywhere.
Vitara ay, I had a sierra which was legendary in sand with stock tyres. Vitara is probably even better – not much more weight but a fair bit more power. Don’t go too big or you’ll really feel the power loss.
Ive had a lot of experience in this area I have driven heavy machinery in mining and transport applications and have been involved in many heavy vehicle recoveries in underground mining. Ive also been driving on sand basically all my life in 2wd and 4wd vehicles.
The only constructive comments I can make as to the debate re narrow vs wide tyres applies to fairly specific conditions using high profile wide tyres. Cant say I have ever been totally stuck in soft sand for a long period of time possibly due to luck but certainly good management and good nouse and problem solving skills.
A few years ago I came across a Prado towing a small caravan bogged in the exit to a dry sandy beach in Qld there were 2 vehicles linked together hopelessly bogged in wet sand trying to tow the combo back down the slope of the beach. I was towing a horse trailer and was having a day with my kids at the beach on the horses I was unwilling to get involved. We all too often became involved in time consuming and sometimes risky recoveries over the years and
As I was “floating” around on the soft sand towing the said horse trailer with an obviously capably set up vehicle the hapless driver of the bogged pajero begged for help even offering $50 for my trouble. The kids basically insisted they were fine …go on get em out! So I unhitched the trailer and dropped my (33×12.50R15 BFG Mud) tyres until they had a healthy bulge I estimate around 10 -12 psi didn’t check as I really didn’t want to do this. I instructed the guy to drop his pressures accordingly on both the Prados tyres and the caravans tyres. Gave bystanders a shovel to clear the sand from in front of the tyres and handed him a handheld 2 way radio so I could instruct him on how to apply power and when to back off, Organised some Maxtracks to put under his front wheels… Well you should have heard the commotion the bystanders calling me an Idiot!!! You can’t tow that up there you will never get it out…blah blah blah . When I swung my vehicle around and hooked a chain (not a snatchstrap) onto the front of the Prado (useless things under there to tow off) a couple of bystanders wandered off muttering. I explained myself quite clearly _ I was not prepared to fart arse around I was having a day with my kids – we are going up and thats it. Finito!
The point of all this is that I towed this vehicle combination albeit slowly but straight up the beach onto hard ground.
In my experience – this would NOT have been possible (nor would I have been so confident) with narrow tyres nor with tyres that did not have some form of aggressive tread and side lugs to assist. Simply due to their assistance as the tyre sinks lower into the soft sand.
Unfortunately we did not video the recovery but to those readers who would offer…. “but….” “must have been wet sand…” or” it must not have been much of a slope….or other skeptical constructive comments.. I will qualify a few things my recovery vehicle was a GQ Nissan Patrol with Front Difflocks it was unloaded, the tyres did bury themselves quite deeply in the sand but at no time did I stop forward momentum – so basically it clawed it way forward at a snails pace.
What worked 1/ Low pressure 2/ High profile 3/ Momentum 4/ Traction to all 4 wheels 5/ Understanding sand driving – i.e. varying engine revs to minimise digging holes but maintain forward momentum. 6/ communication with other driver to stop him digging holes or stopping. 7/ Wide tyres with side lugs adding to traction as they buried. 8/ Knowing my own vehicle.
My point:-
Basically under normal conditions yep I agree wide or narrow makes little difference as long as there is a high profile involved.
Under extreme conditions I am certain (and always have been) wide high tyres will outperform narrow high tyres in soft sand.
Hey Flashnick thanks for sharing your experience, interesting stuff. What makes you think that a skinnier tyre, if adequately aired down and with adequate profile, would not have been able to tow out the stricken vehicle?
