From: pardo@cs.washington.edu (David Keppel)
Subject: Re: Force 40 Brakes
Organization: Computer Science & Engineering, U. of Washington, Seattle
Date: Tue, 7 Apr 92 18:38:14 GMT

As an amateur brake designer I'll add two and a half cents...
Beware that most of my considerations are for braking arbitrary
things, where a bicycle has specific characteristics.

>BRANDT_JOBST/HP1900_01@HPLOMG.HPL.HP.COM writes:
>[Self-energizing brakes are nonlinear, therefore less predictable.
> That's why e.g., motorcycles use disk brakes.]

metcalf@blast.bso.uiuc.edu (Ralph Metcalf) writes:
>[It's for heat dissipation & weight.]

In general, self-amplifying brakes are more sensitive to changes in
the pad friction and the greater the amplification the greater the
sensitivity.  Drum brakes are particularly sensitive because
amplification is achieved using a tecnique that is nonlinear and thus
especially sensitive to changes in pad friction.

Three things (at least) happen when you heat a drum brake.  First, the
drum expands, going from a tube shape to a truncated cone shape.
Second, the drum gets somewhat elastic and is pushed oval by the
forces pusing out the brake shoes.  Third, the brake gets hot and the
pad friction (force vs. drag) changes.

What's most important?  Large truck drum brakes have a steel ring to
help prevent the expansion and ovalizing.  But drums fade in part
because the design is inherently sensitive to small changes in the pad
friction.  The rule is that shoes that wrap only a bit of the drum are
less sensitive than shoes that wrap a large part of the drum.  They
are also less powerful and require more force input, exacerbating
elasticity problems.

Bike rim brakes are disk brakes.  For the Scott SE, the rotor (the
rim) is sitting out in the water and mud and dirt and grit, so the
changes in pad friction are dramatic.  The Scott survives because it
low amplification and so is relatively insenstive to changes in the
pad friction.

Conventinal drum brakes have high wrap angles and and high
amplification.  They survive by protecting the drum from the elements
and using pad materials that are designed to be insensitive to heat.

Conventional disk brakes have no self-amplification.  They survive
because the brake is stiff and you don't waste all your stroke
stretching the brake.  Fundamentally, though, stopping an arbitrarily
large thing with a disk brake requires external power assist.  A bike
rim brake is an exception: it is springy, not stiff, but survives
because it operates at low force.

An unassisted brake as springy as a typical drum can't be strong
because all your energy goes to ``winding up'' the brake mechanism.  A
self-engergizing brake is less predictable than an unassisted brake but
can still survive if it is relatively insenstivie to changes in pad
friction.  Drum brakes with large wrap angles (all conventional drum
brakes) take two hits: first they are self-assisting, second the
self-assist is nonlinear in pad friction and thus even less
predictable.

Drums and disks as we know them are not the only choices.  The
earliest brakes were band brakes, where a strap is wrapped around the
outside of a drum, compressing it.  Such a brake can be very powerful,
but has a large wrap angle and thus fade problems.  It also tends to
be hard to apply smoothly and thus grabby.

A ``normal'' disk brake sequeezes shoes on a disk.  The Ausco-Lambert
design had a ``disk'' of pad material that expanded and rubbed against
a pair of rotor faces pointing inwards.  The mechanism is self-
assisting but doesn't suffer drum nonlinearities.  I've heard that the
brake worked well but when Chrysler built them later (1950's?) they
used soft bearings in the mechanism, had failures, recalled all cars
that used the brake, and simply fitted conventional drums.

A ``normal'' drum expands shoes inside a drum.  Some, like the band
brake squeeze on the outside.  The Murphy Brake pinches the drum.
It uses a self-aligning mechanism so that uneven drum expansion
doesn't cause problems.  It is self-energizing but has small wrap
angles and avoids nonlinearities.  I don't know the disadvantages of
the Murphy brake because all my information comes from the inventor
via a third party.

Disks warp badly when overheated.  A drum distorts in a more
controlled fasion.  Lightness is a goal of brake designs, but a light
brake rotor (disk or drum) will get very hot.  Disks survive with good
cooling.  Convetional drums are heavy and have fade problems.  The
Murphy Brake tries for the best of both with it's low wrap angle.  The
inventor fitted his brakes to semi trucks and ran them dull red hot.
They had some fade but were still smooth.

Note, by the way, that cars and motorcycles mostly use disk brakes or
disks in front for dissipating most engery and drums in the rear.
Trucks use drum brakes almost exclusively, I believe because it is
hard to build a disk brake that can dissipate the energy without
warping.

In summary, remember I don't do this for a living, it's just a hobby
of mine!

	;-D oN  ( Go once.  Stop twice. )  Pardo
Organization: Freshman, IM - H&SS Track, Carnegie Mellon, Pittsburgh, PA