Re: drilled rotors and cooling

[James Montebello <jamesm@domain.elided>]

On Sun, 20 Oct 2002, Richard Welty wrote:
> in this case, drilling or slotting can help. it has NOTHING to do with
> overheating. NOTHING AT ALL. this kind of braking failure feels like you're
> pressing hard, and nothing is happening.
>
> heat related failures are generally fluid becoming compressible inside the
> caliper body. they have a completely different symptom; when you depress
> the brake pedal, it feels mushy and ineffective.
>
> do _not_ confuse heat related failures with failures caused by material
> buildup, they have different causes and different cures.

If course, the issue can become very complicated if you have both failures
happening at once.

The coefficient of friction of any material changes with temperature.
For iron rotors and most pad materials, it will climb with temperature
up to a point, then fall again.  The only commonly used brake materials
that don't have this property are carbon-carbon brakes, which work better
the hotter they get.  Iron rotors worked hard have to be kept below their
temperature limit or it will begin to feel like your brakes are greased.

One other point, if you're having fade problems, REALLY REALLY bleed
your brakes.  Any air in the system will exacerbate heat-related fade,
since the air itself will expand significantly as it gets hotter, and
make a system that feels fine when cool work dangerously poorly when hot.
This goes triple for the rear brakes on transaxle cars, which not only
have serious heat problems caused by their location, but are difficult
to bleed properly, too.

james montebello
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