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Re: Warped front discs

Subject: Re: Warped front discs
Date: Mon, 16 Jul 2001 14:26:03 EDT
In a message dated 16/07/01 10:02:54 AM Pacific Daylight Time, 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 

And you thought brakes were simple, right?


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