Surface area to weight? Remembering they are buried deep in sand, significant bulge, side lugs. I suspect as the tyre goes deeper into the sand total surface area of the tyre inc sidewall bulge may become a factor in stopping the whole show descending even deeper. Also these days fewer people run 15″ rims and fewer run such wide high tyres on 15″. (16s yes but they lose approx 12mm of rubber between rim and ground) Im my case it gave me more ability to run tyes lower. Maybe Buoyancy does become an issue with the wider tyre buried so deep ?? Maybe. So many factors. Main thing is…I personally have never seen a narrow tyre come close to that sort of traction in deep sand. Also from personal experience I knew the vehicle would do it. As mentioned before I was driving 2WDs in sand 40 years ago.(Come to think of it the kids even knew the we could)
I think a lot of confusion reigns when comparing profiles and how much each profile can be aired down – having close to 200mm between the rim and ground (and +\- 300mm of width to increase internal surface area) allows for a lot of latitude in tyre pressure, regardless of load and allows a lot of flattening out on the bottom of the tyre…whereas even 30mm less could make a big difference.
It is a good question!
Side lugs may help, or they make it worse by digging in deeper, depends who you talk to. I don’t reckon it makes much difference. Same with aggressive tread pattern vs normal tread pattern. People make up reasons for and against, I don’t reckon it makes much difference.
It does get complicated as the tyre becomes bogged but I don’t see how it would favour a wide thin contact patch compared to a long narrow contact patch. I’ve never noticed a difference in performance based on tyre dimensions, provided air pressure is right. Like you say, you do need room to be able to adequately deflate the tyre.
Sorry to disagree.
(Speaking specifically about sand driving!)
From what I read here I reckon there is too much variability in driver skill and vehicle configuration for any real conclusions to be drawn. As you mention above – people make up reasons for and against. Some people dig aggressive tyres into sand some know how to float them over sand. What may have held true 20 or more years ago may no longer apply with new tyre technologies. (new compounds, more flexible sidewalls etc etc)
I can only speak from my own personal experience. (I live in outback WA and my childhood and youth were spent on a farm where getting tractors and trailers through mud (or not getting them through mud ) was an annual ritual) I personally use the combinations of vehicle and accessories/tyres that I know will work.
For me, if I found that tall skinny tyres would keep up with my vehicle or get me home each night – I certainly would use them.
Yes there is too much variability. What we need is a good experiment. I’m sure some differences in performance would be identified with a careful enough experiment, but my main point is, you don’t need to rush out and get big fancy tyres if you want to drive on sand. Let them down and go for it. I don’t have fat tyres and I go beach camping nearly every month, often without the backup of another vehicle, on boggy treacherous beaches north and south of Perth. I’ve never had to be recovered by another vehicle but I have recovered vehicles with fatter tyres than me, mainly coz people don’t let the pressure down enough. I putt around in soft sand basically idling whilst people with fat tyres thrash the crap out of their vehicle. It all comes down to pressure.
And skill
Curious thing, this tyre deflating. Prior to the popularisation of 4WD driving, people including me used to get around the bush a lot in things like Chevs, Ford Customlines, Peugeots, Datsuns, even VWs. I drove as a passenger all around central Africa completely off road (not even tracks) through dry creek beds and across deserts and the drivers not once deflated their tyres. One bloke I drove with for several thousand kilometres even admitted to never having used 4WD in his Troopy – never! Not once even ! He was in his 40s and had been driving offroad in Africa since a kid. Do you think it might have more to do with how people drive and less to do with deflating their tyres?
Hey Doug if someone has never deflated their tyres then they have never driven in soft sand. There’s no alternative. The impact tyre pressure has is massive. Soft sand will see you bogged to the axles in about 3 meters if tyres are at highway pressure. Yes how people drive has an impact too. Maybe the best driver will make it 4 meters.
Regarding 2WD vs 4WD it’s a pretty easy system to analyze. If you have double the motive force and you’re not dragging the underbody along the ground then you’re going to be far more capable. It’s universally understood across industry, agriculture, mining and military. There’s no 4WD conspiracy or something. But I do agree that modern culture often over does it with equipment, coz consumers are trained to always buy bigger, fancier, more expensive stuff.
Outback joe you are so stupid why do they sell fatter tires?
I had skinny tires on my cruiser and ran them at 10 psi, then had fatter tires put on and ran also at 10 psi and the difference is so obvious a child could notice
Haha why do they sell fatter tyres, good one. Why do they sell red cars and blue cars? Must be coz the blue ones are better on sand but the red ones are better for freeway driving?
