In article <
[email protected]>,
[email protected] wrote:
> Ryan Cousineau writes:
>
> >> The reason automotive brakes switched to discs from drums is not a power problem but a control
> >> problem. Drum brakes are non-linear in response. That means you cannot tell how much braking
> >> will result from a given pedal pressure because the drum brake is self energizing. The shoes
> >> are literally sucked into engagement and that is why it was common to have brakes completely
> >> lock after driving through a puddle.
>
> > What about trailing-shoe designs? I haven't been lucky enough to spend any time behind the wheel
> > of a vehicle with 4-wheel drums, but my father owned a 1965 Pontiac Custom Sport with
> > mirror-finish drums. They had no fade resistance, apparently because when the drum heats up, it
> > can flare slightly, and as soon as that happens the shoe contact area drops off dramatically.
>
> > I'm not sure if that's the mechanism (could it be just that the expanded drum no longer conforms
> > to the curve worn onto the shoe?), but I'm quite sure that fade resistance is not as good as
> > disc systems that fit in a similar space.
>
> The cause was brake pad material. With a large contact area that a pair of shoes had, they had to
> be fairly soft in order to generate enough drag to stop the car. Softer materials have low vapor
> pressures and outgas and melt at far lower temperatures that the friction material in disc brakes.
> Disc brakes have substantially smaller contact area and an even greater increase in contact
> pressure to make up for that. Today there are probably no disc brakes without power assist, the
> mechanism being so simple and well developed.
On cars, sure. Motorcycles, much lighter than cars, have disproportionately huge brake systems (twin
300+mm dia. rotors being common), and are limited (like bicycles) by geometry more than traction or
brake capacity in simple braking tests. Like bicycles, the brakes are manual, but they typically use
floating discs, presumably to minimize necessary pad clearance and maximize mechanical advantage.
A quick study on the web suggests that American cars started using power discs in the sixties and
stopped using manual discs in the seventies. I assume the Europeans were a bit ahead of this trend,
but possibly some low-end (or manfully sporting; what did Porsche do?) Euromobiles were using manual
discs for longer than that.
Interesting point about drum pad material. Couldn't they use a harder pad material with power-assist
brakes and just dial up the boost? Did drum distortion play no significant role in the drum brake
fade issues?
> Disc brakes on aircraft and racing cars have carbon discs and pads that operate at glowing
> temperatures and produce no brake dust, that ugly reddish-brown sludge that dirties front wheels
> of cars. Their brake wear debris is CO2, a pretty clean substance.
I'm not so sure about that. I used to watch a lot of F1 races (carbon-carbon discs and pads) and
sometimes when those front wheels came off in a pit stop, there would be a cloud of black dust
puffing out, apparently from the brakes. Were the brakes just not up to temperature?
--
Ryan Cousineau,
[email protected] http://www.sfu.ca/~rcousine President, Fabrizio Mazzoleni Fan Club