Lancia Beta Forum

Hi all,
I've just had a googleabout to try to find if there's any information on the subject of tyre pressures for non standard tyre fitments. There isn't. Or at least if there is, I can't find it.
How does one work out the correct tyre pressure for a car fitted with a different size wheel/tyre than the manufacturer intended?
Is there a formula? Or is it just a case of trying to find another car with the same size tyres fitted as standard and, taking into account the weight and weight distribution of the other car, make an educated guess?

Andrew.

Guesswork it is then...

Andrew.

Hi Andrew

I would say so, unless you can find any sensible articles on tyre pressure settings. I would start with the same as for the original manufacturer and see how that goes, changing the pressure up/down and comparing. Sadly you will not see any excess wear issues until much later of course.

Peter

Andrew,

In general the wider the tyre and the lower the profile the higher the pressure required is.

BUT it's usually only a few PSI and you can only do it by what feels right for you.

Agreed. There is a big drop in sidewall from original on my Y10- 155/70/13 to 185/50/14. I will experiment...  :D
Cheers guys.

Andrew.

I come across this all the time with 4wd's as tyre size and type change. A good rule is the 4psi rule, measure the pressure cold and then after about 20-30min driving to get them warmed up you want them to go up by 4psi. This way no matter what tyre you run the result should be the correct pressure. Different tyre types also have different sidewall strenghts and will require different pressure's. If you have access to a decent temp probe you can measure the temp across the tread of the tyre, if it's even across the tyre then happy days, if it's hotter in the middle then the pressure is to high and so on.

Hi

What a simple elegant starting point, really like the test. Temp probes are available fairly cheaply so not an expensive exercise.

Thanks

Peter

Fascinating. I've never heard of this. Where did you discover this theory?

Andrew.

The temperature sort of makes sense although not sure if the difference would be great enough to be conclusive, can't say I'm convinced on the 4psi one what is the physics behind it?

It's been around for a long time and is really quite accurate, especially on 4wd's as tyre size and construction change for different trips or surfaces, not to mention that tyre pressure changes multiple time a day as the terrain changes( i usually run 12psi on sand dunes, 18psi on rock ledges and 24 on rutted climbs) pressure changes are all about getting the right contact patch on the ground, being able to find the right pressure for your tyres mean's you will use less fuel and have less tyre wear. As the tyre carcass moves and flex's it creates heat, as the air inside the tyre heats up it expands and the pressure increase's, if the cold pressure is to low than you get to much sidewall flex and heat therefore a higher pressure increase, conversly if the starting point is to high the sidewall does not move much and less heat is generated resulting in less of a pressure buildup. The sweet spot for tyres (and also interestingly air suspension bellows) is a cold pressure that gives an increase of around 4psi, this gives you the optimum tyre pressure for your driving. Now this can be used for anything, for example if you are racing you are maintaining high speeds and cornering forces so the tyre gets hotter, once again if you play around to find your cold pressure starting point that gives you 4psi increase than you will optimise your contact patch on the road.
Give it a go, you'll be surprised just how much the pressure will change in your tyres after they have warmed up. Now a disclaimer if that 20-30 min drive includes 15min sitting in traffic than that does not count, if your sitting on the freeway at a constant high speed than you may only need 10 min's for the tyre to get up to operating temp, just bare in mind you don't have to be anal about the time thing more just a bit of common sense as to "is the tyre hot".

I have to say the 'science' of this theory, as explained above in laymans terms, makes good common sense to me. I will be doing some experimenting when time allows.

Andrew.

Ditto !!


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