Quote:
Originally Posted by CSBM5
Notice they don't explain the details on rotational mass in the BMW link. For an engineer, it leaps right out at you when you see all the discussions on line -- a very important variable in rotational mass discussions is the radius of gyration. The radius of gyration is the radius where the rotational mass can be considered located in the basic moment of inertia calculation (i.e. I = m k^2, where m is mass and k is the radius of gyration).
Note that the moment of inertia is a function of the radius of gyration to the 2nd power; hence k has a large effect on rotational inertia as it changes.
In the case here a quick estimate of k for the front brake rotor is about 150mm or 5.9". If you wanted to compare mass located at that k versus say the same mass located at roughly the wheel k, it's k1^2/k2^2. If we consider a 19" wheel to have a radius of gyration located at about 8.5", then popping in these approximate numbers yields 0.48; hence the rotor weight is only 48% as important as the wheel weight in this approximate example.
Put another way, wheel weight contributes 2.1 times more to rotational inertia than disc weight.
This is all back-of-the-envelope quick estimates. However, the effect of radius of gyration is very important in moment of inertia calculations and shouldn't be left out of information any source is providing on "rotating mass" changes.
It's somewhat analogous to the way aero drag coefficients are bandied about by car makers, etc. Comparing two different cars, say one with a Cd of 0.30 and the other a Cd of 0.33 is perhaps "interesting" from an aero design perspective, but without the specified frontal area of the two cars, it tells you absolutely nothing about the aerodynamic drag comparison of the two as the car with the lower Cd could actually have a similar or larger aerodynamic drag...you just don't know without specification of the frontal area.
Sorry...pet peeves of two commonly misunderstood (marketing exploited too) things in the car world.
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Thank you for a great breakdown