From: jbrandt@hpl.hp.com (Jobst Brandt)
Newsgroups: rec.bicycles.tech
Subject: Re: Braking oscillation
Date: 16 Nov 1999 17:47:36 GMT

Bob Flumere writes:

>> "Several thousandths of a inch" is "seriously out of parallel"?

> Yes!

> On automobile disk brake rotors, the typical specification for
> maximum "out of parallel" is .00075" (Three quarters of one
> thousandth of an inch).

> More than this will produce a serious pulsation in the braking
> system which will be transmitted to the rest of the structure.
> Automotive brake rotor lathes must machine both sides of a rotor at
> the same time to avoid causing this problem.

This effect is often mis-diagnosed as being warpage or uneven wear
when in fact it is material properties of the disk casting that have
been exposed by wear.  Care in casting these disks is more important
than was initially recognized.  Not only must they be cast with a
central sprew, but there must be control of the wave front as the mold
is filled to prevent segregation and variable thermal hardening.

> Something of seemingly little consequence such as over tightening
> the wheel lug bolts can distort a rotor this much and cause a severe
> brake pulsation that can require re machining of the rotors to
> correct.

I think you'll find that this feature is part of the design and that a
expansion web between the hub and disk is provided.  Only crude
inclusion of foreign debris between hub and axle can cause sufficient
distortion to be measured.

Jobst Brandt      <jbrandt@hpl.hp.com>

From: Bob Flumere <rflumere@tiac.net>
Newsgroups: rec.bicycles.tech
Subject: Re: Braking oscillation
Date: Mon, 15 Nov 1999 21:21:13 -0500

Atanovich@prodigy.net (Mark Atanovich) wrote:



>"Several thousandths of a inch" is "seriously out of parallel"?


Mark:

Yes!

On automobile disk brake rotors, the typical specification for maximum
"out of parallel" is .00075" (Three quarters of one thousandth of an
inch).

More than this will produce a serious pulsation in the braking system
which will be transmitted to the rest of the structure.  Automotive
brake rotor lathes must machine both sides of a rotor at the same time
to avoid causing this problem.

Something of seemingly little consequence such as over tightening the
wheel lug bolts can distort a rotor this much and cause a severe brake
pulsation that can require re machining of the rotors to correct.

Runout (wobble), can be much more severe (.010"-.015") without causing
much of a  problem. (As long as the caliper can float from side to
side, as on a bike).

The brakes on my R2000 (with Mavic Open Pro wheels) vibrate so badly
at higher speeds (25-30 MPH) that riders 25' away in a group can hear
the low pitched "moan" that they generate and I can feel it in the
bike frame as well as in the brake lever.  This is really annoying
when slowing for a left turn while signaling and braking lightly on
the rear.

The pads are obviously being pushed apart and allowed to move back in
toward each other as the wide and narrow spots on the wheel move
between them.  Pushing the bike across a lot at low speeds with the
brake lightly applied feels "lumpy" as the wide and narrow spots on
the rim pass through the caliper.  We tried this at the LBS with
another R2000 and an R4000 with these wheels and found that  the
sensation was the same.  Thus my decision not to bother having the
wheels replaced under the warrantee.

My premise is that even though the Mavic wheels have a machined
braking surface that looks pretty:

A. It is done on a machine that doesn't do both sides at the same
time.
B. The machine is junk.
or,  more likely
C. The spoke tension is distorting the flimsy wheels.

I popped in a set of wheels from one of my other bikes (which BTW do
NOT have machined braking surfaces), and the braking action is as
smooth as glass.

I had an experienced wheel builder relax the spoke tension to what he
considered the absolute minimum but the lack of parallel remained just
about as severe, so we re-tensioned the wheels and I'm living with the
shitty braking action until I decide on another set of wheels.  Note
that the rear has the most distortion and is spoked 2x on the right
and radially on the left. The distortion is at a maximum between the
spoke heads on the right (where the tension is the highest).

I'll be looking for some 32 hole, "non-boutique" wheels with a 3X
spoke pattern for the next set.   I'd much rather stop quietly and
smoothly than look pretty.  <BG>

Bob Flumere
rflumere@tiac.net