Tyre Pressures

A good set of tyres is one of the most expensive upgrades most of us make to our 4WDs – that’s a good reason to look after them.

It’s common knowledge that adjusting your tyre pressures when you come across a change in offroad track surface is a useful technique to gain more traction. Letting down your tyres causes them to bag out a little at the bottom, which increases the surface area of the tyre that is in contact with the road. What also happens is that the tyre becomes longer, which is not so visibly obvious.
As well as providing increased traction, dropping your tyre pressure when driving on uneven surfaces will also decrease tyre and vehicle wear and tear. Reduced pressures will also provide better drivability and increased control in most situations, as well as providing a more comfortable ride and reducing the load on your suspension.

I find it helpful to think of tyre pressures as a percentage. By that I mean if you are driving normally on 36psi, and then let your tyres down to 20psi, you have actually let your tyres down by 45%. If you then find that you need to let your tyres down further and decide that the conditions require you to run them at 16psi then you have actually let them down a further 20% from the 20psi that you were on. By making slight adjustments you make huge changes in percentages, which will provide you the traction that you require.
Now there is no real rule as to what pressure you should run in specific conditions, but I find that 20-22psi is a great starting point, and if you need to, you can decrease pressure further from there.
Some drivers like to drop their tyre pressures down to extremely low pressures when driving on soft sand. One time I found myself stuck on an incoming tide and was not able to get over a particularly soft and steep dune back onto the track. By reducing my tyre pressures down to 6psi, I was able to get that extra bit of flotation that allowed me to creep back over the dune.
The lower you go with the pressures the more conscious of the reduced pressures you have to be, as sudden cornering or rutted tracks can roll the tyre off the rim. I find that you really need to be conscious of this possibility when running pressures of 15psi or lower. If you do need to go lower for any reason, pump your tyres back up as soon as you can.
Another point to remember is that the speeds that we are travelling, and also the weight we are carrying, means we have to adjust our pressures accordingly. We may not be able to run 16psi with a fully loaded 4WD, as rolling a bead off a tyre becomes much more likely. Travelling at higher speeds means that a bulging sidewall will cause excess heat build up in the tyre and cause the tyre to deteriorate. The best advice I can give is to start a bit higher with your pressures and lower them further as necessary.
Reduced tyre pressures do reduce the risk of punctures as the tyre becomes more flexible and is actually more likely to mould around a sharp stick or rock.
Overall, letting down your tyres will decrease your tyre wear by reducing wheel spin and providing better grip. Your risk of punctures is decreased with deflated tyres and you will find that you’re much less likely to get stuck.
See you out there on the tracks.



Effect of adjusting tyre pressures on the size of a tyre’s footprint
This diagram illustrates the effect of reducing your tyre pressures on the size of the footprint of your tyres. Reducing pressures and increasing the size of your tyres footprint spreads the weight of your vehicle over a larger area so when driving on sand, for example, your tyres will drive ‘over the top’ of the sand. If you maintain high pressures and a small footprint, your tyres are more likely to ‘dig down’ into the sand and even get you stuck! Remember, whenever you reduce your pressures, re-inflate to the proper levels as soon as you drive back on to the bitumen.

Qualified motor mechanic Adam Adler has spent half his life under the bonnet of a 4WD and has worked for some of the top accessory companies and workshops. He knows what it takes to get your vehicle out there and back home in one piece. He runs the online aftermarket store www.nutsabout4wd.com.au.

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