Hahaha is that all you have, what about the bit where I said I had skinny tires then fat tires? The fat ones are extremely superior, I even had to change the two back tires with my skinnier spares and had a noticeable drop in performance
I have an old Nissan one and a half ton light truck, it was originally fitted with narrow cheese cutter tyres, Always used to get stuck driving around the farm in all types of conditions and terrain including mud and sand. I decided to put the largest Mudder tyres I could on the rear. This vehicle now performs like a four wheel drive and I have not been stuck since. I am now a believer of wide aggressive tyres for any off road vehicle.
Hey Bill aggressive pattern helps a butt load in mud, which may explain your experience. Also are the tyres bigger diameter? Running same pressure? Same construction type? Old skinny light truck tyres might be pretty stiff which means contact patch is lower than it would be with a more flexible tyre. A few variables to account for.
Hi Joe, fantastic article. I agree with everything you have to say about the contact patch, and you did a great job explaining it. There’s more to the story that I’m struggling with though, and that is how the contact patch size actually prevents you from getting bogged.
It seems we get bogged after a tyre begins to slip, causing it to sink as sand is pushed out from under it. Eventually, the available grip is insufficient to overcome the resistance of sand in front of the tyre, and forward progress stops. Further tyre slip only serves to compound the problem. Momentum helps as there is less time to displace sand at any given place.
Now as I’m sure you’re aware, the amount of available grip has nothing to do with the size of the contact patch – instead, it’s dictated by the coefficient of friction at the tyre/surface interference, and the weight of the vehicle. Given that tyre slip (caused by insufficient grip) appears to be the cause of bogging, how then does increasing the size of the contact patch help?
I guess maybe it’s not the tyre grip giving way, but instead it’s the underlying structure of the sand that’s failing. In that scenario, I can see how spreading the weight over a larger area helps – the lower downward pressure might mean less strain on the sand structure, meaning it can continue to act as a solid – and in turn, bring the tyre grip back into the equation.
I’d be interested to hear your thoughts. If nothing else, it’ll give me something to ponder next time I’m buried to the chassis rails in soft sand.
Hey Jim yeah in sand it’s not lack of friction that’s the problem. Like you allude to it’s sinking into the sand that is the problem. Whether it’s the spinning wheel that causes the tyre to sink, or the sand moving like a fluid and getting squashed out the way, or the sand collapsing, or the sand compacting, either way as you penetrate the surface it makes it harder to advance since you are ploughing through sand rather than riding on the surface. Think of it as trying to dig with a sharp object vs a blunt object. Dig a hole with a shovel, then dig a hole with a soccer ball. Compare results. Like the soccer ball, a soft tyre aint any good at digging holes.
Great read guys! Along with very interesting theories. At the end of the day one thing is certain regarding sand driving.. the larger the diameter the better, not just the contact patch being longer, but the larger diameter can roll over the top of the sand much easier as the angle between the sand and wheel is reduced. imagine a very small diameter wheel simply digging down vs a massive mining dumpster etc.
There actually must be an equation regarding vehicle weight vs tyre width & height. Common sense says if you use a very skinny wheel..lets say it’s 3″ wide, it will bury itself in no time, only because of the weight of the vehicle and the fact you can’t fit a 100″ diameter wheel.
I think therefore tyre width becomes essential in order to provide enough surface area to hold the car above the sand. Now this is where you need your math equation, for example.. to find out what tyre width is required to keep a 2tonne vehicle running smaller 28″ diameter wheels @ say 30psi on soft powder sand. It will get stuck obviously.
Once you have found out the formula to provide the vehicle the ability to stay afloat by increasing the wheel width, you can then basically assume that going wider from that point would provide little benefit.
Now factor in extra weight..ie recovering other vehicles, once again you need to go wider (assuming we keep the tyre pressure @ 30psi) … get my point??
I’m no mathematician, but im sure outbackjoe can possibly formulate this equation. 😉
I would kill for this equation, since I have a 3tonne 4wd camper van and I need to know what tyre width is optimal since I can only go to a max of 31″ diameter… and soft sand is problematic for me due to the weight!
Hey Chris can you explain why a wider tyre is better? Contact area is dependent on pressure, not tyre width.
