better rear brakes

[Bernard Littau]

Hi Richard,

> Motorcycles come with cross drilled rotors primarily for weight reduction
> -
> on these vehicles, there is not a lot of mass to slow so heat dissispation
> is somewhat less of a concern.  However, by lowering the rotational
> intertia
> at each wheel, the bike accelerates and stops more quickly.

What you say is true, but the reason rotors on a motorcycle are made light
is not for acceleration and deceleration -- the effect is minimal.  A bike
needs to lean from one side to the other as it corners.  The rotational
inertia of the whole wheel system wants to prevent this change in angle due
to gyroscopic effects.  Reducing mass is one way of reducing this gyroscopic
effect, and makes the bike much more nimble.

> Also note that motorcycle rotors are typically very thin, low mass, and
> will
> accumulate and dissipate heat at a much quicker rate than automotive
> rotors.

Actually, being thin and having low mass has little to do with the
accumulation and shedding of heat.  The larger the surface area, the better
the head shedding.  The amount of heat put into a rotor is pretty much
independent of the rotor -- it stems from the energy absorbed by the brakes,
which is primarily related to the mass of the vehicle and its change in
speed.

Having less mass means the rotor will gain more temperature for a given
amount of heat.

> I find this an interesting comparison.
> If drilled rotors are good for motorcycles, then why not use them on cars?
> If vented rotors are good for cars, then why not use them on motorcycles?

Everything is a compromise.  Vented rotors will shed heat better, but they
are heavier.

Best,

Bernard Littau
Woodinville, WA
'88 5ktq
'94 K11RS (cross drilled, low mass, floating rotors)