wheel size and performance

[Daveotto]

Engineering logic tells me that if I have the same engine driving different sized wheels (not on the same car!!!) the performance will change. Things like 0 - 60, acceleration from 50 - 70 etc. etc. but I do not see different 0 - 60 values on s line compared to sport when one runs on 18"'s and the other on 17"'s. or is the engine mapping slightly different to accomodate

Can anyone explain this? or provide any data on difference in performance when changing wheel size. Would be nice to think that under same settings a s-line is marginally quicker than a sport version. Or would I get better perforrmance if I changed my s line to 17"'s post delivery.

Anyone got any insight on this.

Dave

 

[pjr]

The rolling distances of the standard wheels, whether 16", 17" or 18" are all within a few mm of each other as the bigger wheels have lower profile tyres. While there are probably differences in the weight distribution between the wheel & tyre for the different sizes, it is probably small enough to not have an effect on acceleration etc.

if you we're to fit wheels that had a smaller circumference e.g. 225/40 x 17 or 225/35 x 17 rather than the normal 225/45 x 17 then (I think) the acceleration would be slightly better, but also your speedometer would need recalibrated as the rolling distance would also have changed.

 

[Sootchucker]

Plus the 18" alloys of the S-Line are machined with a new process called flow forming, (see extract below) meaning the inertia increase usually associated with larger alloys is reduced.

"The optional 18-inch alloy wheels are no heavier than the 17-inch wheels; they are produced by a complex flow-forming technology. In this process, the rim well is rolled out over a cylinder under high pressure and at high temperature. The system shapes the wheel blank in a single work step, during which the material also becomes stronger. This permits thinner wall thickness's so that the wheel is both lighter and stronger."

 

[Daveotto]

The rolling distances of the standard wheels, whether 16", 17" or 18" are all within a few mm of each other as the bigger wheels have lower profile tyres. While there are probably differences in the weight distribution between the wheel & tyre for the different sizes, it is probably small enough to not have an effect on acceleration etc.

if you we're to fit wheels that had a smaller circumference e.g. 225/40 x 17 or 225/35 x 17 rather than the normal 225/45 x 17 then (I think) the acceleration would be slightly better, but also your speedometer would need recalibrated as the rolling distance would also have changed.

Thats great...just done the maths and i reckon rolling distances are only about 15mm difference over the 16" - 18" rim range. That only leaves differences in contact area and how that affects power lay down and friction/resistance over the contact area (but my maths is not good enough to work that one out) and weight of wheels which (according to Sootchucker) is less for 18"'s so some advantage there.

I am sure some smart group of folk at Audi know the answer but sounds like a good project for an engineering degree project to work out theortical differences.