Quote:
Originally Posted by Boss330
@CanAutM3 has already explained that point in a previous post on this thread.
There are quite a few misconceptions/misunderstandings in this thread. A few posters seem to confuse rim diameter and tyre diameter.
Porsche has a staggered setup on the 992 where both rim and tyre diameter is increased on the rear compared to the front. That obviously gives a larger contact area, not due to the increased rim diameter but due to the increased tyre diameter. Hence Porsche's reasoning of increased traction/grip on the rear with this setup.
BMW has, as CanAutM3 has pointed out multiple times, not done this. They have retained a tyre diameter of less than 1% difference between f/r, and only increased rear rim diameter. So no gain in tyre diameter or contact area from a 1" larger rear rim diameter in BMW's case.
Better turn in on the front axle and more stable rear due to a lower sidewall tyre. Best of both worlds!
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I know this is a very old thread I'm bumping... but this is what was screaming out to me the whole time I read this thread. Thank you for highlighting this important point.
Porsche and GM on the Corvette do this 19/20 or 20/21 thing but their rear wheel + tire is a MUCH larger overall diameter. They use the 21" rear to bring the sidewall height down to visually match the front, so it doesn't look like the rear has an incredibly fat sidewall. Bonus is probably also tire availability. The 992 C2S has 245-35-20 on the front and 305-30-21 on the rear. You can see without even calculating that there is a large difference in overall diameter, but they have a visually similar sidewall when you look at pics of the car.
BMW has chosen to maintain a near-identical overall diameter. I understand why the front is 19". I don't necessarily understand why the rear is 20" other than looks and marketing comparison to MB and Audi. I see zero benefit for a lower profile rear tire with identical overall diameter. It would be slightly less compliant and I'm pretty sure that's not an improvement for the rear.
So, the people commenting that Porsche does it and giving Porsche reasoning are missing the essential fact that Porsche and GM increase overall diameter by a large amount. This rationale does not hold up for BMWs matched diameters.