4WD Tyre Pressure for Sand
Driving a 4 x 4 on sand requires an understanding of two simple principles, tyre pressure and momentum and the development of a few driving techniques. We’ll reserve momentum for another article and deal with tyre pressure here.
In deep hungry sand, the absolute key to maintaining forward movement is tyre pressure. Understand how tyre pressure works in sand and you vastly improve your chances of staying out of trouble and driving out of trouble.
If you read the article The Truth About Tyres you will understand we don’t really subscribe to the brand X is better than brand Y mentality. Factors like tyre pressure and technique make a much bigger difference to off road success than being the proud owner of a set of ‘RichYear MudHuggers’ for instance.
InertiaEssentially a car struggles to drive in loose sand because of inertia. Inertia is the tendency of a moving object to keep moving and a stationary object to remain stationary. Sand gets pushed away from a rotating tyre because the tyre finds it easier to move the sand away than to move the vehicle forward. Hopefully four rotating tyres become enough to swing the balance in our favour and generate enough momentum that we begin to move forward. We can easily swing the balance in our favour by the simple act of letting some air out of the tyres. Look at the simple diagram and you begin to understand how this works and why.
By decreasing our wheel height by 25% (by deflation) we can increase our tread footprint by up to 200%. Our tyre becomes similar to the tracks on a bulldozer. We end up with a massive increase in surface contact which means it becomes easier for the rotating tyre to push the vehicle along rather than to push the sand away.
How To Determine Correct Tyre Pressure For Sand
Assuming your tyres are at the correct pressure for driving on the street we can approximately determine the right sand pressure this way.
Park your 4WD on a hard flat surface and measure the distance between the bottom of the rim and the road (a.)
Reduce tyre pressure until this distance is 75% of the original height (b.)
For instance if your distance at (a.) was 100mm then your distance at (b.) becomes 75mm.
(a.) is your street pressure and (b.) is your sand pressure.
Measure and note this pressure and use it as a starting guide next time you drive on sand. No two driving environments and no two vehicles are the same. Factors like the type of sand, vehicle weight and the tyre in particular all play a part in selecting correct tyre pressure and no single formula can work in every situation.
Many people use a rule of thumb approach and drop their pressure to somewhere between 18psi – 22psi as a starting point for sand driving. (If you work in Bar rather than Psi a converter can be found here)
Moisture content, sand temperature and compaction all play a part in traction and if this initial pressure drop doesn’t get you moving then reduce pressure until you are travelling comfortably. Reduce in 3 or 4 psi increments at a time.It is possible to reduce down as low as 10psi without risk of tyre damage. While not recommended for extended driving, it is generally better and safer to drive out of soft sand than to be recovered.
When accelerating, braking and turning with low pressures - do so smoothly and carefully. Over enthusiastic driving makes it possible to roll tyres off rims when running lower pressures.
Likewise high speeds and driving on bitumen are to be avoided if possible. If you must drive on bitumen with reduced air be extra cautious when changing speed and direction.
While it is possible to get by without any equipment and hobble to a service station to reinflate afterwards, a tyre pressure gauge and compressor make life much easier.
If you intend doing lots of sand work you will probably end up owning a compressor costing upwards of $250. While a $50 cigarette lighter model will do the job, it will be slow and as often as not will eventually give up the ghost at the most inconvenient time. Good tyre gauges, however, are relatively cheap and a quality unit will probably outlast you.
Driving in sand can be great fun and can get you to places you would, otherwise, be unlikely to visit. Reducing tyre pressure is the single most effective method to maintaining positive forward motion in the boggy stuff.
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