From: Dave Shaeffer [shaeff_d@bellsouth.net]
Sent: Tuesday, October 23, 2001 12:13 PM
To: 928
Subject: [928] Re: turning rotors

|what's the consensus on turning brand new xdrilled rotors.
|
|Ive heard ALL rotors  from the manufacture/shop have some "warpage" to them
|from sitting around etc etc...and that they would benefit from being
|turned.....fact or urban myth?
|
|Also, after turning (if required)...or on brand new rotors is it a good
|idea to wash them down with hot soap-n- water to remove machining debris?  Or
|can i assume this has been done.
|
|I figure some measures now may prevent some squeaks and bad wear later?
|
|or...should i just "bolt'm up"?
|
|
|TIA
|
|Tony
|1987 S4 Auto 83k
|281-hp 291-ftlbs
|http://members.rennlist.com/v1uhoh
|Las Vegas


Tony, (and all others with "holed" or "slotted" rotors) ....

First, unless the rotors were stored in a vertical position (or in a
horizontal position on a very uneven surface) with a great deal of weight on
them, or subjected to very extreme temperature changes, they shouldn't be
warped. However, some rotors do come from the factory warped and/or out of
round. This is nothing more than a QC failure.

Second, any holed (whether drilled or cast) or slotted rotor cannot be
turned. They must be ground. Without going into too much machinist speak,
this is a function of what is called a interrupted cut. When turning a piece
on a lathe, any interruption of contact between the cutting tool and the
work piece will result in a very rough and uneven surface. Most machine
shops charge about $50.00 _per side_  (that's $100.00 per rotor) to grind
the rotors. For holed rotors, there is an additional problem. All the holes
must be counter sunk (chamfered) and, if grinding process removes enough
material that the counter sinks/chamfers are removed (or under spec) they
must be re-chamfered with will cost an additional amount. Though I won't
swear to it, I believe the same to be true for slotted rotors as well. An
additional problem for drilled hole rotors is that they have a tendency to
develop tiny fissures/striations between the holes, which can (and does, on
occasion) result in the rotor fragmenting.

Bottom line ... unless your car sees extremely harsh track use on a regular
basis, save your money and use solid rotors. So, why do Porsche and other
manufacturers use these types of rotors on cars that (for the most part)
never see a race track, or conditions anywhere near as harsh ??? Can you say
MARKETING ??? After all, they won't be paying to have the damn things
Blanchard ground, you will. I _do_ speak from experience here. I had a set
of x-drilled rotors on my '86 Carrera (courtesy of the PO), when it cam time
to have them ground, I replaced them with solid rotors.
The difference ??? Nada, zip, zero, null ... even under very spirited
driving conditions. Were I to frequently use the car on the track, perhaps
I'd have one set of holed/slotted rotors and race pads for the track, and a
set of solid rotors and street pads for the day to day driving. That would
be the best solution, as both sets would last much longer. For anyone that
drives mostly on the street, or only occasionally on the track, I'd stick
with solid rotors, and just use two different sets of pads. One for the
daily chores, and one for the track days. That's my best advice. Hope it's
of some help, and money savings.

If you're truly concerned about the "true-ness" of the rotors..... First,
use a micrometer (NOT a caliper) and measure the thickness on the rotors at
three to four locations (120^ - 90^) around the rotor, and at 2-3 different
distances from the edge, or the "hat". This will give you a good idea of
whether the rotors are parallel, or flat. Next, set up a dial indicator, and
place it on the face of the rotor, rotate the rotor and note any variations.
Do this 2-3 times starting close to the "hat", and working your way out to
the edge. This will tell you if the rotor has excessive "run-out". These
procedures are described in the workshop manuals I believe. I don't have the
specs committed to memory, but they are also given in the manuals. If you
have any questions regarding these procedures and/or meaning of the specs
etc, please feel free to contact me off-list, and I'll do my best to explain
further.

Best regards,
Dave