Last updated 25/08/2020
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?
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.