From: welty@balltown.cma.com (richard welty)
Subject: them's the brakes
Organization: New York State Institute for Sebastian Cabot Studies
Date: Sat, 12 Sep 1992 17:46:00 GMT

it seems to me that the quickest way to put this whole disk vs. drum
brake discussion to bed is to summarize the basic content of _The
Brake Handbook_ (Fred Puhn, HP Books 328, ISBN 0-89586-232-8)

1) Servos vs power assist

drum brakes have an advantage in that some measure of servo action
(force multiplication) may be designed in.  because of this, disk brakes
require some sort of power assist, unless the vehicle is very light
(e.g., a purpose built race car)  the most common type of drum brake,
the dual servo, is used for its effective force multiplication , as well
as for the fact that the dual servo is the most effective drum brake
design for braking both in forward and in reverse.

2) pedal effort

here is where the servo action bites back.  let us presume that
we are using similar friction materials on our example pads (disk brake)
and shoes (drum brake).  let us also presume that with a CF of 0.50
in our friction material, that all of our sample braking systems
have a pedal effort of 50 lbs.  with heat, it is not uncommon to have the
coefficient of friction of the friction material on cast iron to drop
by, say, 0.1.  in a dual servo system, a drop in CF of 0.1 will lead in
an increase in pedal effort from 50 lbs to 107.6 lbs.  in a disk brake
system, the pedal effort will increase from 50 lbs to 62.5 lbs.  you figure
it out.

``dual servo brakes are most susceptible to the type of brake fade where
a force limit is reached.  thus, if they are ever used in a racing or high
performance application, care must be taken to use linings that do not have
a drastic drop in friction with increasing temperature.''

3) cooling

   a) drums must be very large for good cooling; such large drums have
   the advantage of reducing pedal effort.

   b) vented backing plates can increase cooling, but they also may let
   garbage into the drum to create problems.  some sort of screen is
   mandatory.  if dust enters, however, then it may increase the wear rate
   and cause scoring.

   c) drums are heated at the indside; disks are heated at the outside.
   cooling action for disks commences immediately with use.  drums must
   heat from the inside out before cooling from airflow begins, unless
   venilation via the backing plates is provided.

   d) the expansion of a drum brake when hot increases pedal travel; this
   interacts with the servo action and pedal effort (as discussed above)
   to cause a substantial increase in pedal effort, which is sensed as
   fade.  this does not happen with disk brakes.

   e) with disk brakes, because the surfaces that are directly heated
   are completely exposed, many options for cooling are available such
   as ducted air and ventilated rotors that are not as easy for drums.


4) more on fade

   the gases and dust from the friction material have to go some
   place.  with disk brakes, they may ride between the pad and disk
   briefly, but will be dispersed quickly enough due to the rotation
   of the disk.  this is also, despite the fact that disks appear to be
   more exposed, why disk brakes are less sensitive to water and dirt
   than drums.

   with drums, the drum will start to behave like a bearing surface.
   the gas and dust will ride on the inside of the drum, reducing
   friction and again, causing the brakes to `fade'

so as far as i can tell, there are only two things you give up with
disk brakes:  1) an easy handbrake installation  and 2) you need power
assist of some sort.  other than that, they're a clear win.

richard
-- 
richard welty                            518-393-7228
welty@cabot.balltown.cma.com
  ``if you can read this, mario, you're too close''
    -- bumper sticker seen on a CART safety truck
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