Monday, Apr 07, 2008 at 11:55
Boo Boo
I did
check with the tyre manufacturer and was told to use factory pressures and a 1 psi increase all round would support an additional 70kg in vehicle weight.
I got a bit curious later on and decided to do a bit of calculating. I divided the maximum load one of them would support by the maximum permissable pressure and got the weight 1 psi would support. When I multiplied this figure by four, it came to 69 point something which ties in with a 1 psi increase supporting an additional 70 kg just like the manufacturer said.
When I worked out the weight on each wheel, ignoring the weight of the tyre at this stage, and divided it by the weight 1 psi would support, it came out less than 1 psi off the factory recommended pressures.
I have ordered a different type of tyre with a heavier load rating and have been told by the State distributor that they will need higher pressures. I tried the same calculations and the fully loaded pressures were about 6 psi above factory pressures for the lighter tyres which looked realistic and are about what I would have guessed.
When I get these tyres I will put each end of the car on a weighbridge and do the calculations again. That will give me a good starting point and I will see what happens from then on.
This may be the way pressures are calculated. As sure as can be the factory does not guess and they won't put something in the book that does not work.
Try the same calculations with your tyres and see what happens.
Your pressures do sound about right and from memory are what we used to put in troopies in a Government job I had a few years back. That was with 7.50 x 16 tyres though and yours may be slightly different. Try ringing the tyre manufacturer. It would be interesting to see if it can be worked out mathmatically.
Brian
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