Quote:
Originally Posted by M_Power Rob
The stock CAST rims are heavy (the boat anchor comment), weak (due to being cast) and not recommended for any true performance use.
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They are the same load rating as the forged wheels (this is embossed in the back of one of the spokes, 815kg for all the BMW M2/3/4 wheels AFAIK) the weight is the main difference, but even so that will be more obvious in suspension bump absorption than measurable acceleration difference when comparing 825M and 930M wheels as follows.
Front 930M - 12kg, 925M 10.5kg. Rear 930M - 14kg, 925M 11.5kg
Front MP4S tyre - 11.8kg, rear MP4S tyre - 12.3kg
The tyres have an effective mass centre from a rotational inertia perspective at about 0.33m radius. The wheel barrel has an effective radius of about 0.24m for the front wheel and 0.25m for the rear wheel. The effective radius of the wheel faces is about 0.15m. The wheel barrel is about 60% of the wheel mass. This gives rotational inertias as follows:
Front 930M - 12 x 0.6 x 0.24^2 + 12 x 0.4 x 0.15^2 + 11.8 x 0.33^2 = 1.81 kgm^2.
Front 825M - 10.5 x 0.6 x 0.24^2 + 10.5 x 0.4 x 0.15^2 + 11.8 x 0.33^2 = 1.74 kgm^2 (4% lower than 930M).
Rear 930M - 14 x 0.6 x 0.25^2 + 14 x 0.4 x 0.15^2 + 12.3 x 0.33^2 = 1.99 kgm^2.
Rear 825M - 11.5 x 0.6 x 0.25^2 + 11.5 x 0.4 x 0.15^2 + 12.3 x 0.33^2 = 1.87 kgm^2 (6% lower than 930M).