Calculating Tyre Pressure

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Originally Posted by B.S.F.

isuzurover
Thank you for taking the time to do some 'research'. Maybe I should have mentioned that the tyres are on 16" rims. I am aware that there are a lot of variables, ply rating for a start. However to stop this thread from deteriorating into a 'my tyres are better than your tyres' argument I decided to stick to as few facts as possible.
My 37.5 seemed like a good starting point, a bit on the hard side. I reduced the pressure by 3-4 psi and got even wear (hardly any) over the width of the tyres in
10'000 km.

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The first 3 on my list are 16" as well, the next 2 are 14".
What your method does is calculate an area (in really horrible units - actually an inverse area???).

To get any meaningful value out of it you would need to correct for nonlinearities imposed by section width and rim size. It may work OK as an initial starting point, but so would the value from the tyre placard. In some cases the method could be dangerous. e.g. if I do the calculation for the rear axle load on an unladen 110 county using the first tyres in my list I get 17 psi!!! It would be dangerous to drive on the highway at that pressure.

What is the correct way to calculate tyre pressure for a non standard tyre on a car without placard?

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Originally Posted by B.S.F.

What is the correct way to calculate tyre pressure for a non standard tyre on a car without placard?

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IMHO the best way is to do it visually rather than by calculating. Park the vehicle on flat ground and inflate until the sidewall and contact patch length both look correct. Then drive and check temperatures.

I was told quite a few years ago that NO radial tyre in Australia should be run on hard surfaces below 32PSI,but run at down to 50% on sand,soft mud,etc.


cheers

May I join in this topic?
I call myself a Dutch pigheaded self-declared tyrepressure-specialist, and story began when I got hold of the official formula used by European ETRTO to calculate pressure for a sertain load, and went running with it.

What is used here is the linear calculation , so part of the AT-pressure = part of the maximum load to carry. This is not that bad formula, but official one has a curve in the graphics. For instance 60% of the AT-pressure gives 66% of the maximum load to carry.

Will give a picture I made about the different calculations used, because I found out that the official ETRTO formula even was not that holy as I declared it in the beginning, the American TRA formula's are bad, and give to much deflection of the tire so it can overheat when driving at speed , and damage.

This is the goal of the calculations, that no part of rubber of tire gets a to high temperature so it hardens and crackes in next bendings/deflections.
when driving about 40m/h the tire makes about 10 cycles a second, and every cycle every section of tire deflects and flexes back that 10 times a second wich produces heat. Higher speed > lesser deflection allowed to give the same heatproduction. That is why for higher speed there is a system of highening up the AT-pressure .

Determined my own system wich lowers the loadindex of tire 1 step for every 10km/6.6m/h higher speed then the maximum load is calculated for ( mostly 160km/99m/h) . The other way around is also used by the tire-makers, for instance for a "for trailer use only"tire with N speedrated for 140km/86m/h max speed for wich the maxload is calculated At AT-presssure, gives 2 LI steps higher then Q speedrated ( 160km/99m/h) tire of same sises and AT-pressure ( if you can find them).

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Originally Posted by UncleHo

I was told quite a few years ago that NO radial tyre in Australia should be run on hard surfaces below 32PSI,but run at down to 50% on sand,soft mud,etc.


cheers

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The official ETRTO system gives not to go lower then 1,5bar/21psi, and it was asumed so the tire wont get loose from the rimms when curving.
But nowadays most rimms have 2 humps , wich prefents the tire from doing that.
American TRA-system did not go lower then 26 psi/1.8 barr, but this was because the old formula gave to much deflection then.
America stepped over to the European calculation as late as 2006, but only for normal car tires and XL/reinforced/Extraload and left LT tires to the old less worse calculation wich they use nowadays still.

Have determined my own calculation wich comes close to the lineair calculation here used, wich gives at lower pressures/loads, the same deflection as when maxload/AT-pressure.
Using my system makes it possible to go low in the pressure and still save for the tyre.
with lower pressure better gripp and comfort.
The kind of cars used here have mostly oversised tires of wich one can carry the Maximum permissable axleload of the car. This yustifies a low pressure , even as low as 17 psi here written. And then even for on road, no overheating so no damage to tire.
Also a Radial tire stays with its widht on the ground over a large range of deflection/pressure, so sidewear wont happen that soon.

Have pressure/loadcapacity -lists of Michelin tyre wich gives it for lower speeds too, and for mud/Sand and on track.

Then for in Mud/Sand asumed max speed 20km/12.5m/h about 50% of the pressure given for that speed on road . So as you write.

For on Track it gives max speed 65km/35m?/H and 80/85% of pressure needed for on road that speed.

lowest given is 0.6 bar/ 9 psi , on a tire with AT-pressure( referencepressure) of 6 bar/87 psi. This is low weight on tire then.

This system can also be used on other tires like normal car tires or ofroad tires.

Off road tires have often large profile blocks that cover a part of sidewall.
This stiffens the sidewall so it produces more heat.
but still these tires are given same maximum load for the speed, as a onroad tire . For these ofroad-tyres is best to lower the Loadindex/maximum load as given on sidewall , by 8LI steps/20% of maxloadgiven.This for savety so no part gets overheated.

Thank you for your replies and for the PM, all sorted now.
.W.