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Front tyres 225/40/17 or 215/45/17

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I have found some trackdays wheels on ebay.
Cup 1 replicas
7.5 x 17 et 52 with 205/50/17 with worn Pirellis
9 x 17 et 47 with 255/40/17 with worn Bridgestones.
I have fitted the wheels & tyres to the car today & they fit fine.
I am sprinting the car and will be fitting a new set of
1a Scudo Stunner tyres to these rims.
I am going for the same size rears (255/40/17) but fancy a slightly wider front
tyre to perhaps help the car turn in a bit better.
My options for the front tyres are,
215/45/17
or
225/40/17.
Do you have any suggestions as to what would be most suitable size front tyre?
Also
I am doing a test day at Curborough Sprint Circuit tomorrow.
Looking at the forum everyone seems to suggest 36psi all round is the way to go.
If anyone has any different ideas re tyre pressures for sprinting please let me know.

 
Jason,

You should NOT use a 40 series tyre on a 17" rim. The standard tyre on a 17" 7J rim is a 205/50 and the 225/45 is within a whisker of the same overall diameter so will not interfere with any of the electronics. The 225/40/17 is an inch smaller overall and outside the accepted tolerances.

The choice of 215 or 225 is yours although being a C2 it's likely to make little difference. If it were a C4 the 225 makes a big difference to the inherent understeer.

The perceived wisdom on tyre pressures for 17" wheels is 36 psi all round - for road use at least.

Regards

Dave
 
Jason

I did Curborough last year and I run at 36 all road on 17's.
I only got two laps from a standing start so my tyres didn't get up to temp. so I'd prob. stick at 36psi.

Rob
 
I agree with Dave, DO NOT use a 40 tyre. Porsche recommends for street legal 964 Cup Cars 225 / 45 x 17 front. That´s probably the one for your purpose. The front will be more nervous then on (especially bumpy) straights, but that depends on your setup. (springs, camber) The 215 is fine also, tried it on my standard C2 on track (years ago) and it felt good. No problems with the ABS also with the 215.

Regards,

Hacki
 
ORIGINAL: kdean

a 40 tyre is ok on the rears though right? e.g. 250/40/17
Click to expand...

I think Dave only meant to say no to 40 on the front. 40 on the rear is the standard profile.

Can I throw another spanner into the tyre pressure mystery. I'm going to anyway![8D]

My standard 17 inch wheels on my boxster has exactly the same tyre sizes front and rear as those on the 964 (255/40/17 on the rear and 205/50/17 on the front). The recommended (by the manual) boxster pressures are 36 on the rear and 29 on the front. So why 36 for both front and rear for the 964?
 
I'm sorry for the confusion - the comment about 40 profile tyres WAS intended to be related to the front only. The 255/40 on the rear is fine.

The difference between tyre pressures on a Boxster and a 964 is largely explained by the position of the engine. A 911 has the engine hanging aft of the rear wheels with subsequently substantially more weight on the rear wheels than would be the case with a mid-engined Boxster. I'm assuming that the Boxster is, overall, a little lighter than the 964 and that would explain the lower pressure in the front.

Regards

Dave
 
Thanks for your suggestions. I am going to try 225/45/17 fronts & 255/40/17 rears.
I tested at Curborough on Thursday & started with 36psi all round.
I changed to 34psi front & the car felt more stable & my lap times came down.
 
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