Calculating tyre pressures using formula??

[busbuddy]

In English, that's:
- Take the maximum axle load (e.g. from the handbook)
- Divide it by twice the maximum tyre load (from the side wall)
- Raise that answer to the power of 1.25
- Multiply that answer by the maximum tyre pressure (also on the side wall)

tried this just out of curiosity on mine, fronts are what psi I use but the rears


mine is
1400kg axle load
twice the tyre load = 2060
to the power of 1.25 = 0.61705804537
x 49psi

= 30psi which on my 255/55x18s is a flat

I think I will stick to what I use


if I had the normal 1300kg rear my psi would be 27 either my laptop cant do the math or if you have good weight rated tyres the formula loses the plot
anyone got the max pressure on a standard 185 commercial T3 tyre, working backwards on this formula from the usual tyre pressure for it the max has got to be in the 70s

[CJH]

mine is
1400kg axle load
twice the tyre load = 2060
to the power of 1.25 = 0.61705804537
x 49psi

= 30psi which on my 255/55x18s is a flat

Interesting. So you've got tyres that have a max load of 1030kg and a max pressure of 49psi? I don't have the experience here - is that common, such a high load rating for a 55 profile tyre? What type of tyres are they?

You're right - the formula will tend to give lower pressures for tyres with a higher load rating. But 30psi out of a maximum pressure of 49psi is hardly a flat. What do you run them at?

[busbuddy]

tyres off the last model discovery, run them at 41psi, it is the full load figure in the disco which is only 100kg over the fully loaded in mine, same ones been on for 5 years now and no wear yet


at 30psi they really bulge out, look like low profiles


at least my maths is good

just trying to get my head around the formula as its a question that always pops up with our vans, our weights vary a lot as do tyres available

[CovKid]

Agree with thoughts expressed on this that a formula doesn't always pan out for every tyre/weight scenario. Having gone to big tall tyres, I found I had to inflate them far more than average tyres to get even contact/wear across the width.

I still say start at 35 for the fronts and 40 for the rear, then chalk the tyres and you should be able to ascertain the ideal pressure for that particular setup. No maths involved either. I can see the thinking behind the calculations but tyres are expensive things and personally I wouldn't feel 100% confident in relying solely on an equation to get the best grip/wear out of a set of tyres. Tried that in the past and it wasn't overly successful. Chalk is cheap, quick, and tells you much about your tyre footprint.

[silverbullet]

A cheap probe pyrometer is the surefire way to get it right.

[CovKid]

+10 for that idea. Theres no substitute for ascertaining (in reality) what parts are/not making good contact with the road. Forecourt gauges are not accurate either - discovered that numerous times. I used one once that said 42 and was actually 19