Tyre pressure for over size tyres

[glaggs]

Bored 'cos I'm sat at home with a bad back so I decided to try and find someway of calculating tyre pressures for larger tyres. The theory with tyres is, it is the volume of air that is critical not the pressure. Therefore if you fit larger tyres inflated with the same volume of air the resulting pressure will be lower. Couldn't find any sites that could give a conversion matrix so had to resort to some basic GCSE physics. According to my calculations the Jimny with 31x10.5 -15 tyres fitted should now be running 13.6psi ! Currently they have 20 psi in, and the oe fittment 205/70-15s recommended pressure was 24 psi. With 20 psi in they hardly deform at all at the bottom, so perhaps the calculation is right! Anyone got any links for easy calc. sites?

[chris9119]

Bored 'cos I'm sat at home with a bad back

Were all really hoping that your back gets better ever so quickly............    

[Skibum346]

The theory with tyres is, it is the volume of air that is critical not the pressure.

Think your off on a back pain addled tangent here...

Pressure is a measurement of the amount of air inside a tyres volume.

If a tyre is bigger it takes more air to reach a specified pressure.

I'd have thought it's self balancing... 24psi on a set of 205/70's is still 24 psi in a set of 40" tyres (assuming they are both off the vehicle and there is little significant difference in the physical weight of the tyrte).

for example... Assume your tyre volume is 1 cubic foot and is filed with air at 1 pound per squre inch (if we must work in imperial), the volume of air is 1 cubic foot.

However, Boyles Law says that a gas will compress proportionately to the amount of pressure exerted on it. If you have a 1 cubic foot balloon and double the pressure on it, it will be compressed to 1/2 cubic foot. Increase the pressure by 4, and the volume will drop to 1/4 the size etc.

So if we raise the pressure in our 1 cubic foot tyre to 24 psi there is 24 times the volume of air, 24 cubic feet.

If our tyre was 2 cubic feet in volume 24 psi would mean we need 48 cubic feet of air to attain that pressure.

Geddid?

**Hangs up Mortar Board & Robes and retires to the country... to wait for the challenges from others**

[glaggs]

Correct, to maintain the same pressure you have to increase the volume at a constent temp. But as I origionaly staited, its the volume of air that carries the load (and controls the tyre temp.) If a boy racer fits bigger rims and ultra low profile tyres, and then tries to get away with the same psi he's in the [!Expletive Deleted!] 'cos the tyre will overheat and possibly blow out. So when fitting low profile the pressure should be increased to the point where an equal volume of air is in the tyre.

Reverse the logic and a larger tyre will be at a lower pressure for an equal volume of air.

http://answers.google.com/answers/threadview?id=233041

See the link?
I was hoping to find a matrix or easy calc to work out pressure/volume/tyre size combinations?