Date: Thu Dec 12 05:52:38 1991   
To: z-car@dixie.com
Subject: Re: Brake Diagnosis Problem

The problem you are describing sounds like a rotor problem to me.
Even though your shop told you the thickness is ok, I would still
tend to put new rotors in myself. Why not? You have spent a fait amount
of time and money to renew the brakes in your Z. Spend a little more
and you won't have to worry about the brakes for a good long time.
If you change pads often, your rotors will last a long time. (Pads
are cheap, rotors aren't).

You could have a bad set of pads, but I think the rotors are more
suspect.

John

From: emory!Eng.Sun.COM!Scott.Griffith (Scott Griffith, Sun Microsystems Lumpyware)
X-Source: The Hotrod Mailing list
Date: Apr 1993
Subject: Re: brake rotors (long)
X-Sequence: 5138

On Apr 26, Greg Granville wrote:

> What are disc brake rotors typically made of?
> I've heard that one of the primary causes of rotor warpage at high temps
> is due to the fact that the original castings may not have been properly
> "stress relieved". Can someone describe what this process involves, and
> is it possible to have this process re-applied to a new set of rotors
> prior to installing them on a vehicle that is going to see hard brake
> usage?
> Also, what is the wisdom on milling gas sweeping slots into the surface
> of the rotor. What about drilling holes in them? Will either of these
> pratices cause more problems than they are intended to solve?

Brake rotors are usually your basic gray crummy cast iron. Rotors for
production applications are almost always sand cast, and do have some
built in stresses from uneven cooling- the automakers really are
primarily focussed on cost, so the quality of these guys is pretty
marginal. They are usually overweight to compensate. They have sand
inclusions, bubbles, prosities, and they are usually cast in one piece
with the hub- which leads to some nasty stresses whrn the OD is
incandescent and the hub is cold. It also leads to cooked wheel
bearings, pronto. They almost always exibit quite a lot of core shift,
which leads not only to balance problems but also to premature warpage
(due to one working surface heating more quickly than the other). This
is worsened in brake designs where the pads are asymmetrical, like my
Mustang- the driven pad is smaller than the slave pad, so the heat
loads differ on the two faces of the rotor.

The aftermarket rotors available for racing applications (Coleman,
Brembo, and other vendors) are usually a better class of materials-
some of the ASTM high-temperature cast irons are the norm. The best of
these are usually vacuum degassed to eliminate porosities that can
lead to cracking. These are also designed to run on a separate hub
adapter (the hat), which eliminates one major source of stresses and
also cuts the heat transfer to the hub and wheel bearings.

Warpage is usually caused by differential heating, and it's not clear
to me that any sort of stress relief can be made to work for any long
period of time. I have warped and cracked more stock Detroit rotors on
my track car than I care to think about, and I can't think of any
mechanical or thermal stress relief technique that might help them. I
routinely used to run the rotors well up into the bright orange
incandescent range- any rotor, no matter the quality of the original
casting, will begin to behave pretty erratically at these temps!

The best way to keep rotors alive is to keep them cool. _Massive_
quantities of air ducted right into the rotor eye, and kept there with
a blanking plate that more or less fills the eye, is the ideal. The
vanes will pump the air through the rotor and help with the airflow.
Wheels that help exhaust the hot air outboard help as well- my best
results on the stock rotors have been with the old Kelsey-Hayes
3-blade "blower" wheels- they were ugly as sin, but they moved a lot
of air out of the wheel wells. Keeping the rotors cool will also pay
amazing benefits in pad life. My finding has been that pad wear, even
with carbon race pads, goes up exponentially once rotor temps reach
about 800degF.

The next best way to keep rotors alive is to heat-cycle them very
gently. Take a lap to heat them up to working temps, and use the
cooloff lap to cool them gently. If you're on the street, and intend
on some heavy driving, the same rule applies- heat them gradually. An
absolutely guaranteed rotor killer is to do a max-effort high speed
threshold braking maneuver on cold rotors. Cast iron's thermal
conductivity is really pretty poor- so the surface will go up to
_insane_ temps, with the vanes between the faces still dead cold. The
thermal stresses are immense, and the stock low-quality casting won't
take it. Surface heat checking is the best you can hope for! I refer
to this kind of stop as "the rotor giving its life to save yours".

The last thing to help keep rotors alive is to work on your braking
technique. As you roll onto the brakes entering a corner, come on them
progressively and smoothly, and _slowly_ increase the pressure until
you get to the threshold. Luckily, this is also the best way to keep
the car balanced entering a corner, so it's a useful exercise in that
respect as well. If your braking style is one of planting th car's
nose _all at once_ when you get to the braking zone, you might want to
try working on braking a little easier, a little earlier at the start.
You'll find that you can then brake harder later in the corner, and
the wight will transfer more gently. 

It's a subtle thing, but what you're trying to do is to spread the
heat load in the brakes out over a little bit longer time- and more
improtantly, reduce its peak value. For the massive, 11/10s, fully
committed, hairy outbraking move, then do what you've gotta. But if
you aren't under pressure to pass or be passed, work on being gentle
and progressive with the pedal.

Drilled and milled rotors are great for racing, if done right. I'm
running nothing but carbon pads these days, on a very heavy car for
road racing, so playing with the rotors is not good for me. Drilling
can aid in cooling, and aids in fade control by providing a place for
the high temperature gases to escape to, but can lead to warpage and
cracking much earlier if the car is seriously underbraked. The holes
need to be drilled and chamfered in such a way as to avoid creating
new stress raisers, and I have never figured out how to do that very
well.  Milling consists of cutting a shallow groove in the rotor face,
also to provide a gas escape- I like this better, since it creates
less in the way of a stress raiser if a radiused groove is used. These
days, I just slot the _pads_ for fade control, and leave the rotor
alone. IMHO, for an underbraked car on stock rotors, drilling or
milling is a waste-you'll be throwing them away very soon, and the
removal of metal is almost certain to make that sooner.

Before I did good brake ducts, I used to go through a set of carbon
pads a day and a set of rotors in a weekend on my nice overweight
Mustang GT track car. With ducting, I get 4 days out of pads, and
about 4 out of rotors (using the stock rotors). I've since gone to
custom Coleman directionally vented rotors on custom hats, and I don't
have enough data yet to predict long term life, but it looks like I
might be able to keep pads on the car about 4 days, and the rotors
might last a season.  Which is the next best thing to _immortal_, to
me. I've got a pile of rotors that are cracked so badly you can see
light through 'em in the garage...

If you are going to run a stock 1-piece rotor hard, you _must_ use a
good high temp synthetic grease (Mobil One or Redline) on the wheel
bearings, and keep after them. Any stock grease will melt out and
blow by your grease retainer, leaving you with dry bearings. Been
there, done that, back when I first started playing with the car on
the track.

Hope this helps!

-skod

--
Scott Griffith, Sun Microsystems Lumpyware
expatriate SCCA New England Region Flagging/Communications worker
(and sometimes driver, of anything that turns both right and left,
and can pass tech...) Return Path : skod@sun.COM