Hi,

Would replacing the 205/45/17 tyres on a P1 with 215/40/17 be detrimental to handling?

And - Is sidewall flex purely a negative thing?

Probably a question for Simon de Banke.

NBS, the change you're suggesting is very common - mainly because of more tyre choice in 215/40x17. Bridgestone S02pp's are still widely regarded as the best (if you can still get them). I went from PZero 205/45x17 to SO2pp's in 215/40x17 and the difference in wet grip is quite extraordinary. Toyo Proxis and Goodyear F1's are also rated.

Sideways de Bonkers is definately the man for this sort of question but the difference between these two sizes is really very slight.

Why not ask him about the size of the contact patch between your regular road rubber and the space-saver spare? That question had most people scratching their heads and was a real eye-opener http://bbs.scoobynet.co.uk/wink.gif

Richard.

You are allowed a bit of "leeway" when fitting tyres to wheels, they don't have to be exactly the same size. Going from a 205/45/17 to a 215/40/17 won't be a problem.

Sidewall flex is reduced with lower-profile tyres. The upside is that the tyre becomes a stiffer spring, so reacts quicker to direction changes which may correspond to an improvement in handling. The downside is a stiffer spring equals a harder ride.

On smooth surfaces, lower profile is better, on rough surfaces, a higher profile may be better.

Excellent advice from David and also from hoppy.

IMHO you should stick to the tyre size that the wheel is designed for though. As this is also the wheel size that the tyre is designed for.

You will have more tyre wall flex by fitting a wider tyre to the same wheel (all other things being equal), but the main problem is not the amount of flex but the way it flexes as the natural pivot point of the imaginary join between the sidewall and the tread band is in a different place in relation to the rim seats.

This means that the sidewalls are slaying outwards more than they would normally be causing all sorts of strange things to go on.

In fairness.. it probably won't make a huge difference on the road, but I would personally always recomend against it as the tyre just cannot work in the same way as it was intended.

Hoppy.. don't start off the contact patch thing agian!!! http://bbs.scoobynet.co.uk/biggrin.gif

Cheers

Simon

What about MY01. Standard fitment is 215/45 x 17, whereas the STi has a 225/45 x 17.

I wondered whether it would be worth considering the wider size when I need to replace.

Any thoughts?

Kev S

Thanks for the advice.

This prompts the further question of rim widths.

Are UK300 rims (215/40/17) the same as the PI (205/45/17) and what width are STi (225/45/17)?

Am I right in thinking that the aspect ratio of 215/40/17 and 205/45/17 are very similar and therefore the sidewall height will be also be very similar and therefore the spring effect will also be similar?

NBS

You will get more road noise from a lower profile tyre, and a stiffer ride.

DavidRB got it exactly, on rutty sufaces you want sidewall flex so the tyres don't skip over the rutts.