From: bense@oasys.dt.navy.mil (Ron Bense)
Newsgroups: rec.autos
Subject: Re: front disc brake question
Date: 9 Sep 92 19:45:25 GMT
Reply-To: bense@oasys.dt.navy.mil (Ronald Bense)
Organization: Carderock Division, NSWC, Bethesda, MD

In rec.autos, lester@naomi.b23b.ingr.com (Les Bartel) writes:

[my portion deleted to save space...]

>I have done a number of brake jobs, and I have observed that there is less
>metal for heat to conduct to the slave cylinder on drum brakes than there
[...]
>is that there is less metal in the heat conduction path to the piston.
>So, how can the brake fluid boil faster in drum brakes than in disk brakes?

It is not only the connectivity, but radiant heat, and lack of a cooling
medium. Heat transfer works through more than metal to metal connections.
BTW, the stuff does boil, although hot components can cause a glazing
of pads (or shoes), they will not cause the extreme fade experienced
in drums. (I have never experienced fade in disc brakes, but I also
haven't raced them on a track. Then again, I never raced drums around
a track either. :)  I've worked on both also, and I can't stand drum
brakes.

>Regarding boiling brake fluid reducing hydraulic pressure to nil, I have
>experienced brake fade in heavy braking.  I have never experienced boiling
>brake fluid.  The feeling I got was not one of lost hydraulic pressure

They pressure won't be completely lost, as gas does pressurize, and
boiling causes much of the heat energy to go into producing gas (have
since forgotten technical term - heat of vaporization?) thus cooling
parts a bit until the heat builds again.

>(no loss of pedal), but a loss of braking.  This is caused by a loss of
>friction due to hot pads, front rotors and rear drums.  Mostly front
>rotors since the rear brakes don't do much of the braking.

Pads get hot very quickly. Perhaps you'd like to explain why disc brakes
continue to work even after driving on the brakes at fifty miles an
hour on the freeway with them smoking away? Try this with drums, and
you wouldn't have much braking power. (Don't try this unless you have
money to waste on new rotors and pads, as it is likely to warp the rotors.)

>My arguement here is that brake fade is due to hot components -
>not boiling fluid.  Boiling fluid cannot cause brake fade.  It can cause

Sorry, but a course in thermo and another in heat transfer would show
you why this is false. Yes, I'm an ME, although my specialization is
in structural analysis, I had to take all those type of courses.

>loss of brakes due to gas (ie not liquid) in the brake line.  This can
>be observed by loss of pedal or a spongy pedal.  I hope this sets the
>record straight so that someone else may be able to diagnose failed
>brakes a little more accurately.

I hope I have shown you enough information that you will be able to
help others diagnose their problems better in the future. As an extra,
liquids are in general considered to be incompressible. Therefore, as
soon as you have any fade what so ever in the pedal motion, you have
gas (as in liquids, solids, and gasses) in the brake lines. This can
be caused by air in the lines, or, in the case we're talking about,
boiling brake fluid. Remeber, someone just posted that DOT 3 boils at
400F. That really isn't that hot when you consider the energy needed
to stop nearly two tons of mass from the rather rapid clip of 55 mph.
It all gets converted to heat. Science, what a wonderous world it does
open....

Ron

Potassium Benzoate included as a preserver.
Newsgroups: rec.autos