UFO Rotors - $$

Bernie Benz b.benz at charter.net
Thu May 1 16:19:11 EDT 2003

Tom,  I've heard of this rust belt phenomum and experienced it a long time
ago on a Lancia.  I cleaned it up with hard brake useage sacrificing a set
of cheap, hard pads.  If the rust pitting is uneven around the circumference
of the rotor, this may very well be your once around pulsation, but it
should be uneven and inconsistant, inasmuch as the wheels are not turning in
sync all the time, so will be in and out of phase.

IMO, rotor wear below the speced limit is no problem, excepting for the pro
wrench's liability concerns, but otherwise it is designed to sell
replacement rotors.  If not below spec, machining will force it there, and
sell the new rotors.

When the active rotor surface is worn below the original surface one has the
best of all wear sensors.  When the pad wears to the point that the backing
plate rubs the unworn rotor annular rings, this sensor turns on loudly,
indicating that the active pad thickness is now equal to the previous rotor
wear.  BTDT, but I still check the system twice a year with seasonal wheel
changes, but I don't change pads until I hear noise.  "No one can forget the
sound!", quote a fellow lister.


> From: "Tom Mullane" <tmullane at snet.net>
> Date: Thu, 1 May 2003 14:34:15 -0400
> To: "Bernie Benz" <b.benz at charter.net>
> Cc: "200q20V mailing list" <200q20v at audifans.com>
> Subject: Re: UFO Rotors - $$
> Bernie,
> The current issue is not warpage, but rust.  Parts of the contact surface of
> the rotor have rusted; or rather, it seems like the contact surface is too
> hard for water to penetrate, but it kind of gets "behind" the shiny, glazed
> surface and causes a delamination of the rotor where parts actually flake
> off.  On the inboard side of the rotor, the pad contacts only a ring about
> an inch wide in the center of the contact area. This car sat for months at a
> time in a Boston garage between uses, possibly the origin of this
> phenomenon.
> Thanks for the paper shim tip.  It may be useful it the new rotors have some
> runout.
> Tom
> ----- Original Message -----
> From: "Bernie Benz" <b.benz at charter.net>
> To: "Tom Mullane" <tmullane at snet.net>
> Cc: "200q20V mailing list" <200q20v at audifans.com>
> Sent: Thursday, May 01, 2003 1:54 PM
> Subject: Re: UFO Rotors - $$
>> Tom,
>> IMO rotors, UFO or otherwise, should not be turned unless they are badly
>> warped, inasmuch as they tend to wear true and therefore used rotors will
>> always be better than new or machined, warpage not withstanding.  Some on
>> this list have talked about a "dewarping procedure" that they have used
>> sucessfully, but I've never had the problem so don't know about it.  Rust
>> does not contribute to warpage and should not affect the braking surface
>> unless the car has been parked below high tide elevation for the winter.
>> Measure the runout of each rotor, the max spec being <.002"  If you do not
>> have access to a dial indicator, A post-it is about .0025", so if stuck to
>> the low runout side of the outer breaking dia on the inboard rotor
> surface,
>> it should not go under a pointer fixed to just touch the high side of the
>> runout.  Simple test, I'll bet you'll find no runout problem.  Or, if
>> necessary, they can be shimmed into spec with paper shims at the hub
>> interface, BTDT on G60s.
>> But do overhaul your calipers and caliper slides!  A dirty but zero cost
>> job, and may well be the cause of your problem.
>> DFI if IAB,
>> Bernie Benz
>> Gardnerville, NV
>> '90 90Q-20V  140K
>> '91 200Q-20V 140K
>> Ford "Quatro" Tractor
>> Other Misc. Toys

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