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btcc93
06-07-09, 17:29
I did my first ever sprint yesterday and was way off the pace, numerous things to sort out on the car. But for what is left of this season need to not get involved in any major expendature, so will start off with things that will cost zilch. Which brings me to tyre pressures as a starter, the car is stripped out and a six point cage in so the weight is considerably lighter than the makers intended noticably on the back where the springs fitted should lower the car by 35mm from standard but in fact only lower by around 7mm. Manufacturers recommended tyre pressures for a standard unladen car are 30psi all round but I have always on the road run at 28 on the back and 30 on the front. This seems to be way off on the track tho, any suggestions from the wealth of knowledge on the forum would be much appreciated.

JamesK
07-07-09, 19:43
As a rule of thumb i'd say always run the heavy (or driven) end a bit harder than the other.

Comparitive reduction in psi will make one end grippier than the other.

There is also the trade off between stability and grip - higher psi at faster venues etc

J

grahamb
08-07-09, 11:46
Try 24psi all round as a good starter, and lower to see if there is any improvement.

I was told by a biker that a good rule of thumb is that the tyre pressure you are aiming for, results in a warm pressure close to the cold pressure.

That is, with the tyres cold, set a pressure, go for a run, then check the pressures again. If they're different, let them cool, try lowering the pressure another psi, then go back out for another run.

Hard to do, particularly with variable conditions, but you're aiming for stability (ie limiting the amount of pressure increase during a run).

You could of course use Nitrogen in the tyres, in which case the pressures shouldn't change at all.

I run around 18psi - 20psi with the Fiesta.

Razorbill
08-07-09, 15:26
It would help to know some details -

Make and model of car
Weight & weight distribution
2 or 4 wheel drive
Power output
Tyre sizes
A1, B1 or slicks

Theoretically the faster the car is to be driven, the more pressure you need to stop the tyre flexing too much, so if your std pressure is, say, 30 psi, then on a fast track you may run 40 psi. But then you've reduced the car's weight...but by how much?

You need to find somebody with an identical car, or similar car, and use their experience to give you a starting point.

Guessing the pressures could be expensive, and I don't mean in tyre wear!!

R

btcc93
08-07-09, 19:19
Thanks for the replies folks. The car is a Mk1 Mondeo 4 door, don't ask why it is a long story but if you saw it you might understand, so finding a similar car and spec is going to be like the search for rocking horse manure. Yep I agree I need to get it corner weighed, but as I only finished it on Friday of last week I have not had the chance to get that done. So it is front wheel drive, powerwise it is a 2ltr on Jenveys but a rebuilt engine with 1,600miles on it thus it has only so far been mapped to 6,500 revs, peak bhp came at 5,800 at which the output was 165 brake. Tyres are 215 40 17 Zeta Linea on 7" rims

btcc93
02-09-09, 12:20
Sorry to resurect this one with a bit of a twist, Next year I was thinking of getting a set of either R888's or A048's to use in the rare event of us having some dry weather. Now my query is I have spare sets of both 16" and 17" rims available which size would I be better to use?