Tyre pressures

Hi Willem.

What brand of tyres are they, and what is the maximum load rating and at what pressure?

Cheers,

Peter

Geez Peter, I had to go out into the cold with a torch to read the requested info...LOL

Cooper STT .. Maximum load rating 1550kgs at 80 psi cold

Is there a secret formula?

Cheers

Geez Willem, sorry about that, remind me to buy you a warming ale if we ever catch up lol.

The load rating @ maximum pressure is the key to every tyre. I'm no mathematician, but by my figures (assumed only), your tyres would be running a tad soft. This is how I work tyre pressures out, using your figures.

Tyre maximum load rating 1550kg, divided by 80psi = 19.375 kilo per psi.

19.375 kilo per psi X 35psi = 678.125 kilo's per tyre

678.125 kilo's per tyre X 2 = 1,356.25 per axle x 2 axles = 2,712.5 kilo's total vehicle load.

This presumes you have a perfect 50/50 load distribution between the front and back axles, which you won't have. A weighbridge when loaded will give you the right figures.

I always add 6psi to the load figure I get for highway running, and generally this will always give you about a 4psi heat build up after an hours running.

On dirt road, I just drop this 6psi and go back to my original figures. Mud, sand and rough surfaces are a different ball game, but the formula above will give you the best traction, mileage and fuel economy out of the tyres, the way the tyre designers intended.

I think this is where a lot of people get confused. The original Bridgestones on my GU III had the maximum load/ pressure @ only 36psi, not 80psi like the Coopers, but the formula works equally as well on Bridgestone and any other make of tyre.

Sorry about being long winded, I'll make up for it over that ale ;-)

Cheers,

Peter

Thanks for that Peter. It is mid boggling....lol

So 40psi should be the psi I should run while on the bitumen according to your calculations and the weight of my vehicle. My Tyre bloke and Coopers retailers suggested 35psi not knowing the load of the truck.

I find the STT's pretty good in all aspects of offroad and quite quiet on the black top. When driving longer distances(say 100km) on very low pressures the sidewalls do seem to scuff a bit and some lettering may be scrubbed off.

Time will tell if my methods are detrimental to the longevity of the tyre....:o)

Holy Toledo Batman ! Scrubbing the lettering off the sidewall :-o You have been doing some serious cornering ;-)

IMHO try 40psi in the front and 44psi in the rear when fully loaded and see how that goes. Remember, at these pressures the tyre is only at 1/2 load carrying capacity.

BTW, cold tyre pressure is messured at 20 degrees C.

Catchya on the track :-)

Yep the camp photo is of dommett - can't wait to get back there again, the other is about 30k up river at 'sandy beach'.. as for the pressures, looking like going with the 25 psi give or take, but we'll be going slow. Good to catch up with you the other day, catch up soon

Andrew

PS - apparently the road to dommett is getting graded at the moment and is open.

Andrew
I think you will fined that they are all cold temp settings
as it would be to hard to get the tyre to the same hot temp to test

Richard

When we travel from bitumen to dirt we will drop pressures 10 psi front & rear (but I run split and nower tyres) 50 psi rear 40 psi front

Mine were hot temps.

I find that the Coopers warm up 2-3psi only so that that amount off.

Thanks Willem and Richard. I just needed a bit of conformation on the hot/cold measurements as I will be in the situation of leaving on the blacktop, and dropping the pressures into the journey, ie starting with cold highway pressures, then dropping to hot dirt road pressures.

Cheers Andrew