Wheels and tyres.

**DakDak** · 21-09-2012 02:14 PM

Hi all,

Question,

Say you have two rims. One 17x8.5+30 and the other 17x9+35
One both these rims you have the same tyre 245/40/r17

Would one perform better than the other? As in more grip, therefore handling better?

The 8.5 would balloon out and have more tyre surface to road?
I see drag cars run massive tyres and small rims. This is for traction, does it only apply to the straight and not the twisties?

I hear stiffer tyre walls are necessary for hard/tricky corners slaloms, but wouldnt high end tyres have stiff sidewalls anyway? Plus the side wall is 40mm isnt that considered low enough already? I know you can go lower like 30 but I'm just asking about 245/40

Im also aware of the basics of unsprung weight. So wouldnt it be better going the 17x8.5+30, as its weight is lighter, however the slight the difference maybe?

**chargeR** · 21-09-2012 03:17 PM

A tyre will generally perform better under lateral loads on a wider wheel, within certain bounds. Emilio from 949racing has shown this with his testing of wider and wider wheels on the same tyre. Which is why limiting wheel size, not tyre size, is a common parity measure in many race series. Tyres are very complex though, and even those whose profession it is to produce these things do not have a full quantitative understanding of them, and resort to engineering judgement and intuition.

Personally I went from a 275/40 to a 245/40 tyre on a 17x9.5" wheel and found that the smaller tyre did seem to have less ultimate lateral grip, but was much more responsive and ultimately faster. So I will always err on the side of the wider wheel for a given tyre. I also made a change from a 9.5' to a 10" front wheel and thought that the increase in grip was perceptible. An extra 0.5 or 1" of wheel width doesn't add very much unsprung weight at all either, and in the balance of wheel/tyre size and unsprung weight I'd say that in most situations it is worth compromising on mass for the tyre and wheel size gain.