Sunday, November 03, 2013

Tire pressures

One of the mysteries about off-road driving is just what exactly to set one's tyre pressures to and because it genuinely does make a huge difference to the performance of your vehicle it is important to get it correct.  What I have found is that there is a tendency amongst people to lower pressures more than required because the folk lore states that when off road; lower the pressure and surely more of a good thing is better.

So herewith are roland's rules.
1.  Don't believe everything your read or are told, use some common sense.  I've read recommendations about tyre pressures which would certainly have landed me in trouble had I followed them and vice versa.  The problem is that between wheel size, the specific tyre fitted, tubeless vs tubed, the vehicle itself, the loading of the vehicle, the type of terrain and the weather condition there is so much variation that any general recommendations need to be used in conjunction with a good dose of common sense.
2.  Having said that, for my Fortuner 4x4 fitted with OEM Bridgestone dueller tubeless tyres when going overland I subscribe to the following rules' of thumb.
  • 2.1 bar - tar and excellent gravel roads.
  • 1.8 bar - Normal gravel roads with corrugations, rocky roads or rock and sand mixed.  Even short distances of heavy sand are fine at 1.8 bar.
  • 1.5 bar - Heavy sand for long periods of time with no or very little rock.  Very corrugated dirt roads without any rocks.
  • 1.2 bar - Emergency in very heavy sand.  Need to be very careful of anything sharp like rocks or thorns or sticks which will go through the sidewall.
3.  When driving with lower than tar road pressures be aware that you shouldn't do any sharp turns and this applies especially at the lower pressures.  If you do, you risk rolling the tyre off the rim bead.
4.  The pressures are measured when the tyre is cold and if the car is heavily loaded I add an extra 0.1 bar onto the above pressures.
5.  I try and drive with the pressures as high as I can get away with in the conditions.  I figure that no sidewall is going to be as tough as the tread itself and the lower the tyre pressure, the more the sidewall is exposed to being cut or punctured by rocks, sticks or thorns.
6. Drive carefully and avoid driving over sticks, rocks and thorns  as much as possible and I am particularly careful to try to make sure that the track of the rear wheels and/or trailer,  whose turning circle is tighter than the front of the car, doesn't go over the obstacles that I carefully avoided with the front wheels.

So that's it;  I'm not sure about what to do in mud but I think I would tend to have the tyres harder rather than softer in the hope that they would 'cut' through the mud to find hard ground.  This would obviously only work if the mud wasn't too deep and my tyres too wide.  In really deep mud I'm not so sure and I would probably try and avoid the obstacle altogether if possible.

A quick disclaimer that this is purely my opinion as a part time 4x4 overland driver and what I have used successfully thus far.

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