The science of sand driving in your 4WD

With all the u-beaut 4WD beach driving in NSW and QLD (and our other states, of course), have you ever thought a little deeper about sand driving in your 4×4? How does your vehicle move through sand? What are the forces acting on it, and what recovery forces are required? Let’s dive a little deeper and get a bit sciency!

It’s always interesting to understand exactly how and why your vehicle behaves a certain way in specific terrain. It can make you a better driver and lead to safer recoveries. To that end, we thought we’d dust off our lab coats and look at the science of sand driving.

Why? Because too many yarns focus on dropping tyre pressures and nothing else. Here we’ll look at different types of sand, the physics involved in driving on sand, and the recovery weights. This article won’t be light and breezy, but you will learn stuff.

Sand driving techniques

Now we need to explore how all this translates to car setup and driving techniques. You all know you need to drop tyre pressures, but why? Traction, most would say, but that’s only part of the reason. Do you know what the other reason is? It’s to reduce rolling resistance. This is going to get a little heavy and there’ll be some maths, so stay with me.

See, when you stand on a soft surface, you sink in. That’s why you leave footprints in the sand but not on concrete. And when a wheel sinks into the ground, it has to push through the terrain, which is your rolling resistance. If the wheel can’t overcome the rolling resistance, it’ll spin and dig in, becoming a vicious cycle. That’s why skilled sand drivers quickly back off as soon as they feel excessive wheelspin. It allows them to find the happy medium between slip and grip. It’s also why we suggest that if your wheels are spinning in sand, then you should back off and drop pressures slightly to find the happy sand driving medium.

Why do we drop tyre pressures?

Reducing tyre pressures increases the tyre’s contact patch by lengthening it, not making it wider. This reduces the amount the wheel sinks in and thus reduces rolling resistance. There is also a traction improvement, as there’s more tyre in contact with the ground. The brain-hurting part is that reducing tyre pressures physically increases the rolling resistance of the tyre. However, that increase is outweighed by the fact that you’re not sinking in as much.

How can I determine the optimum tyre pressure?

This is tricky. It’s possible to determine an approximate pressure, but it all depends on the type of sand you’re driving on. Just know that you definitely should drop your tyre pressures on sand. From our experience, though, you can take a good stab.

The correct pressures are very dependent on the nature of the sand, for example, density, any gradient, and your vehicle, particularly the tyre diameter and weight. However, as a very broad rule, any sand worthy of the name would warrant tyre pressures at 20psi to begin with. If you find you’re struggling to drive across the sand, then don’t hesitate to drop tyre pressures by 2psi every time down to about 10psi.

Remember also that your tyres will heat up as they deform more when rotating due to the lower pressures. Also perhaps the ambient heat as the day warms up. That means you may need to bleed air off throughout the day to keep your tyres at the ‘right’ pressure. Starting with 15psi could see you at 20psi by lunchtime. Conversely, in desert conditions, you may have ended the day on 18psi and, come the morning, your tyres could be at 13psi because they’ve cooled down. Leave them there as they’ll soon heat up and the air inside will expand.

NOTE: Be aware that going too low can actually be detrimental to the ‘flotation’ we’re trying to achieve. As the tyre bags beyond optimum, it can ‘well’ in the centre and actually create a little lump that it’s constantly pushing against, making the going that much harder.

Is wheelspin bad?

I mentioned wanting to kill excessive wheelspin when driving in sand, but that’s not exactly correct. Every car tyre has ‘slip’. When a car moves, it has a measure of wheelspin, which means that if it is a driven or undriven but unbraked wheel, it rotates further than the vehicle travels, and if it is a braked wheel, it rotates less than the distance travelled. The extreme examples of this are a burnout. And when a wheel slips a lot, we call it ‘wheelspin’.

So, ‘slip’ is necessary?

