From: BRANDT_JOBST/HP1900_01@HPLOMG.HPL.HP.COM
Newsgroups: rec.bicycles
Subject: Re: Need information and experiences about drum brakes2 Resp
Date: 11 Sep 92 21:07:11 GMT

Detlef Adolph writes:

> At the University of Hannover we want to construct a new hydraulic
> drum brake, eventually with "integral function" of front and back wheel.
> Therefore we would like to get some answers to the following points:
>
> 1) We need drawings and material information of good drum brakes
> 2) Who has experiences with the use of drum brakes
> 3) What do you think about hydraulic in connection with drum brakes?

A drum brake is used on bicycles primarily where the conventional rim
brake is fatally ineffective either because ice and snow preclude drying
off the rim or as with a tandem, the rims become dangerously hot and
cause tire failure.  Bicycles being so sensitive to weight as they are,
cannot be burdened with hardware that is avoidable.  Hence bicycles have
brakes that for most vehicles would be considered unacceptable.

Being in a place of learning, you should have no trouble finding detailed
drawings of moped and light motorcycle brakes with performance data and
dimensions.

For tandems that are used in mountains that have long descents with tight
curves, a drum brake is a necessity regardless of speed because a large
part of the kinetic energy must be dissipated by braking, there being
substantially less wind drag per mass of the bike with riders.  The
lower the speed the greater the portion of this energy must be absorbed
by the brakes.

On two wheeled vehicles, skidding the front wheel even for the briefest
time causes unrecoverable loss of control and for this reason running
both brakes off the same lever is generally avoided.  The exception is
with tandems that use one brake to absorb over-speed and the other to
do the hard braking into curves.  Tandems, being long with respect to
the height of the center of gravity from the road, do not have as much
weight transfer to the front wheel as single bicycles and are therefore,
also more able to brake with the rear wheel where a single bicycle
would skid the rear wheel.  However, putting both brakes on one lever
exacerbates the conventional problem of the hand having insufficient
strength to stop the front wheel.  With both brakes the brake
effectiveness is half of that.

Hydraulic brakes have two major advantages over cables in cable housings,
they have practically no friction losses (that are substantial in cable
brakes) and they easily achieve any mechanical advantage desired merely
by the piston ratios.  So for lack of frictional losses a hydraulic brake,
all other things being equal could have a 3.5:1 or 3:1 ratio instead of
the conventional 4:1 common to practically all bicycle brakes.

Drum brakes have servo effect that helps increase braking force but
reduces control, the amount of servo being dependent on the friction
coefficient, while disks have no servo are linear and require more force
to apply.  This is why disk brakes are nearly always power brakes except
in the lightest cars.

You will notice that trucks do not use disk brakes because they cannot
dissipate enough energy.  As long as the drum is relatively narrow with
respect to the friction surface of a disk, the disk can dissipate more
energy.  Once the drum gets ~400 mm wide as they do on trucks, there is
no competition from disks.

Assuming there is no contact of the brake lining to the friction surface
when not in use, the drum usually has the advantage for bicycles by a
wide margin because it requires a far lower mechanical advantage and
therefore, will not be as spongy or run out of travel nearly as soon.

Your effort on the bicycle drum/disk brake should bring you some interesting
problems, all difficult to solve.  Having looked at many such brakes, I am
convinced many people have tried but for lack of time or enthusiasm settled
for mediocre compromises.  Make sure you understand what has been done
before starting any design yourself.

I wish you inspiration.

jobst_brandt@hplabs.hp.com
Newsgroups: rec.bicycles