Re: drums Vs. discs...pros n' cons..

[Jerry Casper <gremlingts@xxxx>]

 Ok, here's my 2 cents worth on the subject. 

- Drum brakes are imperfect due to certain design
constraints. The drum brake pad contacts the surface a
little at a time, it's literally "levered" into the
drum via one dual-action piston. The much-maligned
dual-cylinder ( 4-piston) brakes would shove the shoe
into the drum in a much straighter line, and cause
quicker braking, but they are apparently balky 
operating when old. Might have worked ok when
new....but I digress.
- The drum brakes fade also, due to their enclosed
nature, which builds up heat with repeated use. Disc
brakes aren't enclosed, and the more modern rotors are
vented and sometimes have cooling holes drilled thru
them, too, on extremely high-end sports cars. So discs
may get hotter sooner due to quicker stopping ability
( and therefor more heat build-up ), but they
dissipate heat much better and quicker.
- Using a pad on the side of a rotating surface
seems to be more efficient slowing down rotation than
at the outer (faster) circumference location. I'm not
up on the physics aspects, so can't explain WHY it
seems to work better, but it does seem that applying
pressure on the side of a rotating object is easier
than pressing down on the edge. Probably has something
to do with the torque curve and application of
pressure to the centrifugal gradient of rotating mass
( ok,I just made all that stuff up...I don't KNOW
! :D )
- Lastly, drum brakes when wet can either grab or
fade dangerously, as the water has to ride underneath
a longer pad surface around the circumference, and if
the water buildup is sufficient, I'd imagine it could
hydroplane some, due to the levering action. With disc
brakes, they are applied to the side surface, not the
rotating on-end side, and literally wipe the surface
much better, like a windshield wiper, so they are not
as affected by water as a drum brake shoe would be. 
I guess the two biggest advantages are that 1) disc
brakes are applied immediately over the entire pad
surface, whereas the shoe pad has to be levered
gradually onto a larger surface, and that takes time.
and 2) the side pressure vs. outer circumference brake
pressure stopping ability is more efficient on discs
than drums. 
Hope this has been somewhat useful ( or less than
100 percent useless ) in helping explain disc brake
use. FYI, Drum brakes were used almost exclusively in
NASCAR racing up until around '71, when Penske used
Girling discs on the Matador to win the Riverside ? CA
roadrace, as it allowed them to ride the brakes in the
curves, then mash the gas coming out and get a quicker
jump. After that, other teams started using disc
brakes as well, so AMC actually improved NASCAR racing
competition at that point! :D 

Jerry Casper


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---------------------------------------------------------
Jerry Casper , owner of 16 AMCs 
7 Gremlins / 2 Javelins / 1 Hornet / 1 Pacer
3 Matadors ( 2 coupes ) / 1 Spirit / 
1/2 Eagle
1/2 Jeep Wagoneer

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