correct tyre pressure method

[wemorgan]

I've read quite a few posts where people have asked to know the correct tyre pressure for their new wheel-tyre configurations.

I've read this method in a race car setup handbook, maybe it's of some help.

Using an infrared thermometer like this measure the inner edge , centre and outer edge temperatures of each warm tyre.

For correct camber the two edges of the tread should have equal temperatures.
For correct tyre pressure the temperature should be equal to the average of the edges.

eg.

Inner edge, Centre, Outer edge
150, 165, 150, Pressure High, Camber OK
155, 150, 145, Pressure OK, Negative Camber
150, 155, 160, Pressure OK, Positive Camber
140, 140, 160, Pressure Low, Positive Camber

 

[BTB 500]

Interesting. I've got a gun-type IR thermometer, might give it a whirl.

Not sure that road use heats tyres up enough for this to be reliable though?

 

[lynall]

Thats clever out with the ir gun tomorrow



Lynall

 

[V12]

Any results? Interested to know whether car tyres generate enough heat for this to be a viable method of checking? Also concerned by the varied driving that could affect these measurements, if you drive round a hard twisty country road you could easily heat more of the edges than the centre...hhmmmm.

 

[Number_Cruncher]

While the thermometer measurement will give an even contact, that isn't the be all and end all of setting tyre pressures. The so called cornering stiffness of a tyre is pressure dependent, and this affects the handling balance of the car.

So, be especially wary if the method means that you are increasing the front pressures, and reducing the rear, because that reduces the stability of the car.

Edit: So, in this respect, I would say that the method suggested is not THE CORRECT method, but is a method which might work well, but if used, should be used with caution. It would be better to stick to standard specs than to de-stabilise the car.

 

[Rory]

I've read this method in a race car setup handbook, maybe it's of some help.

I read somwhere that an old racing car tip to check pressures was to put a wide chalk band across the tyre and check that it wore off evenly.

My Merc tyres certainly wear unevenly - edges and the front and centres at the rear - so I keep meaning to try this but haven't got around to it.

 

[wemorgan]

While the thermometer measurement will give an even contact, that isn't the be all and end all of setting tyre pressures. The so called cornering stiffness of a tyre is pressure dependent, and this affects the handling balance of the car.

So, be especially wary if the method means that you are increasing the front pressures, and reducing the rear, because that reduces the stability of the car.

Edit: So, in this respect, I would say that the method suggested is not THE CORRECT method, but is a method which might work well, but if used, should be used with caution. It would be better to stick to standard specs than to de-stabilise the car.

Sorry, I hoped I'd made it clear that this method was only really where no standard tyre pressure was available or for improving race car dynamics.

It's not necessarily true to say that increasing front pressure and reducing the rear will reduce stability. Since if you did not know the correct pressure to begin with, then it is possible to improve stability by making the above changes. It'll all depends where you are beginning from.

 

[Number_Cruncher]

>>It's not necessarily true

I would humbly beg to differ. Reducing rear tyre pressures (or increasing fronts) will always increase rear slip angles relative to front tyre slip angles, which moves the car towards an unstable condition.

 

[wemorgan]

>>It's not necessarily true

I would humbly beg to differ. Reducing rear tyre pressures (or increasing fronts) will always increase rear slip angles relative to front tyre slip angles, which moves the car towards an unstable condition.

But if you had no idea what the correct pressure were meant to be, then it is quite possible that the starting pressures were significantly too high. Hence reducing them towards the 'correct' pressure would improve stability.

The tyre temperature method would show that the centre temperature was too high compared to the edges. With low rear tyre grip the car would overstear. Reducing the pressure would increase the contact patch and improve stability.

If you don't know what pressure you are beginning from, how can you debate what a change of pressure would do?

 

[Number_Cruncher]

With standard tyres, MBs are set up to run on the centre of the rear tyres - this isn't a mistake on MB's part. This is because the small sacrifice in uniformity of contact is more than made up for by the increase in rear cornering stiffness, which promotes understeer - i.e., promotes stability. The temperature method would in the standard case give the wrong result, and I see no reason why it wouldn't give the wrong result on modified tyre setups.

 

[BlackC55]

This is only really aplicable on race tyres under exteme load.

 

[lynall]

Well measured mine last night after 12 miles.

Nsf 25,27,30 Osf 28,25,24
Nsr 28,25,24 Osr 30,27,26

Measured as if looking from above the vehicle.

Not sure what it means but car had a full 4 wheel track a few months ago with a couple of new bolts to get it within tolerance, tyres now seem to be wearing perfectly flat.



Lynall

 

[wemorgan]

Thanks Lynall.

Interesting to read that the centre pressures are near enough the average of the outer pressures.

I wonder why the NSR is the only tyre where the outer temp is hotter than the inner. Does this indicate that your journey had more right hand corners than left?

Do you know whether the garage measured the camber as well as the track?

.....I really should get around to doing it myself.