# Re: Al Rotors - The definitive answer (Looong)

```>The first thing to look at is the rotor/pad interface.  The rotor/pad's
>job is to overcome the rotation of the tire/wheel/rotor/hub/etc
>(everything that's rotating) period.  The rotor/pad does not directly
>stop linear momentum of the car!  So, Eric, you are correct in saying
>that a lighter rotor would reduce rotational inertia of the rotating
>mass.  But you're dead wrong in saying that the mass difference from an
>aluminum rotor to an iron one will make a significant difference in
>stopping force due to reduced rotational inertia.  This effect will be
>totally insignificant.  Glen is right on the money saying that the wheel
>and tire have a much greater effect on this phenomenon.  The one thing
>Glen left out is the fact that the tire also has a frictional force
>acting on it that wants to continue to rotate the wheel/tire/etc.  This
>force is much greater than the rotational inertia of the rotational mass,
>and much much greater than the rotational inertia of the wimpy little
>rotor!  So where does this frictional force on the tire come from.
>Simple - multiply the normal force on the tire (gee, would that be
>weight?!!! as in MASS times acceleration) times the coefficient of
>friction between the tire contact patch and road surface.  Again,
>integrate the weight of the car over the contact patch - thus if the
>contact patch is bigger, you get more overall normal force.  Also note
>here that the rubber compound of the tire plays a key role in coefficient
>of friction.  So, the heavier the car, the larger the normal force for a
>given Cf and tire patch.  Thus a larger frictional force trying to keep
>the tire/wheel/etc. spinning.

Jeremy,

Nice try but.....We once did a test on the Camel Lights car for Hitco
Ind. A huge supplier of Carbon, Metal Matrix, Al, and Cast Iron brakes
for the racing community.  What we found does not support your views but
supports my views. Not only did the car stop faster (Shorter) with a
lighter rotor (All other aspects normilized) but the car Accelerated
FASTER with the lighter rotors.....  On a race car the size of the lights
car (2300lbs 585hp) the wheel weighs 8lbs and the tire weighs 12lbs for a
total of 20lbs.  a Iron Sephricone Rotor for this car weighs 16lbs (With
Top Hat) 0-100mph came up in (Let me get out my notes...) in 8 Seconds
with the gears that we had in for the test.  With the Carbon Rotors That
weighed 6lbs with Top Hat 0-100mph came up in 7secs......... In the same
distance that the Iron rotors took to reach 100mph the Carbon rotor's
helped the car get to 120mph...... Rotational Inertia plays a HUGE PART
is this equation.  If it did not we would'nt have spent \$3500 Per Race on
a 8lbs Clutch and flywheel.  Ever lighten a cars flywheel?? HUGE
Difference in Wrap up speed.  Rotational Inertia once again.  Do the
math......

Later!

Eric Fletcher
'87 5KCSTQIA2RSR2B