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Re: Cross drilling rotors

On Dec 21, Erik Frank wrote:

> I'm about to set up my brake rotors in the mill to cross drill them  I have 
> read in _Tune To Win_ about hole patterns, and slotting, etc.   What I'm 
> unsure about, is what size to make the holes.   Can someone who has some 
> rotors that are cross-drilled, or who has drilled some in the past, please 
> tell me what size hole they used?

I'd like to also toss in my two cents on the side of not drilling. You
didn't tell us what flavor of car you're running, or what style of
braking your tracks favor, but let's assume that you're running a
fairly heavy sedan. Drilling does allow you to increase your brake
torque slightly, and avoid fade problems if you run pads that are
prone to outgassing, but it does drastically shorten rotor life if you
see peak rotor temps over 1000degF. Many folks who have run them have
found that the reduction of thermal mass results in significantly
higher peak temps, and speeds up the process of cracking. Even if a
lot of time is spent chamfering the holes on the working faces, the
unfinished inner sides of the holes into the rough sand-cast vent
passages still provide some fine stress raisers, and the rotors crack
prematurely.

For the last couple of seasons, I've been running slotted rotors, and
I've had excellent results, even on my fat car on heavy-braking
tracks. The easiest and best way to do it is with a 3/8" ball end
mill, and just cut shallow chordal slots maybe .030-.040 deep, two
places, from the OD to the inner swept radius and back to the OD. This
seems to do every bit as well as a drilled rotor in terms of brake
torque and fade resistance, but it takes far less than a tenth of the
time to do.  And the smooth circular form of the groove presents
little in the way of additional stress raisers, resulting in much more
satisfactory rotor life. The slot also provides a nice rotor wear
indicator. When it's gone, so are the rotors!

Your mileage may vary, but unless you have a sizable budget, and a lot
of spare time to spend drilling and chamfering holes on rotors that
you'll throw away fairly soon, slotting seems to have the advantage.

- -skod

- --
Scott Griffith, Sun Microsystems Lumpyware
expatriate SCCA New England Region Flagging/Communications worker
(and driver, of anything that turns both right and left,
and can pass tech...) Return Path : [email protected]