From: BRANDT_JOBST/HP1900_01@HPLOMG.HPL.HP.COM
Newsgroups: rec.bicycles
Subject: Re: Force 40 Brakes
Date: 7 Apr 92 19:23:52 GMT
Article-I.D.: hplomg.9204071923.AA18203

Ralph Metcalf writes:

> No. Drum brakes can be configured either self-energizing or non
> self-energizing; I have both kinds on my motorcycle (1982 Yam.
> XS400SJ).

All internal shoe drum brakes have servo effect and some have more than
others.  I suppose you have heard of double leading shoe drum brakes.
The notation means that both shoes are sucked into the drum by forward
rotation in contrast to conventional drums that have one shoe in each
direction from a common pivot.  Cars generally used these on rear
brakes because otherwise you couldn't stop on a hill since the servo
action of leading shoes works against them in reverse.

> Be careful about the reason why motorcycles use disk brakes; the two
> biggest reasons are heat and weight.

I think you will find that not much weight is saved by disks and heat
is the major cause of a variable friction coefficient.  That is why
motorcycle drum brakes were made inordinately large; to keep the
temperature down.  The last drums used were as big as racing car drums
just to avoid the unpredictable brake effect.

> The rotating mass required for drum brakes is enormous compared to
> disks.  Harder to spin up, but, more inportantly, much harder to slow
> down.  With increasingly larger drums, a larger percentage of the
> braking power is consumed in slowing down the braking *system*.  The
> law of diminishing returns definately applies here.

This was not a driving design consideration because rotational inertia
on a racing bike is not a significant effect.  The brake function is
the principal reason disks are in use and this is primarily because
disks completely decouple rotational motion and brake caliper forces
that act at right angles to the rotor.

> Secondly, heat dissipation is important to prevent fading and warped
> surfaces.  Rotors can be ventillated, slotted or drilled to help
> dissipate heat (also reduces rotational inertia).  Drum brakes are
> *very* hard to cool, so overheating becomes one of the limiting
> factors on braking performance.

As I mentioned before, heat was not a problem to dissipate.  The
reason for huge drum brakes on motorcycles was to keep the temperature
down in order to operate at constant response.  These brakes had no
power assist so the variability was tied to a small hand lever motion
and large brake shoes.  Disks have no servo effect and can run with
brake pads in contact without significant drag.  A drum brake in
contact causes unwanted braking and can lock up when the shoes get
hot and expand into the drum.  The shoes are aluminum the drum liner
is cast iron.

> I don't agree that drum brakes are more powerful; this is definately
> not my experience!  Unfortunately, I am not qualified to give
> definitive reasons why this is so.  I'm sure some of the performance
> motorcycle people can give better and more complete explanations about
> the reasons for using disk brakes.  I do not want to start any sort of
> rec.motorcycles <--> rec.bicycles flame war, so readers are on their
> own for a better answer.

I don't think you will find drum brakes displaced from buses and
trucks because drum brakes dissipate heat well, have large surface
area, and use relatively simple mechanisms to operate.  Disk brakes
are often touted for things that they appear to do but in fact do not,
rather than the more complex mechanical and control problems that are
the driving force behind them.  Aircraft brakes are an example of a
mechanism that violates most of the common ideas about brakes.  There
is no cooling to speak of and they are multi stator and rotor clutch
assemblies made of carbon composites that aren't even good conductors.
If you look at the wheels of an airliner you cannot even see the
brakes they are so deeply embedded in the wheels.  The difference is
that they make only one stop and store the energy to be bleed off over
a long time.

This is all interesting but has little to do with bicycle brakes that
will probably remain as exposes disk brakes with myriad cleaver
mechanisms that appear to multiply force.  This cannot happen without
power assist or by introducing undesirable servo assist.

jobst_brandt@hplabs.hp.com