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Quote: Originally posted by arc systems on 19/6/2008

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However 4psi for all tyres cannot be totally correct as different tyres have different volumes of air starting at different pressures, all then cannot then be correct at 4psi although it would perhaps be the maximum variation.

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The cold pressure matters, but volume doesn't if you make the simplifying assumption that, for a particular tyre, the volume doesn't change with pressure and it cancels in the calculations.  This is probably a reasonable assumption once you get somewhere near to the correct pressure. 

If you assume a cold temperature of 20 degC  (293degK), a hot temperature of say 50 degC (323degK) and a cold pressure of say 40psi then the hot pressure will be .........

P = 40 x 323/293 = 44.1 psi which is near enough a 4psi increase.

If it works well enough for this example, it's probably a reasonable rule of thumb.

Cheers
Andrew

I do wish I was as good as you at maths/physics but I'm not and bow to your figures, not being funny, I agree with what your saying. What I've been trying to make sense of is the variation in pressure tables from calculated pressures.

So, if not volume how about surface area of a tyre that would have a higher volume? given the fact temperature rise is always going to be better dissipated by a larger surface area ?

The fact is any one tyres pressure table does not show a linear pressure/load variation while I can't believe it's not linear, at least at a static temperature?

Quote: Originally posted by arc systems on 20/6/2008

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So, if not volume how about surface area of a tyre that would have a higher volume? given the fact temperature rise is always going to be better dissipated by a larger surface area

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Well yes ............. but I don't see that it would have an effect.  If a bigger tyre was used because the load was greater then the footprint would be larger too and the heat generated would be greater at any given speed.   The point I was making about volumes is that you can reasonably assume that they are constant - for the same tyre and hence they don't affect the calculation.  If P1,T1 are the cold values and P2, T2 are the hot values, The gas law is  PV = RT     So ... 

P1 x V = R xT1
-----------     --------
P2 x V    R x T2

and V and the gas constant R cancel leaving P1/P2 = T1/T2 , which is what I used to calculate the change in pressure in the previous post.

The one problem that does affect the accuracy of any calculated pressure is that we don't know the design running temperature of the tyres used - manufacturers don't seem to divulge that, at least not on web sites I've seen - though we do know that racing tyres tend to run at 80-90degC.  One technique that I understand that they use in racing is to vary the pressure to ensure that the tyre temperature is the same across the tread - indicating that all the tread is doing the same amount of work.  I've seen tyre pyrometers for sale on Ebay - but in my book, setting tyre temperatures that way is a bit on the obsessive side.  OK for racers but not for caravannners maybe. 

You could regard the *correct* pressure as being that at which gives even tread wear across the tyre.  I had an Isuzu Trooper for 7 years and did over 100,000 miles in it.  I religiously set the tyre pressures to those recommended by Isuzu - but the middle of the tread on the rear tyres always had more wear than the outside of the tread, showing that the tyre had been overinflated. I lowered the inflation pressure by 2psi and the wear was even after that.  Seemingly manufactuers don't always get it right. 

Cheers
Andrew