There is lot of myth in these posts. First of all, BMW does not develop M cars anymore for track use. Well they advertise this as a capable track car, but they itself have admitted that most of the M cars will never see the track. Just go through the posts and you see that most of them use them as daily drivers (why on earth I dont understand)
Additionally, they have strict EU rules.
I watched the BMW video in the first post with irony. The reason why the went with wider tyres is the weight of the car. Its much heavier than the prev gen and to offer similar performance, they needed much more grip. This is the only reason. Yes sure, different chassis, different suspension etc... but when we get more abstract or look at things higher level, the weight of the car is the issue.
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Originally Posted by MineralGreyMetallic
Will the 20" wheel with smaller/firmer tire sidewall help alleviate or exacerbate understeer, or too hard to know?
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You alleviate understeer with proper suspension and camber. 305R19 or 305R21 wont make much difference. R21 has considerable amount of more mass so on track it will perform worse than R19. But sure, lower sidewall equals stiffer tyre. You will a bit better lap time but imo this is just compensation of poor suspension.
If you are facing understeer on track and are already going relatively fast then you already are at level where you need coilovers and camber plates. G87 original suspension setup is crap (for track)
For my track setup Im going with R19 wheels and 305 tyre (both front and back). It seems the best setup. Best mass ratio, wide range of semis and slicks, coilovers allow to lower the car meaning the center of gravity will be also much lower than on 21 inch wheel. Add front aero details and you have wayyyyy more downforce at front.