# Re: A6 styling re A4/A8

```STEADIRIC wrote-
>>Sure, less is better, but c'mon man!
>>You tryin' to tell me that the relative rotational mass of the prowler
>>brakes Vs that of the viper brakes accounts for the better braking *more*
>>than does the relative mass of the vehicle and relative amounts of swept
>>area?
>
>Yes I am, do the math.......  Don't know the math, LEARN IT if your going
>to pontificate.  I know it.......  Graydon knows it........ Scott knows
>it.........

I'm gonna pontificate. Where's my mitre?

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.

End of Audience. Get out of my basilica.....  ;-)

>
>Later!
>
>
>Eric Fletcher
>'87 5KCSTQIA2RSR2B
>

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```

• Follow-Ups:
• Al rotors
• From: "Graydon D. Stuckey" <graydon@apollo.gmi.edu>