dadba said:
Running 35 inch BFG muds on normal on road off road use. Can I see if anyone of you would be putting the same psi in as I have bin doing. Cos when Ive asked various 4x4 type peeps in the past I seem to get different responses.
So lets see if we agree on a normal running psi. Wot dya think chaps ??? :?
I came across a post on another forum (an Australian Nissan Patrol forum) that suggested the use of the "Four pound rule" for setting tyre pressures. Apparently, this originally came from Michelin.
What you do is ..........
* measure your tyre pressure cold
* run at speed for at least half an hour (I suppose a motorway trip would be ideal) to allow the tyre temperature to stabilise
* measure the hot tyre pressure
If your cold tyre pressure was correct to start with, then you should expect a pressure increase of about 4psi when hot. If the increase is more than 4psi then the tyres are getting hotter than they should - so you should increase the cold pressure by a bit, which will reduce the temperature rise. OTOH, if the increase was less than 4psi then you should reduce it by a bit.
If you've been running them for several thousand miles, you'll be able to tell by measuring the tread depth in the middle and near the edges of the tread. If it wears more in the middle, then your tyre pressures are too high - and too low if the tread wears more at the edges.
Another way is to do what the racers do. Get yourself a tyre pyrometer (I've seen some on Ebay). Run the motor at speed & measure the tyre temperature in the middle of the tread and at the edges. What you are looking for is all of the tread doing pretty much the same amount of work and hence being at about the same temperature. If its hotter in the middle then reduce the tyre pressure a bit - or increase a bit it the edges are hotter.
As I've just changed tyres to a (slightly) different size, I thought I'd try out the four pound rule sometime and see if it tells me anything useful. For now, I've just set the tyres at stock pressures - 29psi front and 32psi rear.
Cheers
Andrew
