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Another tyre pressure question
norfolkluego - 9/7/10 at 09:09 PM

We're sprinting at Debden on Sunday and using R888s for the first time. Simple question, what should the pressures be, low to get the contact patch as big as possible or pumped up to keep the tyre walls stiff in the corners.
It's a Velocity, weighing in about 550kg on 13x7 wheels.

JoelP - 9/7/10 at 09:17 PM

16 to 20 hot. Experiment!

PAUL FISHER - 9/7/10 at 09:46 PM

At 550kgs and 13" rims,so I guess your running 60 profile tyres, so I would start with 22 psi cold, around 26 psi hot, and see how it feels, you might find you have to drop a couple of psi out at either front or rear, but I dont think you will be far off at those pressures.

RazMan - 9/7/10 at 11:26 PM

Is there a formula for working out tyre pressures? The reason I ask is that I recently discovered two sidewall splits in my rear tyres so I suspect that my pressures were a bit on the low side. Having just bought new tyres I wouldn't like to see them go the same way.

nitram38 - 10/7/10 at 03:57 AM

quote:
Originally posted by RazMan
Is there a formula for working out tyre pressures? The reason I ask is that I recently discovered two sidewall splits in my rear tyres so I suspect that my pressures were a bit on the low side. Having just bought new tyres I wouldn't like to see them go the same way.


Easiest way is drive on a straight road and use an infra red/laser thermometer when you pull overe to check the tyre temps across the tread.
Too hot on the outer edges, too low pressure, hot in the middle, over inflated, even temp across the tread just right

[Edited on 10/7/2010 by nitram38]

Worzey - 10/7/10 at 08:19 AM

At Lydden last week I used 22psi in my R888's as advised by Gerald Dale Motorsports who setup the geometry and suspension on my Velocity.

I used to run 20psi but the new setup and tyre pressures where amazing. I've never had it feel so well balanced on a trackday before.

Mark Allanson - 10/7/10 at 08:39 AM

quote:
Originally posted by RazMan
Is there a formula for working out tyre pressures? The reason I ask is that I recently discovered two sidewall splits in my rear tyres so I suspect that my pressures were a bit on the low side. Having just bought new tyres I wouldn't like to see them go the same way.


I have heard this one - no ideal if it is correct but it sounds straight...


Would out the tyre footprint in sq in (my 185's are approx 7.25"x4.5" so thats 33 sq in.

My car weighs 700kgs plus 2 100kg passengers which is 1980lbs, I have 48-54 weight distribution so thats 950lbs at the front and 1030lbs at the back.

So the front tyre has 425lbs on 33sq ins which gives 13psi which seems really low! (But I have guessed the footprint)

RazMan - 10/7/10 at 09:53 AM

quote:
Originally posted by nitram38
Easiest way is drive on a straight road and use an infra red/laser thermometer when you pull overe to check the tyre temps across the tread.
Too hot on the outer edges, too low pressure, hot in the middle, over inflated, even temp across the tread just right


I can see the logic there but wouldn't camber come into the equation? ie. negative camber would increase the loading on the inside edge of the tyre on a straight road, therefore causing the surface temp to rise.

RazMan - 10/7/10 at 10:08 AM

quote:
Originally posted by Mark Allanson
So the front tyre has 425lbs on 33sq ins which gives 13psi which seems really low! (But I have guessed the footprint)


Not too far out though. Tyre pressures certainly affect the footprint and the best way to measure the footprint is to slide pieces of paper under the tyre from 4 directions, then measure the distances between the paper - in the absence of any fancy formulas it should be pretty close.

However I would have thought there must be a way of calculating the 'sweet spot' for achieving a good balance between footprint area, contact pressure and grip - I have Googled like crazy but can't seem to find anything on this


[Edited on 10-7-10 by RazMan]

norfolkluego - 13/7/10 at 12:17 AM

Took your advice and experimented and found that at 22 the grip was amazing, shame that the engine decided to eat itself just as we found that out