For your scenario of heavy vehicle you need a large tyre in all dimensions to give you the room to be able to deform the tyre to run reduced pressure. This means a small rim, high profile tyre, large diameter and wide as well. The dimensions do not directly help, but they give you the room to run low pressure, since a smaller tyre would have the rim pinching the tyre. It’s the low pressure that gives you the flotation.
Interesting commentary and discussion above. Tire design and load capacity certainly must play a role here. The stock load range E Bridgestone tires on my FJ do not SEEM to deform as quickly as LT tires might of the same size at the same pressure. As a result it would be necessary to deflate stiffer tires more to obtain the same contact patch. Depending on the weight of the vehicle the deflection required to achieve the patch may not be obtainable before tire shedding became an issue. Perhaps a skinnier tire with a lower load capacity will deform more than a heavier, wider tire – supporting your conclusion? I did not notice load capacity to be mentioned above.
Hi Joe, what do you think about these? http://www.righttracksystemsinc.com/wp-content/uploads/2015/10/8-1024×768.jpg I’m making a set, I think the contact area will at least double.
They be good for specialized off road work. I reckon speed will be severely limited.
Thanks, yeah we’re not in a rush. Just something that’s going to work better that lowered tyres on the sand.
Hi OutbackJoe. I really liked your post. I would like to translate it into Portuguese, would you give permission?
Hi João that should be ok if you give me a reference link back to my original article. What website will you post the translation?
or would you like me to host it on outbackjoe.com?
Joe, interesting article backed up with strong arguments – but i am lost somewhere on the terminology used! Does this inclue RIM size?
My question: if I use a 295/35/R21 vs 255/55/R18 and keep the psi at 10 – my car will perform well regardless of which tyre of the two I have on?
Hey Ray I touch on it in the article when referring to tyre profile. Large rims / low profile don’t give you much room for the tyre to deform. That means you can’t reduce the pressure as much coz you’ll pinch the tyre between the rim and ground, possibly damaging the tyre and the rim.
Hi Joe. I read your original article above and the comments that follow. I agree that tyre pressure is the biggest factor affecting tyre flotation on soft sand, and yes the physics you quote are right, with the assumptions and qualifications you have made.
Narrower tyres will have almost the same contact area as wider tyres for the same tyre pressure, apart from a few less significant factors you mention.
But (don’t you hate that word), from my experience driving lighter 2WD VW beach buggies and heavier 4WDs in soft sand, wider tyres, especially with round shoulders and relatively smooth (not chunky) tread, will perform better to ‘float’ on sand without churning it up or sinking in. A tyre with good flotation won’t even break up the sand surface and sink into the sand, but rather will stay on top. Wider, almost-round profile tyres (farm machinery tyres) have been commonly used for VW beach buggies because they give really good flotation.
Also, as you say, wider tyres allow lower pressures to be run without as much unwanted distortion of the tyre towards the rim and elsewhere, and so give more flexibility with tyre pressure range that can be used without damaging the tyres or rims.
The one factor I haven’t seen discussed above is that the narrow tyre ‘tread length’ of the contact area will tend to not be flat on the sand – it will want to ‘bunch-up’ (ripple or have waves) along its length – because the tread is longer than the straight-line length of the contact area. The lower the tyre pressure, the worse this will be. The curved circumference of the tread when it is not flattened is longer than the straight line length of the contact area when the tyre tread is flattened (assuming flat sand) and so the tread will try to bunch up. I suspect that this reduces the effective contact area under that tread area, or it churns the sand a bit, reducing the tendency to float. It may cause uneven contact pressure along the contact area. Have you ever tried to flatten a length of tyre tread when the tyre is off a rim? It doesn’t want to flatten. Tyre flexibility will affect this, but tyres flex more easily across a tyre and around the shoulders and the sides, than along the tread full of steel cords.
Anyway, pressure is the biggest factor affecting flotation, but the sum of all of the other factors would tend to be significant enough to persuade me to stick with moderately wider tyres with roundish shoulders and less aggressive tread (like a wider highway pattern) for soft sand work.