The softer or less traction the surface has, the more slip is needed for optimum efficiency. For sand, slip ratios are around 15-40%, which means the tyre rotates 15-40% more than it would if the car travelled forwards precisely the circumference of the tyre. That is one reason why fuel consumption rises when driving in the sand, and another is the increased rolling resistance of both the deflated tyre and the tyre sinking into the sand. Interestingly, rolling resistance in sand increases with speed to a point, then it decreases. This is because the tyre needs to move sand out of the way, and that’s easier at slow speeds, but at high speeds the tyre can surf on top of the sand, similar to the drag reduction seen in a fast boat when it planes on top of the water.

So, how much force is required to move a vehicle in the sand?

We need to make it clear that we’re not talking about a snatched weight – more on that in a bit. Rather, we’re talking about the amount of effort required to overcome the rolling resistance and move a vehicle rather than physically drag the vehicle out of a bog. And it’s worth noting that the starting force/effort required is generally 2-2.5 times more than the sustained force/effort.

On flat bitumen with tyres at road pressure, I measured (with a load cell) the amount of effort required to move (sustained) a vehicle weighing 2600kg at 2.3% of the vehicle’s weight or 60kg. The starting effort/force required, as mentioned, is around double this sustained amount, so 120kg. Drop the tyre pressures to 20psi (such as you would when driving in sand), and that rises to 80kg or 3% of the vehicle weight. With a starting effort/force of more than 160kg.

Let’s just pause for a moment…

Let’s just press pause for a moment. We’re talking about a ‘force’ here, so the correct unit is Newtons, but we can all conceptualise kilograms (this is how things like snatch straps, shackles and more are usually rated), so we’ll use those instead. Unpause.

On firm, flat sand, the force/effort rose to 7% of the vehicle’s weight at 20psi, or more than twice the force required compared to bitumen. However, when following existing wheel marks, the force/effort was half, but this was still more than that of bitumen. And that is because the compressed sand creates less drag.

That result also shows why you often get stuck when turning on the sand. It’s because each of your four wheels has to create a path through the sand (increasing the rolling resistance), as opposed to the rear two following in the tracks of the fronts.

What about snatched weight?

Some years ago, Unsealed4X4 used a load cell to measure the snatched weight experienced when snatching a vehicle bogged in sand. The snatcher was a Nissan GU Patrol and the snatchee was an accessorised Toyota Prado weighing 2840kg, the location was Stockton Beach but that’s not really that important. With a load cell attached to the snatch strap the load recorded to pull the Prado out of its predicament was 1605kg or, more than half the weight of the vehicle. For reference, to bog the vehicle, the tyres were left at highway pressures as you can see in the photos.

What’s a load cell? Essentially, it’s just a block of alloy with two shackles on either side with some electro-trickery on the inside that measures the pulling force in increments of 5kg up to a maximum (our unit) of 10,000kg.

What is sand?

Sounds simple, right? It isn’t. Sand is made up of tiny grains that are not connected, a bit like a handful of marbles, if you catch my drift. The hardness, or not, of sand, is affected by things like its shape and the amount of water it holds.

Dry sand grains can move freely against each other, damp or wet grains, on the other hand, are unable to move freely because the water molecules have a viscous surface tension which acts a bit like weak glue, sticking the grains of sand together. You’ve probably seen this effect when building sandcastles on the beach. There’s an optimum amount of water for best sandcastle building.

The softness of sand is also affected by what else is in it, so vegetated deserts have sand that is partially bound by vegetation. And then there’s compression. The more sand, or any soft terrain, is compressed, the harder it becomes, which is why it’s much easier to travel in existing wheel tracks rather than make new ones.

How are dunes formed?

At a macro level, expanses of dunes are exactly like corrugations on a dirt road, only far more substantial. Both form in the same way. Imagine an area of perfectly flat sand with the wind blowing on it, moving the top layer of loose sand. Then there’s a little imperfection, maybe a rise, a stick, anything, and the sand collects around it, forming a tiny ramp. Sand blows up the slope, increasing the sand on the slope, growing it. Over the crest, the wind drops sand, with the finest grains dropping over the top. The result is a steeper leeward (opposite to windward) side, and finer, softer sand particles. That is why sand dunes are typically easier to drive heading in the direction of the prevailing winds.

Images by Robert Pepper, Brett Hemmings and Kumho Tyres.

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