Ess_Three said:
Sorry...but your retort is equally poop.
It's the terms 'bigger' and 'smaller' that can cause confusion.
Bigger wheels do not mean 'smaller' tyres as the tyre's width is the commonly accepted measurement of size 205, 225, 295 etc.
Bigger wheels mean smaller sidewalls as you were intimating...but not always smaller (width) tyres...
Which means the original poster was correct in their comment that larger wheels generally means larger (wider) tyres...for example going from a 195/55 tyre on a 6" wide 15" wheel up to 18" wheels will mean going up to a 7.5" or 8.0" wheel as you don't get too many 6.5" wide 18" wheels...so you'll need to fit a 215 or 225 tyre to keep things safe.
Hence, bigger tyres...width wise anyway.
I'll let you have that one. The assumption on bigger diameter was wrong.
Ess_Three said:
Again, point out that whilst you ar correct in what you say, the original post referred to bigger (the assumption being 'wider') tyres giving more traction, which they do.
OK...in the wet wide tyres can be worse depending on pressures and comploud, but in general terms, wider tyres = better traction and more grip.
wider tyres cannot provide more grip for the same weight car. The improvement in cornering ability is because you have a bigger wheel which means you have a smaller sidewall. the wider tyre is just an aesthetic bonus and in the long run does provide vehicle stability.
Ess_Three said:
All true...
Except larger (wider) wheels will cause more drag (aerodynamic drag, bigger frontal area) and more frictional drag (more rubber in contact with the road), both of which WILL affect the top speed in a negative way.
aerodynamic drag on a tyre - negligible, but ok if you're being picky. I think the pretty much flat front of an audi would negate this anyway.
frictional drag - no more than on the narrower tyre. This comes back to the F=MA yes there is more rubber in contact with the road, but the force exerted by any part of it is smaller. For the same rubber compound it would be the same.
So to answer the original question, yes the ride will be harsher - but not too harsh on 18's, you'd need to go to 19's to suffer.
The handling improvement would depend on the width of tyre/profile you ellected for:
225/40R18 has a sidewall of 90mm
215/45R17 has a sidewall of 96mm
215/40R18 has a sidewall of 86mm (and gives a nice euro-stretched look).
however the smaller sidewall handling differences can easily be cancelled out by poor choice of tyres, worn suspension etc. Or alternatively more benefit could be gained from uprated suspension and great tyre choice.
At the end of the day on a road car that's driven every day it's about looks over tiny performance details, as the minute performance advantages you'd pick up from some tweaks will not be noticeable in every day driving.