Hi Joe. I thought that you might be interested in some historical context. Ralph Bagnold and colleagues started the practice of lowering tyre pressures back in the 1930s while driving model A Fords across the Libyan desert. No wide tires there. Bagnold headed up the Long Range Desert Force during the war. Apart from his practical experience with driving on sand, he was also a physicist and pioneered concepts of dense granular flow as well as many other aspects of granular materials. I use his theories in my daily work. I’ll bet he would have nailed the maths!
This link has a video of some of his exploits. https://youtu.be/K1HHMrp07tE
Hi Joe, while not being a mathematician, I do have a fair amount of sand and beach driving experience and it is my belief that a wider tyre is far superior than a thin one, especially if the desired effect is to spread the vehicles weight across the sand.
The following link relates to a mine proof vehicle (The “Pookie”) designed during the Rhodesian bush war in the 70’s. It’s primary function was to be able to drive over personnel land mines without detonating them, so as to be able to detect them and therfore have them disarmed before the army was brought through. The vehicle was fitted with second hand Formula 1 tyres to spread out the load.
One wonders who was the first brave person to actually test out this theory !
Back in South Africa when driving on the beach was permitted (yes its banned now !!!) nearly every second fisherman owned a VW based beach buggy. No 4×4 fitted, just very wide tyres let down to a few PSI.
I cant remember seeing many bogged buggies !!!
Similarly golf carts use a very small diameter wheel, BUT it is very wide so as to reduce damage to sensitive golf courses.
I leave you to draw your own conclusions as to the effectiveness of wide verses thin tyres in these scenarios.
https://en.wikipedia.org/wiki/Pookie
Sorry, didn’t read it all. Not all sand is the same but I live on a sand island with no paved roads. I like experimenting with tyre types and sizes. In order of importance I rate them this way.
1. Pressure
2. Pressure
3. Pressure
4. Mud terrain over any other tread pattern, I normally get into arguments over this but the difference is significant,
5. Soft sidewall, cheap tyres are usually better, not mud expelling designs just square faced chunks about 50/50 tread/gaps
6. Largest OD – always go bigger in the direction you want grip, taller = forward reverse traction, wider = cornering grip
7. Once you have the tallest you can go, then go wider but always taller over width but width helps if it bulges out from the rim
To their embarrassment I can drive in 2wd around guys stuck in 4wd’s, the only change is tyres otherwise its all stock. I get a kick out of pulling out all these jacked up 4wds in my lowly Pajero.
People who do all the recoveries run massive Mud Terrain 35s etc and they never get bogged in sand.
If you have two identical vehicles. One is running 4x 200mm wide tyres and the other 300mm wide tyres. Both at 20psi and both are same overall diameter on same size rim with same sidewall. The wide tyre will have 33% more contact patch at any psi. It will be harder to power through as your pushing more sand but as long as you have sufficient power thats not the issue. The weight is spread over 33% more area so its going to sink less. This is easily tested to see which has more contact patch at 20psi, trace both tyres on the ground.
As far as grip or smooth goes. Taking off a smooth tyre will more likely slip and start spinning first however once spinning the big tred tyre will dig faster. Also trying to use recovery boards with a bald tyre is a nightmare as they just cant grip it and pull the vehicle free. (This I know from experience) so agressive tred is better. Look at the sand driving tyres for motorbikes.. they have paddles on them which is as agressive as it gets.
This must be one of the longest threads I have ever scrolled through! Don’t claim to have read it all. In fact, read hardly any of it! But I did see ‘Graham’s’ comment, and approved the response. Ref tyre size vs pressure: I once measured the contact area of the tyres in my Landcruiser and worked out the contact pressure at 36 psi – what a dolt! That was the air pressure at the time, which has to be the same as the contact pressure. (Way up above here there is mention of the sidewall providing support, but that contributes such a small amount you can forget it – wouldn’t change that 36 by as much as 1). It’s easy to explain if you understand the concept of a free-body diagram. But then I am an engineer, so I won’t! But you can believe me. So, if you halve the pressure in the tyre, you double the contact area – and have half the contact pressure on the sand/mud/gloop. And the increased area is all in line with the tyre – the tread is not going to stretch sideways!! If the side-wall flops onto ‘terra infirma’ you need to put some air back in.
