Calculating tyre pressures using formula??

[Cruz]

When I first fiited my 16" alloys I put in 35psi front and 45 psi rear. The rear wheels have worn out more in the middle which pointed to them being over-inflated.

If I wanted to (finally) calculate the real pressures at maximum van weight capacity would I use this formula for each axle ??

Max load per tyre x 2 (stamped on tyre) = max load on that axle
then
Max axle tyre load/max rear permissable axle load = ratio
then
max psi of tyre/ratio = psi to inflate to

So using the details from the westfalia spec sheet for a high top joker

Front axle permissable load = 1100kg
Rear Axle permsisable load = 1300kg
and my Tyre max weight 750kg @50psi

front axle
750kg x 2 = 1500kg max tyre load
1500/1100 = 1.36
50psi / 1.36 = 36psi

rear axle
750kg x2 = 1500kg max tyre load
1500/1300= 1.15
50psi/1.15 = 43psi

So as I am never going to be running at max weights I should be fine using a small rounding down factor to say 34 fronts and 40 rears?

[CJH]

These was some discussion about a formula in a previous thread - I posted at the end of that thread with the ETRTO's (European Tyre and Rim Technical Organisation) formula of:

Tyre pressure = Max tyre pressure x (max axle load/(2 x max tyre load))^1.25

I think this is similar to your formula but with a factor of safety built in.

[CJH]

That formula gives 34/42 - so very close to your values.

[Cruz]

Cheers I will set them at 34 front and 40 rear and keep my eye on the rears this time, especially the o/s rear where the lpg tank is.

[silverbullet]

Very useful. To clarify that the last term is "to the power of 1.25"?

[CovKid]

Or if you want to get them bang on, do it with chalk (did mine this way)

https://www.youtube.com/watch?v=ojGsavK0Uwc" onclick="window.open(this.href);return false;

Windy video, but it explains how to do it. Ensures you get even wear and maximum grip. Calculations get you part the way there (sometimes) but no substitute for chalking to check they're right.

[HarryMann]

Vid doesn't work for me Ralph.
Also to confirm that last term is:

Max axle load / 2 × max tyre load

all to power of 1.25

[HarryMann]

Ah working now must be my connection..

Think there may be some provisos on the chalking method to avoid too low pressures or unstable front/rear combinations.

[CJH]

To clarify that the last term is "to the power of 1.25"?

Also to confirm that last term is:

Max axle load / 2 × max tyre load

all to power of 1.25

Yes, to both (although I think HarryMann was answering the question, not asking another one).

Tyre pressure = Max tyre pressure x (max axle load/(2 x max tyre load))^1.25

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)

The first two steps give you the proportion of the maximum load per tyre that you are using. The third step builds in a factor of safety. The final step applies the resulting load proportion to the tyre's maximum pressure.

[ghost123uk]

Raise that answer to the power of 1.25

In the interests of completeness, is there an easy way to do that ^^^ without a suitable calculator?

[CJH]

Raise that answer to the power of 1.25

In the interests of completeness, is there an easy way to do that ^^^ without a suitable calculator?

Good question. I guess you don't need telling that you can type it into a Google search box (e.g. type in (1100/1500)^1.25 and you should get 0.6786).

If your calculator has a square root button, but not an X^y button, then you can do it another way:
X^1.25 is the same as taking the square root of X, then taking the square root of that, then multiplying that by X.

But actually for most practical cases, this step doesn't do very much.

X should always be a number less than 1. If it isn't then your tyres aren't rated high enough!

With my tyres, which are rated at 850kg, and a front axle load of 1100kg, X is approximately 0.65. Raising that to the power 1.25 reduces it to 0.58, so it's giving about a 10% factor of safety.

The closer that number is to 1, the smaller the factor of safety. So for a rear axle 'heavy duty' load of 1400kg, X is 0.82, which reduces to 0.78, which is a 5% factor of safety.

So maybe a way to do this roughly would be to:
- knock 5% off for any ratio between 0.8 and 1
- knock 10% off for any ratio between 0.65 and 0.8
- knock 15% off for any ratio between 0.5 and 0.65

[scottbott]

lost me there,I think I must be getting thicker the older I get,I will have to sit down to that and really concentrate to work my tyres out

[CJH]

I think there's a danger here that this formula will be seen as a way to get the 'correct' answer, whereas I think it should just provide a starting point. I'm not a tyre expert by any means, but what little checking I've done with this formula has always got an answer close to the manufacturer's figures, but not exactly the same.

If you use it just to get you close, then note that you can skip that X^1.25 step and the resulting tyre pressure will probably only be a few psi higher - that's all. So maybe just skip that step and knock a couple of psi off to get your starting point.

If the formula is still a problem, remember you can always use the spreadsheet here.

[CovKid]

This is where the chalk method helps I think. You'll at least know what the contact area is and can assess whether they're wearing evenly. Whether that, or indeed a formula is truly accurate is debatable but using both should get you very close.

[silverbullet]

Camber and toe settings will also affect wear patterns e.g. a lot of negative camber (for whatever reason) and a very wide tyre will always wear the inner shoulder of the tyre more than the outer.
I would say in that instance the best you could hope for is an even wear gradient across the tyre.