A. Drum brakes for many years have been self actuating, or self energizing.
This does not mean they deploy themselves, but that they help once deployed.
Kinda like a power brake booster. Once the shoes make contact with the drum,
they cock around the backing plate, being stopped by whatever the design uses
(I'm used to Spitfires, so they use the adjuster pin). The shoe up against the
stop then gets a boost from the secondary shoe jamming its backside. And yes, I
should be able to remember which is called leading and which is called trailing,
but as I type, I can't recall which is which.
This sort of brake action is far different then a simple drum brake, like many
motorcycles run on the rear wheel. In that case, the shoes are pinned on one
end, and have a simple cam that pushes them both out on the other end. This
type
cannot self energize, and takes quite a bit more pedal force to achieve the same
amount of friction. They are also much more controlable.
B. This all leads right into the problem with releasing self energizing drum
brakes. Once the secondary shoe is helping the primary shoe, you've got to
release quite a bit of hydraulic pressure to get the secondary shoe to quit
helping. You've got to get it away from the drum, then it lets the primary shoe
release the drum. This sort of action is very hard to modulate. It's sorta
like
trying to release a vice-grip gently, as opposed to simply relaxing your grip on
a pair of regular pliers (ala a caliper). It certainly can be done, but it is
much more difficult to do with control and speed.
C. A rotor and caliper system are not in any way shape or form self energizing.
So the pedal pressure is directly and pretty much linearly related to the
braking
action. You are also not overcoming any return springs, or sliding along a
backing plate. Hence the clarity of pedal feel. Add in the elimination of a
self energizing design, and you now have superior pedal control.
I am not trying to make you eat your words. Drum brakes are quite decent for
many applications. Most of my vehicles have some drums on them. And, under
certain conditions, a drum brake can be superior (exposed discs in the rain can
scare you silly waiting for them to sheer the water off prior to working).
For what it costs to achieve a full disc conversion, the money could be better
spent on buying superior pads and shoes (Carbotec, Ferodo, etc). But if one has
done that, and made sure the lines and such are in tip top shape, then the final
step is to go full disc, eliminating the drums.
My words also assume the brakes are in working shape. A set of glazed pads are
inferior to a set of good pads. A leaky caliper is inferior to a well
maintained
drum, etc.
Daniel1312@aol.com wrote:
> Hi Nolan,
>
> You make 3 very interesting points below which I have edited to read: A, B,
> and C. I would be grateful if you can expand upon them for me/the list. I
> suppose A + B = C? Convince me and I will eat my words! :-|
>
> Daniel1312
>
> In a message dated 23/11/00 20:10:03 Pacific Standard Time, npenney@erols.com
> writes:
>
> << A car with disc brakes is much easier to control at the limits;
>
> A. Being less prone to the self actuating lock of a drum brake.
>
> B. Requiring far less drop in hydraulic pressure to release when locked
> then a drum brake.
>
> C. There is also the benefit of superior feedback and feel at the
> pedal. >>
|