I worked that out because I had happily driven up a wet track, and the next day a 20 ton excavator got well ‘bogged’ in it. When I went to see it, the tracks were completely buried in the ooze, just the cabin and arm poking out of the slush. Granite country, all ‘Deco’. The excavator was still working, and could drag itself along with the bucket. But the tracks on the excavator were some 8 metres long and 600mm? 800?? mm wide – anyway, I worked out the ‘contact pressure’ was something like 3 psi. So why didn’t it just float over the top? My Cruiser had about 12x the contact pressure. So how was it that I had not got bogged, and he had? It’s the way tracked vehicles steer. The Deco gets a hard crust maybe 3 or 4 inches thick, and driving over that is OK. But with the excavator, the tracks slew over the top when it steers, and ‘bust the crust’ (such poetry!) and, if soaked after a good, sustained down-pour, what’s underneath is just porridge. I had the cruiser well bogged in it a few years later when it suddenly broke through. Have known a tractor do the same – driving along free as a bird then, no warning, up to the axles in ‘porridge’.
Happy days!!
Hi Joe, good article. I have to agree with you completely – tyre pressure is everything. I would be interested in a follow on article comparing fat v thin on rock and mud terrain. You have shown, correctly, that they have similar performance in sand. Therefore in mud would the ability of the thin tyre at higher pressure be an advantage to ‘cut through’ to the firmer surface below, something the fat tyre may not be able to do? Given that most militaries around the world go for thin tyres I have to believe they are the best true all terrain tyre choice even is they don’t look sexy enough for many 🙂 Personnel I run 255/100R16 XZL tyres, I have found them to be extremely capable on all terrains, I suspect the single steel side wall ply allows more flex than more convention multiple ply sideway tyres and so the reduced pressure performance is superior.
Nice Article. About ‘Mountain’ bike tyres, the tread often appears to have solid chucks on the other side; which increases grip be that in rough or sand conditions. Especially true for the moment the tyre gets uneven downward or side pressure; it deforms while traversing changing surfaces. The foregoing discussions have assumed a continuous and linear soft sand patch; whereas surface conditions vary while travelling. The higher walled tyre design deforms in a way that puts studs of hard rubber with good spacing between, that act like a canoe-paddle momentarily while pushing sand outwards about 45 degrees.
I have a question. For travel in the desert with frequent unpredictable soft-sand; would the following be of any help?
1. Two rears tyres (as seen on trucks) in place of one?
2. Strips of mat, such as shade-cloth, to feed under the front and then back tyres if stuck?
Hi David yeah two rear tyres would help with heavy loads. It would allow lower pressure to be run. Chucking crap under the tyres helps too. You can buy recovery tracks for that purpose as well.
I’m all for you outback Joe! My oldman and I went out yesterday in some wet conditions which consisted of slippery hard-pack clay, deep water with clay bottom and rutted muddy soil. Now even though these tracks aren’t hard and iv done them in my Holden VP wagon once before, they are a completely different story in the wet! Out where I live the clay is not to be taken for granted and can make any experienced off-roader look like a fool even on long flat farming roads. Now I’ll give you a run down on the vehicles and what we were running to give you an idea. My Dad was in his 1991 Toyota land cruiser with a 1HZ 4.2l motor….I was in my 1994 Toyota 80 series land cruiser with the same 1HZ 4.2l motor. Now with tyres my Dad was running 265/75r16 witch isn’t exactly wide but its wider then my 235/85r16 so I think we can call it wide vs skinny. Now we were both running brand new tyres and they are both 50/50 tread designs and are very similar looking with very similar tread depth! My car is weighing in at about 2.2.t and same with my dad’s troopy (2.2t) and we were both running about 40ips. Now my Dad tyres are 8 ply and my tyres are 10 ply so there is a difference in tyre construction. Now before people mention driving style you gotta remember I’m 18 and my dad’s 59 now my dad’s the one that made me the driver I am today. I’ve been passenger in the troopy around Australia and through and around the centre more time then I can count so I think it’s fair to say that our driving styles are close enough together that we virtually are the same driver and like to let the vehicle do the work….slow and steady wins the race. So what I noticed yesterday is that my skinny’s kicked the ass of the wider tyres. I cut through the crap and hit the hard-pack and found that the skinny’s were able to drive their own lines where the wider tyres seemed to follow more lines and ruts then what it should have been making. All the numbers and calculations aside I always thought as a young kid that wide tyres were better….bigger means better but after my little journey yesterday I can tell you I will never be going to a wide tyre as my skinny’s make a night and day difference!! Also noticed my skinny’s clean themselves out better without having to spin them up but the wide tyres just filled up with mud and lost traction….oh and my Dad also has a locker in the front where I don’t and I still preformed way better….now given the tyres are the only thing in contact with the ground I came to the conclusion that skinny’s and way better than wide tyres any day of the week in any terrain. Thanks Again!
