Nice one Ian. Exactly what I wanted to read about Now...where was I? 17s or 18s???

 

How about taking a tape measure and measuring from the ground to the centre of the wheel. As finding the centre can be a bit difficult, measure to the top and to the bottom of the 'H' logo plug thing, add the two readings and divide by 2. This will give you your actual rolling radius for the car at rest, alliowing an unemotional comparison of with and without added bling.

 

Hmmm, as you drive the tyres warm up, as does the inside them. Hot air will expand, increasing radius. Does this mean that the 18" wheels with a lower profile will suffer less radial expansion in real terms...

 

If you'd cared to work out the volume of air inside the tyre and the amount of heat generated in the tyre via hysteresis (of course dependent on the compound of the tyre) then you could probably establish whether a wider tyre with a lower profile expands more or less proportionately than a narrower tyre with higher profile.

If.

 

But you need to take in to account the steel bands that run around the circumference of the tyre to supress this type of effect, of course.

 

You also need to consider tyre expansion due to the centripetal force of the spinning wheel.

 

... and then there's tyre growth at speed - much more important than you'd think, different carcass construction etc. etc. etc.

All tyres come with a 'rollling radius' or circumference figure (or sometimes revs per mile) - you want to try and match these with the standard tyres.

As for rolling radius and track times - changing the rolling radius is just changing the final drive. Longer ratios = less accelaration, BUT only if you are in the same gear. If longer gearing lets you hold the car near peak power when others are changing up - you will be faster. No-one wins Le Mans on sprint gearing

By-the-by (and please don't trust my maths on this - my degree was long ago) I worked out that changing to Spoon light weight 16" rims was equivalent to +30 rear wheel hp - If it's right they are beginning to look quite cheap!!

 

... arrgghhh what Richieh said - must type faster

 

Hmmm, on that basis JamieS2K and Co are driving the equivalent of a Clio 182 engined S2000

 

My big fear is I've got the decimal place wrong - it would be a pi$$er to spend