Thread: Calculating Tyre Pressure

Would you not use the pressures on the tyre placard?

Which is usually around 36-40 so you are reasonably correct.

Jonesfam

Are you incorporating tyre depth & wear artifacts into your assessment ?

The maximum load, and the pressure it is rated at, are dictated by the overall strength of the tyre. I do not think that there is any good chain of reasoning to say that this has any bearing on the pressure to be used at lower than maximum loads, as at these loads the overall strength is above that needed.

Other factors will influence the desirable pressure at less than maximum load, but the maximum load and pressure are simply limits.

John

Another factor is the acurracy of your tyre gauge.
Last year, on Fraser while waiting for the Ngkala Rocks track to clear, two mates and I compared the six gauges we had between us and none agreed. The two closest were within 5 psi. These weren't your el cheapo gauges either.
I posted some time ago on either here or the other forum I'm on, as to how to how to check the accuracy of your gauge but to no avail.
Who trusts servo gauges & how accurate is the car companies gauge at original inflation?
Steve

Originally Posted by B.S.F.

On my tyres it says
Max. load single 1230kg @ 97psi.
The weight on my rear axle is 950kg
97psi :1230 x 950= 74.9 = 75psi 75:2 (2 wheels per axle) = 37.5
That's the pressure I've been using for the last 2 years. It feels pretty solid.
Are my calculations flawed?
I'm not interested in what tyres or pressures you are using, and I'm familiar with the 4psi rule.
.W.

I agree with your answer of 37.5psi, and the way you got to it.

I then use the 4psi rule to fine tune the pressure.

75psi 75:2 (2 wheels per axle) = 37.5


Isnt the pressure per tyre and not per axle
. so it would be still 74.9 PSI. using your calculation. using only the method of a ratio 1230:923 Kg


Ive come across people telling me about the method of calculation for tandem axle and wasn't understanding of the logic till now. more axle lower pressure. Ive normally just pointed to the shape of the sidewall and the tread contact or tyre wear.


Tyre Placard pressure only accurate when using the original supplied tyres. Often complained about by manufactures. Commercial vehicles often the tyres are run at low pressure compared to the indicator on the sidewall. My HINO has worn out the Middle of the tyre running at half pressure.

Originally Posted by B.S.F.

On my tyres it says
Max. load single 1230kg @ 97psi.
The weight on my rear axle is 950kg
97psi :1230 x 950= 74.9 = 75psi 75:2 (2 wheels per axle) = 37.5
That's the pressure I've been using for the last 2 years. It feels pretty solid.
Are my calculations flawed?
I'm not interested in what tyres or pressures you are using, and I'm familiar with the 4psi rule.
.W.

I've never heard of this calculation before, so I ran some numbers on the tyres on my Caravan.

It runs Toyo HO8 LT's. The max load and pressure on the tyre is 850Kg and 65PSI. The van weighs 1,300Kg but has a draw bar weight of 120Kg, so there's 590Kg on each tyre.

Running the calculation - (65/850)x590 I get 45PSI - Guess what? I run them at 45! . When they are warm, they usually sit on about 50, so almost spot on according to the 4PSI rule (6PSI for LT tyres)

There must be something to that - I'm going to check some others on the Landies...

Not sure if you're right or wrong, but it is pretty spooky that the calc works on the first try - When I first fitted them, I just pumped them up until the looked right - which was around the 45 to 50 mark, after a bit of fiddling, I found 45 gave me a slightly better towing experience on bumpy roads, so I have left them at that.

Firstly, it is more mathematically/physically correct to divide your axle weight in 2 and use that value, rather than dividing the pressure in two.

However, there are a lot of nonlinearities your method ignores.

Here are a list of all the tyres I have and those listed here, with a calculation for your axle load:

kg | psi | psi/kg | psi for 950 kg axle load
1500 | 65 | 0.04 | 20.6
1380 | 80 | 0.06 | 27.5
1360 | 65 | 0.05 | 22.7
560 | 51 | 0.09 | 43.3
515 | 51 | 0.10 | 47.0
1230 | 97 | 0.08 | 37.5
850 | 65 | 0.08 | 36.3



Now the first 3 tyres on that list I run at ~40 psi with a similar axle load to you. Wear has been even and they have lasted a long time. At 20psi I am sure they would have died a quick death!!!