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Old 16th April 2009, 02:59 PM   #1 (permalink)
jae
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A Basic Guide to Tyre Pressures

I've been looking for the correct pressure to run my new tyres at, and found rather a lot of not much. So I went hunting, and didn't find a lot more, until I stumbled across this, which is what I am using. Thought you folks might like to use it as well.

The baseline method from Oscar Pereda of BF Goodrich. He suggests this as a good starting point if you have abolutely no guide as to what pressure to use in your tyres. This starting point should be regarded for road use only.

(Vehicle Weight in pounds imperial/100) + 2 psi at heavier end + 2 psi all around if suspension and alignment are stock.

Example:

Stock 911, 3,000 lb.
(3000/100) = 30 psi
Add 2 psi all around = 32 psi
Add 2 psi to heavy end = 34 psi at rear

With modified suspension, the result would be 30 psi front, 32 psi rear.

SO:

Stock R33 GTR = 1530kg = 3366lb.

3366/100 = 33.66
Non standard suspension, so only adding 2psi to heavy end.
36 psi front, 34 psi rear.

But I'm a fat bastad, so that adds 240lb (let's be creative with the scales, eh?).

Adding 2.4 and rounding down for the tyre's stated maximum pressure (40 psi) gives 36 at the rear and 38 at the front.

PSI to Bar is 14.5038:1
Pounds (lb) to kg is 2.2:1

NOTE: Compensation for driver weight is my own addition, not mentioned in Oscar's baseline calc.


In all fairness, I should point out that my R33 GTR on 265/35/18 was running 34 front and 32 rear and that was superb. But me new tyres are bigger with a lower profile, so this is why I went looking.
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Last edited by moleman; 16th April 2009 at 04:15 PM..
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Old 16th April 2009, 03:08 PM   #2 (permalink)
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Adding to FAQ a sticking it.

Nice one Jae.
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Old 16th April 2009, 03:32 PM   #3 (permalink)
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38psi on a 33?!?!

are you shitting me?

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Old 16th April 2009, 03:36 PM   #4 (permalink)
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38psi on a 33?!?!

are you shitting me?

mook
I would say that's a bit much as well and that it would make quite an uncomfy ride on low profile tyres. Interesting read nevertheless.
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Old 16th April 2009, 03:37 PM   #5 (permalink)
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i run mine on about 28/30psi
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Old 16th April 2009, 03:42 PM   #6 (permalink)
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I'll let you know how it goes.

Theoretically 36f/34r using the base calculation, but I added compensation for fat bloke at the wheel, which may be unnecessary unless car is four up with a bootfull.

If you're on 18s, Mook, you really should try 34 front & 32 rear. Seriously.
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Old 16th April 2009, 03:52 PM   #7 (permalink)
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I'll let you know how it goes.

Theoretically 36f/34r using the base calculation, but I added compensation for fat bloke at the wheel, which may be unnecessary unless car is four up with a bootfull.

If you're on 18s, Mook, you really should try 34 front & 32 rear. Seriously.
are those cold pressures?
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Old 16th April 2009, 03:55 PM   #8 (permalink)
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fwiw 34f/32r is what I use (cold, 275/35/18).
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Old 16th April 2009, 03:57 PM   #9 (permalink)
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are those cold pressures?
Yes.
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Old 16th April 2009, 04:04 PM   #10 (permalink)
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and to think i took air out at the nurburgring
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Old 16th April 2009, 04:10 PM   #11 (permalink)
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My 33 on Road 30 all round on track they are set around 24/26 Cold

Never had any wear probs

I agree with Mook 38 is just to High
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Old 16th April 2009, 05:01 PM   #12 (permalink)
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Even on 19's with 30 profile? Hmmm. I may run it the other way over, start at 36f/34r and see how I do.
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Old 16th April 2009, 08:37 PM   #13 (permalink)
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I'd imagine track use is totally different from road use; I'd expect much more heating on track (although with the way jae drives this may not be true )
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Old 17th April 2009, 01:22 AM   #14 (permalink)
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Forgot to say i,m on 18,s as well 265s
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Old 17th April 2009, 08:37 AM   #15 (permalink)
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As this equation does not have a tyre size compensation, I would think the author is talking about standard size tyres. Smaller (narrower) tyres need more pressure, therefore I would expect most performance cars to use lower pressures than the guide suggests.
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Old 17th April 2009, 09:02 AM   #16 (permalink)
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and to think i took air out at the nurburgring
You should ALWAYS lower the pressures when on track
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Old 17th April 2009, 09:24 AM   #17 (permalink)
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Don't agree with that. Seen too many pics of cars cornering hard practically on the rims because air has been let out.

Had a long discussion about with Colin CATDT at the Nurburgring.
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Old 17th April 2009, 09:27 AM   #18 (permalink)
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Pressure goes up with temperature - if you leave your pressures the same as on the road and they're on the highside the tyres will overheat due to be worked a lot harder than they would be on the road

10 years of bike trackdays taught me that I doubt cars work different in that respect

We're talking 2-3psi max here, but its enough to make a difference
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Old 17th April 2009, 09:43 AM   #19 (permalink)
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I'll take your 10 years of track days and raise it with Colin's 30 years of industry testing and what it's taught him.

You don't let air out. You run the car with road pressure, then when hot, you check it's not too high and let air out if it is.
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Old 17th April 2009, 10:01 AM   #20 (permalink)
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It will almost always be too high if you're running your usual road pressures and you're driving hard on track, especially with normal tyres

I use one of those gun things you point at the tyre to get the readings
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