Personally I don’t think the conclusion that skinny tyres are better in any terrain, is warranted. My feeling is that it will always depend on the surface. I can certainly see how narrow might beat wider, IF the surface is a slimy mud with a firmer base just underneath. If, in contrast, it is 2m of mud in a tidal mangrove flat (like where one of my brothers stuck his troopy years back), then something which prevents you breaking through will be better. Sand, has different considerations of its own.
Cheers.
Where is the conclusion that skinny is better? I’m not trying to suggest that. There’s many hypothesis, all untested. The point is a vehicle with adequately deflated tyres will go well in sand regardless of tyre width.
I am not expert in physics or sand driving, but these are my thoughts.
The calculations about Pressure, Area, and Force are invalid. If we are talking about air pressure in an inflated tyre, then the Area is comprised of the inside surface of the tyre and wheel, and the Force is the total force pressing on those surfaces.
Those are being confused with the Force applied by the mass of the car, and the Area being the contact patch of tyre on ground (or sand or whatever). So while the physics equation is true, it is being incorrectly applied.
Consider this: if Area = Force / Pressure, where Area is the contact patch, and Pressure is tyre pressure, and Force is the mass of the car, then if you deflate your tyre the contact patch becomes infinite. Obviously, that contact patch size is very constrained.
Further, if the tyre width makes no/little difference, then that would mean a 40mm wide tyre would be just as good (which it obviously wouldnt). Or a 1000mm wide tyre would provide no extra flotation (which I think it obviously would).
The Area ie the contact patch, is highly constrained by being a tyre, and even correctly applied, seems to me that constraint, and the rapidly changing role of the sidewall itself, makes the analysis meaningless.
I have been watching a YT video where contact patch length was measured at different tyre pressures. Moving from 38psi to 16psi increased that by a bit less than 10%. Yet the car drives MUCH better over sand. Well, I think it obvious that is NOT due significantly due to increased contact patch. There is something far more complex going on eg the shape of the contact patch being flatter and less round, and the way that interacts with sand.
How do those full size tour buses get around on Fraser? Well, they have dual rear tyres for one. That is essentially a wider tyre. It doubles the Area, and halves the Pressure, in the sense of pressure between tyre surface and sand. If you move from a “skinny” 4wd tyre to a wider one, then as I see it, correctly applying A = F / P, then there is fundamentally more A, F is the same and P – the pressure between tyre and sand – is lower. If you make the tyre Area smaller or increase the mass of the car, F, then Pressure will increase, and the tyre can only do one thing – sink lower into the sand to a point where it is firmer. I call that “bogged”. I understand that 4wds can be effectively driven on sand on thinner tyres vs wider tyres, but my impression is that the difference between those tyre widths isnt much, and other factors ie the complex interplay of tyre, its shape on the sand, and the sand, really confuse the matter.
My 2c, as they say.
Hey Alexander the pressure formula is being applied correctly, there is no confusion. The pressure in the tyre is the same as the pressure holding up the weight of the car via the contract patch. An infinitely large and flexible tyre would behave exactly according to the equation. However sidewall contribution and the finite space in a tyre and eventually rim contribution all play a role. It’s all explained in the article.
So, if you air down to 0psi, is there 0psi between the tyre and the ground?
