In the previous blog post I mentioned that I got very stuck in the sand during my recent trip to Namibia. I certainly did not enjoy all of the digging. It was very hot and while digging, I promised myself that I would investigate the science behind sand driving when I returned home. Understanding why your vehicle gets stuck despite your best efforts will go a long way to prevent this from happening in future. Most of my focus was on tyre pressure but, as I found to my dismay, this was not enough.
Dropping your tyre pressures when negotiating sand is a given. Most of us are under the impression that this improves traction which is essentially true. According to the experts however, the actual reason for dropping your tyre pressures is to reduce rolling resistance. When your tyres are too hard, you sink into the sand and you now have to try and push through the soft sand rather than drive on top of it. Reducing your tyre pressure enlarges your tyre footprint. It gets fractionally wider but much longer. The bigger footprint prevents the tyre from sinking into the sand thus reducing the rolling resistance. There is also an increase in traction due to more of the tyre patch being in contact with the ground. If your vehicle tyres however cannot overcome the rolling resistance of the sand, they will spin and dig into the sand increasing the rolling resistance as you sink in.
With these basics in mind, the question has to be “What is the correct tyre pressure?” It depends on many things including vehicle type and weight, tyre type and size, type and temperature of the sand and even the time of day. Wet beach sand is easier to drive on than soft, hot dune sand. Dropping your tyre pressures in the cool of the morning before setting off will almost certainly require another deflation later in the day as the terrain, ambient temperature and the tyres themselves get hotter. There is thus no one correct pressure for all sand conditions. I have also found that even as little as 0.2 of a BAR in pressure can be the difference between struggling and plain sailing.
The experts also say that a certain amount of wheelspin is required for your tyres to overcome the rolling resistance in sand, enough to overcome it but not too much as to induce digging in. This is one of the reasons why fuel consumption in soft sand is higher than on firm surfaces. The speed at which you travel on sand is also important. Rolling resistance increases with an increase in speed up to a point where it becomes less and the going gets easier.
A couple of other things to bear in mind when sand driving are that it is advisable to follow in the tracks of the vehicle in front of you. The sand in these tracks has already been somewhat compacted by the vehicle in front making your progress a little easier. Also, turning in soft sand immediately increases the rolling resistance. If you have to turn, increase your engine power to try and maintain constant speed and traction in a safe manner. Beware however of un-beading a tyre from the rim. If you have to stop, roll to a stop without using your brakes and if possible, face the front of the vehicle downhill. This will make pulling away a bit easier. As with most challenging things in life, practice makes perfect. I guess I’ll be heading back to the Namib Dunes real soon then.