Re: [ST] Tyre pressures for track day

["Philip Las Gourgues" <philip.l@xxxxxxxxxxxxxx>]

Chris wrote:
>
> I've heard it both ways. I don't really understand the thinking behind
> lowering your tyre pressure on a track.

It's to stop the tyres overheating. With modern tyres the main thing that
effects tempreture is the area of contact patch on the road. Sidewall flex
has a negligable impact on tyre pressures. Lower pressures give you a larger
contact patch on the road. Therefore that contact patch doesn't have to work
as hard and the tyres run cooler. Tyre pressures that you should choose for
the track also allow for the fact that the tyre is going to heat up pretty
quickly and hense the pressure will also rise. Therefore you run the cold
pressure lower than what is ideal. On the road you usually don't ride hard
enough to get any real heat into the tyres so you run them at higher
pressures to start with. This also reduces wear due to the tyre not
deforming against the road surface as much. On the track the wear due
deformation is insignificant compared to that cause by tearing, overheating
and slippage that would be caused by running the higher pressures.

> Arguments that I've heard for lowering pressure is that it increases your
> traction and stick.
>
> Arguments that I've heard against lowering pressure is that it increases
> tyre heat which does temporarily increase traction, but will cause
> delamination and catastrophic tyre failure.

This argument is actually backwards. It used to apply to old steel belted
tyres where sidewall flex would generate a significant proportion of the
heat. You'd have to lower the tyre pressures to absolutely stupid levels to
cause the tyre to delaminate.

> My *unprofessional* opinion sides with the latter argument. I'd suggest
> buying some race tyres if you want more stick rather than trashing your
> street tyres by lowering the pressure.

It's true that the tyres are probably going to cop a beating on the track
because you tend to ride harder. I guess if you can afford a sticky pair of
track tyres then that's great but it would get pretty expensive getting them
changed every time. Also I don't really think the ST is enough of a track
bike to be worth running different sets of tyres.

The ideal way to set tyre pressures is to measure the change in pressure
between hot and cold. On the road you generally don't get enough heat into
the tyres for this method to work well (if you followed it you would
probably end up running 60psi in an attempt to get a big enough change) but
if you ride hard on a twisty road or are doing a track session it will
minimise tyre wear and damage. You're aiming to get a 6 psi increase in
pressure at the rear and a 4 psi increase at the front. This is just a
starting point though and you can adjust the tyres as required to get the
bike handling the way you want. Higher pressures will give a bigger pressure
increase from cold to hot and conversly lower pressures will give a samller
increase. On a bike like the ST 32 front and 34 rear is probably a good
starting point for the cold pressures.

Hope this helps.

Phil
'02 S3


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