# Re: A6 styling re A4/A8

```>AHEM, Eric, the rest of us don't _ know_ it!
>
>It is not manifestly obvious to me how this can be the case. Perhaps you
>could share the math with the rest of us....?
>
>Before you accuse me of being lazy, I've briefly discussed the problem with
>a physicist here. the upshot of our admittedly naive investigation was that
>aluminum rotors would probably make for somewhat diminished stopping due to
>the specific heat of the (light) aluminum rotor vs the (heavy) iron rotor.
>Decelerating at the same rate, the aluminum rotor will get 40% hotter.
>
>                                Al              Fe
>Density g/cm^3          2.7            7.86
>Sp. Heat J/(g*degK)     .9              .44
>
>with these numbers, I calculate that for equal volumes (we simply swap
>aluminum for iron) the iron rotor will be 2.9 times more massive. for equal
>masses, the iron rotor will heat 1 degK for each .48 degK the aluminum
>rotor heats. 2.9*.48= 1.392.  So, a rotor of Al equal in volume to the
>original Fe rotor will be increase in temperature 40% more than the iron
>rotor for a given deceleration (energy input). There. I did the math.
>
>regarding the kinetic energy of the rotors themselves, which I think is the
>factor Eric is alluding to, it is  (m w^2 r^2) I think, where m is the mass
>of the ring, w is the angular velocity, and r is the radius of the
>idealized ring. Frankly, I (waving my hands) think this will pale in
>comparison the energy of a 3000 lb car travelling at 100 kmph or so.

Yes but for the same heat rejection as an Iron rotor an aluminum rotor
weighs 31.5% less....  A metal matrix rotor 40% lighter and a carbon
rotor 70% lighter..... There are books on the subject.......

Later!

Eric Fletcher
'87 5KCSTQIA2RSR2B