If what you are saying is true, then if you halve the tyre pressure, then the pressure between the tyre contact patch and the sand would have also, and the mass of the car stays the same, the area of the contact patch would have to double. Obviously, that is not the case.
Read the article it’s explained in there.
Joe, do you mean the article above, on this page, or some other one? thanks
Yes the article above. Especially the bit about area changing less than the formula predicts. The formula is not accurate, and it’s not even important. It is just a tool to help come to the conclusion that similarly constructed tyres of different dimensions have similar contact patch when at same pressure.
This is a fascinating debate. Floatation on soft sand is the key, and that can only be achieved by increasing the size of the contact patch on the ground to reduce the effective pressure per square inch that the vehicle mass exerts on the sand. Once up “on” the sand, skinny or wide will float down a soft beach relatively effortlessly at slow speed.
I’ve never used excessively large or wide tyres, but have used two tyre sizes predominantly for soft sand driving in the coastal regions north and south of Perth in Western Australia. These are a 235/85r16 with a 120/118 load rating and then 265/75r16 with a 123/120 load rating. Initially when I started running the 265’s on sand at the same pressures as the 235’s, i was shocked at how poorly the 265’s performed. After taking into account that the 265’s are actually a stiffer tyre, reducing pressures a little bit more restored the floatation contact patches that I was getting with the 235’s.
These two tyres are the same rolling diameter, and the same sidewall height. If the 235 does have an advantage over the slightly wider tyre, it’s because it’s lower load rating allows for a slightly larger footprint at an identical pressure. 18psi as an initial go to pressure with the 235/85r16, is on par with 15psi as the initial go to pressure with the 265/75r16.
If the tyres are compared at the same pressure, the skinny wins. If they are compared at pressures that give the same footprint area, then there is little between them.
After 4 years on the 265’s, I will be reverting back to the 235’s. Not because there’s any measurable performance gains off-road, but I do want to be able to reduce un-sprung weight a little. The front suspension of my Triton does not have the weight in it, that a heavier live axle vehicle has, so the heavier tyres have affected on-road more than I like.
Hey Joe, I’m thinking of changing my tyres from 255/55/r18 to 255/65/r18 – essentially a higher sidewall. Is there merit in doing this? or should I just stick with what I’ve got?
Yeah man. Better ground clearance, doesn’t get bogged as easy in soft sand. Just gotta weight it up against speedo going out of whack, taller overall gearing, slower acceleration, worse engine braking, less effective brakes and increased fuel consumption.
outbackjoe, interesting topic. I have a different point of view based on my experience in soft sand at Lake Power back in the early 1990’s.. My old Isuzu trooper on micky thompson bajas 31 inch tires. No airing down. No lockers. When I had 10 inch wide tires, I could not drive up the hill out of the lake without burying the truck to the axles and had to air down to about 12-15 psi to struggle my way out(then limp back to page gas station to air up). I then moved up to 12.5″ wide tires same diameter and could struggle my way uphill away from the lake without getting stuck, without airing down. The only difference was the width of the tire. It definitely helped me float over the sand. The skinny tires buried me every time. The wider tires didn’t. And this is over a few years of use. I do think the width makes a bit of difference. At least it did for me.
Think about this, no one has done(that I could find) a sandy beach video of the same truck running skinnies vs wides of the same brand without airing down. See which one will go further up a mild sand hill. That would be a good test. Then air both down to 15 psi, to see how that affects each. Maybe when aired down, the difference may not be as great, who knows
interesting theories all..
observing reality on Fraser Island (K’gari). Suzuki Jimnies will go anywhere due to genuine 4wd and low weight – even with novices drivers and road treads. Mud grabber tyres are a menace as they rip up the sand surface when it’s soft. When becoming bogged in soft sand, releasing a couple pounds air pressure in tyres sure helps. Finally - the buses stop for no one so get out of the way. Actually, the buses will use your straps to drag you off the track if you are bogged ( is ‘bogged’ the right word in sand?) and in their way – then quickly unhook and drive on leaving you to your own devices. The bus may drive forward and reverse a couple of times to hardpack a runway before dragging you out.
if you drive automatic, best use the manual ‘paddles’ to pick correct gear.