In a message dated 16/07/01 10:02:54 AM Pacific Daylight Time,
owner-mgs-digest@autox.team.net writes:
> For what its worth, brake rotor warping is a perennial problem for many
> vehicles using original parts as well as aftermarket units. BMW in
> particular can't seem to make their replacement rotors last more than 10 or
> 15k (even though the originals *usually* go 50 or 60k.
>
You just can't expect a replacement disc that sells for small change (I've
seen $15 MGB rotors) to represent decent quality compared to OEM. Problem is
that most owners won't pay for OEM, assuming that they were still available.
Many OEM rotors were cast from Meehanite, a dense fine grain cast iron.
Similar alloys of 'grey' cast iron are still used (the Meehanite had some
early cracking problems). I suspect that some of the after-market rotors are
made from melted down Trabant chassis or somesuch, looking at the quality of
some I have seen.
It was so bad that when I was working my way through university, at a parts
and brake shop, we would automatically take a few thou skim of brand new
discs as the proportion that were uneven and would give pedal pulsation right
out of the box was unacceptable.
You should ideally change both rotors at the same time. It takes up to 200
brake applications to bed in some pads - you will get uneven braking unless
you turn or replace the other disc.
The warping of rotors by over-heating can be ameliorated in part by heat
treating the discs, but this adds to the cost and is normally only done in
special racing applications. A heat -treated rotor is much less likely to
warp (the warping results from something called 'residual stresses' that can
cause cracking or further warping the next time over-heating occurs.
A fairly new technique, popular with the rally guys, is cryogenically treated
rotors - these are reputed to reduce microcracking and surface rusting
significantly.
And you thought brakes were simple, right?
Bill
///
/// mgs@autox.team.net mailing list
/// (If they are dupes, this trailer may also catch them.)
|