Exploding Rotors: why you do not drill

[AudiBiTurbo at aol.com]

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In a message dated 6/20/2003 9:12:00 PM Eastern Daylight Time,
brett at cloud9.net writes:

> ...which is yet another reason why cross-drilled rotors, which are
> known for cracking when overheated and have less thermal mass, had no
> business on that car.
>
> Brett

I think that's rather strong, but with hindsight, unarguable.  I've worked on
older (1987 era) 911 Turbos with factory cast "crossdrilled" rotors which
failed in the same way.  I've also personally handled Porsche 996, 993, and GT2
rotors all developing stress cracks.  What causes them?
Most people immediately jump to "bad manufacturing" or uneven casting.  In
most cases this isn't the reason IMHO.  You see too many cryogenically treated
rotors and ads for this to be an uncommon occurrence.
It happens because there is tremendous friction induced heat in a rotor and
every feature (holes, slots, etc.) are a stress riser for the base material
(the metal composite).  Add on top of this age (lots of heat cycles) and possibly
rain, and you can easily get a weak, crystalline grain structure which is
prone to fracture.  Invariably with tracked cars the cracks will be visible to
the naked eye before catastrophic failure.
Brakes are the number 3 wear item on the car, behind gas and oil.  Many
racers or driving enthusiasts replace their rotors as a matter of maintenance.  In
fact, similar failures happen with wheel lug bolts and most drivers don't give
it a second thought to still using original lug bolts forever.  This doesn't
mean that the lug bolts or rotors are bad, it just means that the car owner
needs to replace these parts as a matter of ongoing maintenance.
Personally, I recommend gas slotted rotors to my customers.  The slots are
only on the surface (and therefore if cracks develop, they won't be "blind") and
do a better job at wiping the pads and expelling gasses.  In fact, look at
CART and F1.  I never see "drilled" rotors there!  On a street driven car,
"drilled" rotors can be better because they are effective at wiping and tend to get
hot faster due to less thermal mass.  Brakes work best with some level of
heat in them (which is dependent on compounds, etc.).
Lastly, don't overdrive the car.  Stabbing at the brakes, abruptly getting on
them (late braking) and/or abruptly getting off of them can seriously
thermally stress the system.  Being smooth means every driver action is smooth, such
as squeezing on and off of the brakes.  This allows for more even heat build
up, and much less localized stress on the pads, calipers, and rotors.  At Lime
Rock Park I was fortunate enough to drive a student's stock (except
non-runflat, but street tires) Mini Cooper S.  In the run group session with mostly solo
students, I lapped every car, including Porsches, S4s, and BMW race cars...
in fact, most cars I lapped twice!  OK, not every car... a 2001 996 Twin Turbo
did pass!  I was very smooth, and often starting braking around 600 to 700
feet from Big Bend.  Hardly braking deep!  I was gentle on the equipment (except
tires) and was duly rewarded.  Had I been late braking, I would've either
faded the pads, warped rotors, or broken something.  Smooth pays.
In this case, I won't speculate what happened.  It's probably a combination
of a lot of things.  Suffice it to say we should all think harder about
thermally cycling metal, doing closer inspections of our critical systems, and be
more inclined to replace thermally stressed parts with new on a more frequent
basis.

It sounds like we're generally all in agreement but I wanted to get the focus
back to us all paying more attention to our cool mistresses... um... I
mean... "cars!"

More miles and smiles to all,
Mark Rosenkrantz
AudiBiTurbo at aol.com