Why Wide Tyres Don’t Help In Sand

Last updated 01/02/2017

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 style four wheel drives 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 difference in sand performance due to tyre dimensions. But I have always noticed a MASSIVE difference due to tyre pressure.

How can this be?

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 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, as the narrow tyre will deform more and so increase the risk of pinching the tyre tread between the rim and the ground. In this case a wider tyre may offer an advantage since it can be run at lower pressure. However, for a typically weighted car with a tyre with reasonable profile, 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. So a wide tyre may not offer any benefit.

What about 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 and thus affording more edges to bite into the surface to provide additional traction. Long contact patch or wider contact patch doesn’t matter much in my opinion – at the same pressure they provide the same area and apply the same force to the ground. There are other theories like a narrower tyre has stiffer sidewalls and thus doesn’t spread as much as a wider tyre, or a wider tyre creates a bigger “bow wave” and has to push more sand out the way so inhibits movement when compared to a narrow tyre which has lower frontal resistance. It’s all either unsubstantiated or simply doesn’t really make much difference compared to tyre pressure alone. In any case it’s pretty much irrelevant based on field observations. Any tyre at correct pressure will perform well in sand. Contact patch is mainly dictated by pressure and real world experience indicates that vehicles with skinny tyres perform just as well in sand as vehicles with wide tyres.

There are similar theories regarding best sand tyres in terms of tread pattern, sidewall construction, 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 logical. 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. 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.

See also:

Tyre Pressure Guide

How to Drive on Sand

Bridgestone Dueler D694LT Review

How to Catch Barramundi

Touring / Travelling Phone and Internet Setup

back to 4WD, Touring and Camping

more articles by outbackjoe

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63 replies »

  1. 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.

  2. 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?

  3. 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

  4. 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.

  5. 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?

  6. 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.

  7. 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.

  8. “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.

  9. 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.

    • 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.

  10. 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.

  11. 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?

  12. 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.

  13. 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?

  14. 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.

  15. 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.

  16. 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.

  17. 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.

  18. 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

  19. 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.

  20. 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.

  21. 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.

  22. 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.

  23. Hi OutbackJoe. I really liked your post. I would like to translate it into Portuguese, would you give permission?

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