Should I change tyre pressures and use differential locks on dirt roads?

QUESTION

Hi John,

You helped me with a question on Troopy vs Prado engines about a year ago, so I thought I'd try my luck a second time.

I bought the Troopy primarily for payload reasons, and I've just done a camping tour from Adelaide up to Innaminka and across to Birdsville, taking all the dirt tracks we could and spurning the bitumen as much as possible. Never did come across Dingo Piss Creek, so it must be higher up in QLD somewhere.

I like to lock the centre diff on dirt roads to give all wheel drive (Troopy being normally 2WD) which seems to 'settle' the car a bit. Andrew St Pierre White thinks it's a good idea to do this too.

But the tyre pressures front to rear are quite different. I run 22PSI in the front and 32PSI in the rear on dirt. Which made me wonder whether there would be an appreciable difference in the rolling radius front to rear, which might induce transmission wind up.

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Should your Troopy be in 4H...

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...driving on this?

So my questions are:

Would the diameter of the tyres be different at different pressures? I am guessing for a set load, yes; more pressure would give a bigger diameter. Is it significant, and if so, does the relative lack of tyre friction on a dirt road dissipate this? Or does the heavier rear load 'compensate' for the slightly bigger diameter because the higher force squishes the sidewall down a bit more?

So in the end both front and rear are effectively the same diameter, because the different pressures are supporting different loads.

I emailed BF Goodrich to get the load/pressure table for my KM3 tyres, which they sent me. The load tables are specific for a tyre size, and mine are 255/85R16, and they are Light Truck.

I put that into a graph to make it easier to work with:

And I got my car weighed at the local weighbridge. Front axle was 1390kg, rear was 1850kg, luckily I was below the Toyota GVM of 3300kg!

So cold temps (from the graph) would be 25PSI and 38PSI (cold). Which is quite a difference front to rear, and quite a way from most people's idea of “35PSI all round mate - never had a problem”.

Thanks,

David.

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ANSWER

David,

You’re thinking too hard when you should be watching 4WD porn like the rest of the off-roading community.

Tyres aren’t balloons, dude - the circumference doesn’t stretch with pressure. Overlapping woven steel belts and thick, relatively hard rubber are inflexible, lengthwise and there’s not that much pressure variation. The sidewall flexes, but the rolling circumference doesn’t change. You can measure this with a piece of chalk and roughly 100 inches of road.

Pressure is just the internal load (force) per unit area of tyre, right? 22psi is 2.5 atmospheres (absolute) and 32psi is 3.2 atmospheres (absolute) - it’s only a 28 per cent variation of a fairly low pressure in a fairly circumferentially rigid container.

Pressure is about supporting the applied load (and durability plus ride quality on rough surfaces). Tyres are specifically designed to do this without changing circumference, because this would mess significantly with speedo accuracy everywhere, obviously.

Using the centre diff on a hard dirt road with good grip is a mistake, and yeah - it will ’settle’ the car a bit (or you could just drive conservatively).

Unfortunately, it also increases the strain on transmission components and reduces their service life.
In soft conditions like bulldust, it’s fine.

So I’d only being engaging the centre diff when it’s slippery, just like it says in the manual.

JC

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