Re: Brake rotor warping - yet again!

[C2slayman@domain.elided]

Hi John,
       I'd like to congratulate you on measuring rotor warp with a dial 
indicator.  It's nice to see actual diagnostic work from time to time vs 
speculation.

In a message dated 12/24/02 12:04:57 AM Central Standard Time, "John 
Fielding" <johnf@domain.elided> writes:
> 
> There is another common fault which is to do with the varying porosity of 
> cast iron.  The
> minute particles of scrubbed off pad embed into the rotor surface.  As long 
> as the
> porosity is uniform this doesn't normally cause a problem.  However, if the 
> porosity
> varies the amount of pad material also varies.  This gives a sympton 
> similar to rotor
> warpage, but it isn't quite the same.  True rotor warpage causes 
> "pad-knock-back" as it is
> commonly called.  Varying porosity cause the brakes to judder or snatch as 
> the rotor
> rotates against the pad.  Knock-back can be felt through the brake pedal 
> and the steering,
> not as well on a servo or power steering assisted car as the older ones 
> with plain braking
> and steering systems.  Pad varying "stiction" doesn't cause any sensation 
> in the brake
> pedal but the vehicle judders under braking and can be felt through the 
> steering. As the
> pad material builds up on the rotor the symptons get worse.
> 
>        This is an interesting theory.  I'd like to add to it.  I once had 
> horrendous (sp?) judder on my '88 Volvo Wagon.  I went to turn the rotors 
> and they were too thin.  So I bought new rotors and still had the problem, 
> reduced only slightly.  I then checked the rack and pinion and found it 
> sloppy.  After replacing the steering unit, the problem vanished 
> completely.  So I attributed the problem to slight variation in the rotor 
> friction.  As the friction varied, one rotor would drag more than another, 
> pulling the steering.  As the rotors varied back and forth on their 
> friction, this would cause the steering  to oscillate.  So I buy your 
> theory of "stiction" as being another possible source.


Cheers,
Charlie
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