Tyre pressures on dirt/sand.

Bush cops drive fleet cars….. I’ll stick with Adam Plate’s and other’s rules of thirds. One third out on dirt, second third out on sand - roughly. Tyre chipping is as much an issue for overinflated tyres on outback graded dirt with chipped rock - who can afford that these days haha.

Well this ought to take the heat off the catch can thread.

Tyre pressure management is a bit more complex than just letting a bit of air out in my experience. Speed and tyre pressure are linked and as pressures drop so must speed. On a road of variable surface it easy to take advantage of a good stretch and increase speed but at low tyre pressures heat will quickly build and thats when the fun starts. I am a strong advocate for managing pressures and many years of experience has been my teacher. It is a bit of trial and error to find the right pressure for the conditions and things like weight and tyre structure / quality come into it. I can sympathise with the 4wd instructor when training novices giving that advice.

The tyres fitted to new vehicles are often less suitable to rough roads and lower tyre pressures but they are nice and quiet on freeways at 40psi.

I live on an unsealed road. Like many country roads it doesn't warrant reducing tyre pressures to drive over. Outback roads are a whole different matter. When you only carry one spare and you are hundreds of ks from help a flat tyre becomes a lot more significant.

Off to get some popcorn now and watch as this thread unfolds. :)

Kenell

I would say the bush cop is full of shit, any tyre damage to police car tyres is not out of his hip pocket, but to the government.

First thing is the cop a qualified off road driving instructor?

Suggest you give the person a copy of the late Adam Plate Outback Tyre Pressures

When he was alive, if you spoke to him it was his pet talk

You often see photos on various media of police vehicles hopelessly bogged on various beaches here in Queensland and being helped out by members of the public . A picture is worth a thousand words.

Easier to put the spare on than wait for and fit a new spring, especially if the bolts were fitted before the long-range tank.

Michelin publish speed Vs Pressure Vs load carried data for the tyres I use on the OKA.
I have converted the Michelin data to a chart for my own use. I have use it religiously for 20 years.
Blue represents highway speeds, red is 65kph, yellow is 20kph.

Cheers,
Peter
OKA196 motorhome

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An "ex-bush cop who basically said the tyres don't need adjusting on dirt but maybe a small deflation on sand."??

To some degree he may be right…… for a police 4WD vehicle which is lightly loaded compared to a touring 4wd loaded to the max.
Airing down on a dry unsealed road may not be essential but useful for comfort, with reduced speed. But on soft sand, lowering tyre pressure is essential to increase tyre contact area and so lessen sinking.
However the degree of pressure reduction is directly related to vehicle total weight, just as Peter's chart indicates.

Good info from Adam P. I met him Adam at a motorcycle crash years back after I left working on a station. These people generally used the 30/70 rule. NOTHING OVER 30 PSI in tyres & NOTHING OVER 70 KMH, OFTEN LESS! If you look at the tyres of these outback residents, all are chipped and the surface shredded. They rely on their tyres everyday. They do have a sensible attitude to tyres.

I don't let tyres down on general dirt roads because you're usually still driving at a reasonable but sensible speed so you want to have control of the vehicle without tyre walls rolling more than needed affecting handling.
For slower rough or rocky tracks a tyre reduction helps.
For sand 18psi I find is a good place to start that gets you a long way and makes driving easier you can go lower if required.

Hi Stephen H15,

The correct tyre pressure is a combination of a few different factors, tyre construction, tyre load rating, the weight the tyre is supporting and the road conditions. Anyone telling you a specific tyre pressure without knowing these parameters is just guessing. Another thing to remember is that even someone with the same vehicle, and the same tyre, may require a different tyre pressure than you owing to the weight difference of their vehicle when fully loaded. This also includes your caravan tyre pressures.

In order to get the correct tyre pressure for your vehicle, you really need to know what weight each wheel is supporting by having your fully loaded vehicle weighed. Then, go on line and find a Tyre Pressure Calculator, G&S Chassis has a good one, enter the details requested, and it will give you a starting point for sealed road driving. From there you can adjust according to the road conditions. Monitoring the tyre temperature whilst driving can help to “fine tune” the pressures.

Rough gravel and corrugations generally require less pressure than sealed or good gravel roads, and sand requires less pressure again. Some experienced off road drivers recommend 10 to 15 psi lower than Highway pressure for corrugated roads, and 15 to 20 psi lower than highway pressure for sand. However, these numbers are not “cast in stone”, and should be used as a guide only.

Remember, when you lower tyre pressure, you also need to lower your speed. Driving at high speed with low pressure will cause the tyre to overheat, that may lead to tyre damage, or even a blowout, leading to an accident. Another thing to remember is that the more weight you are carrying, you cannot afford to lower tyre pressures too low, or you may “break the bead” and “roll” the tyre off the rim.

Macca.