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# Al rotors

On Mon, 12 Feb 1996, Robert S. Cohen 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.....  ;-)

Robert, Glen, Eric, at al,

I never meant this to be a big heated argument.  I was merely
stating what I heard from Team Prowler.  I didn't do the test myself, nor
was I in attendance when the test was done, so take it with a grain of
salt, not a Congressional seal of approval.

My point was that the ALuminum rotors reduce the rotating inertia
of the wheel/rotor assemblies, and that has a profound influence on the
braking of the car.  How much is not open to discussion really unless you
have real numbers to back you up.  I don't, so I haven't really argued
it.  I would also point out that the rotors are NOT made of simple
aluminum.  They are a pretty exotic alloy (possibly even a composite)
that takes this kind of abuse very well.

I'd love to run the numbers, but being a lazy engineer, I don't
have all the formulas memorized, and I would have to go back to my texts,
and look up the formulas.

Unfortunately I don't have time for that today.  I have a sick
Quattro sulking in the garage, and a little boy that wants to play :)

Later,
Graydon D. Stuckey
graydon@apollo.gmi.edu
Flint, Michigan   USA
'86 Audi 5000 CS Turbo Quattro, GDS Racing Stage II
'85 Mazda RX7 GS 12A-leaning-towards-a-13B-soon