[FC] FC brake UPGRADES
Rad Davis
corvanatics@corvair.org
Sun Oct 12 00:06:01 2003
I'll pitch in here...
First, a quick definition: The coefficient of friction is (in practical
terms) the amount of friction you get for a certain amount of pressure
applied to the two sliding surfaces. So, for example, an ice cube has a
very low coefficient of friction on a polished floor. By comparison, a
piece of 36 grit sandpaper with the same contact area to the floor has a
much higher coefficient of friction.
Brake friction materials are always a compromise. Cold pedal effort, hot
effectiveness, and lining life all are in some degree of inverse relation
to each other.
Particularly, operating temperature is a matter of compromise. If the
lining is compounded to allow light braking effort when the brakes are
cold, it usually won't do as good a job when the brakes get really
hot--things that grab the cast iron rotor or drum well at room temperature
tend to melt or boil at high temperatures, and liquids make lousy brake
friction surfaces.
Contrarily, metallic linings have incredibly high pedal effort for not much
stop when cold--metal on metal just isn't that grippy at room temperature,
but work very well and consistently once they get hot. They also deliver
pretty good lining life--the lining usually wears away at the same rate (or
slower) that the drum/disc does, unless the drum/rotor was designed with
this sort of lining in mind.
Most brake linings seen on cars made in the last 20 years or so are
designed to have best braking (best coefficient of friction) somewhere in
the "getting hot" temperature range. That is, the pedal effort required
for a particular amount of friction at the brake decreases as the lining
and drum/rotor goes from ambient temperature to what might be regarded as a
"normal high working temperature." If you keep getting the brake hotter
after that point, the brakes "fade" -- the friction generated with a
particular amount of pedal effort decreases.
With drum brakes, there's an extra element of excitement--the drum grows as
it gets hot. If the shoes make a nice even fit with a cold drum, there
will be less shoe contact with the hot drum because the shoes probably
haven't grown exactly the same amount. Accordingly, drum brakes have a
reputation for very "exciting" fade characteristics. They also (in factory
trim) take longer to cool back off than discs do because there's not really
an effective way to get air into the inside of the brake assembly to cool
things off.
Yes, there were really good reasons why GM chose to use drums on all
Corvairs, but that's a discussion that's beyond the scope of this thread.
Speaking as a guy who's dragged a fair amount of things a fair distance
with my Greenbrier, and now lives in a pretty mountainous area, I'd have to
say that I've always found the brakes adequate. Indeed, it's possible to
lock up all four wheels on an empty FC with the brakes in good order. I
can't recall ever wishing for more brakes than I had. I will note,
incidentally, that a deluxe Greenbrier like mine is the heaviest FC
delivered from the factory.
Powerglide guys need not fear that they have less braking. I can state
with some authority that a loaded manual trans FC doesn't get particularly
much braking from the engine when going down a 5-7% grade in either fourth
or third. It's the brakes or nothing.
Driving technique is definitely the most important component to getting
stopped or slowed when you want to without drama. The longer the linings
are in contact with the drum, the hotter the rubbing surface of the lining
gets. Eventually (in a second or two), a gas cushion forms that keeps the
shoe from getting a good grip on the drum. It's easily fixed, though--just
let off the brakes for a second, then reapply. In that second, the
spinning drum sweeps cool (well, relatively) air over the hottest part of
the lining, and gives it time to conduct some heat away to the rest of the
lining and the shoe itself.
Yes, it takes a little while to get the technique. You obviously have to
brake a little harder than you might otherwise have chosen in order to make
up for that half second when there's no braking at all. There are other
benefits, though--you're unlikely to warp a drum, and your shoes will last
much longer than before.
As an illustration of how effective this technique is, I offer the example
of my daily driver: a 1994 Geo Prizm (Toyota Corolla by another name) with
195,000 miles on it, of which I've done 160,000, mostly in Western
Pennsylvania (home of the Allegheny and Laurel mountains). In that time, I
have installed one set of front pads and one set of rear shoes, The rotors
are un-warped and have never been turned. Further, I have no reason to
believe that I'll ever need to turn them... I *was* careful to bed the new
linings in properly to avoid glazing.
Can the FC brakes be pushed beyond their limits? Yes. All brakes
can. But good driving technique (as Ken said) will go a long way toward
pushing the limits out past where you are likely to go.
--Rad Davis
At 01:08 PM 10/10/2003 -0700, you wrote:
>Some more qeustions. How do metalic brakes help stop brake fade, is the
>coefficient of friction higher on them? or does the meterial resist/disipate
>heat better (or is that what a higher coefficeient of friction is?) Also,
>the
>proper way to drive a brier (or any car) in hills is to apply the brakes hard
>till you slow then let off of them, is that correct? What about for highway
>and freeway braking or just regular driving for that matter? The short
>wheelbase does have some effect on the drving requirements, yeah? What
>about a
>power brake booster, would that have helped the brier that lost its brakes in
>the mountains?
>What are some other ways I can help cool the brakes-I know that if I drive it
>properly this would not be necccesary.
>Thanks
>Harry
>
>
>Quoting Keith Hammett <khammett@stainlessfab.com>:
>
> > Brake Fade is when the brake pads or shoes heat up to a point that the
> > coefficient of friction is no longer effective between the pads/shoes and
> > the rotor/drum. Fluid type (air/DOT3/Silicone) does not matter for brake
> > fade! If you have ever driven in the mountains then you have seen the
> > emergency ramps (most people think they are just for trucks, wrong), most
> > vehicles that are pulled from these have no brake problems, except that the
> > driver over used his brakes heating them up and losing friction!
> >
> > Boiling brake fluid is when you have heated the fluid in the
> > caliper/cylinder from way over using your brakes. This will occur if you
> > continue to use your brakes after fading has occurred. After letting
> brakes
> > cool one will usually have spongy brakes from the air that gets trapped in
> > the lines. I would recommend that if you have boiled the brake fluid that
> > you change it.
> >
> > This summer when we went up Pikes Peak the ranger at the bottom told me to
> > make plenty of stops on the way down that minivans tend to over heat their
> > brakes. I drove straight down and at the brake check station the ranger
> > that checked the brake temperature said that I was in great shape and to
> > continue on. I still sat there for about 15 minutes to allow some cooling
> > before proceeding. Knowing how to drive is the best way to avoid Brake
> Fade
> > and boiling of brake fluid. I did this with less than 70% of brake pads
> > left on the van. BTW the van is a 2000 windstar.
> >
> > Keith Hammett
> >
> >
> >
> > -----Original Message-----
> > From: corvanatics-admin@corvair.org
> > [mailto:corvanatics-admin@corvair.org]On Behalf Of srmarti
> > Sent: Friday, October 10, 2003 7:19 AM
> > To: corvanatics@corvair.org
> > Subject: RE: [FC] FC brake UPGRADES
> >
> >
> > > please put dual master cylinder on any fc , or any corvair ,for that
> > > matter.make sure that everything is in good condition. I watched
> > > the greenbrier ahead
> > > of me on a mountain in vt lose its braking due to heat years ago
> > > , heard them
> > > on cb saying they lost them, and we had thier kids riding with us. the
> > > brakes faded out from heat. i think that a dual cylinder would
> > > have saved them.
> > > they did get hurt. not good memories at all. also silicone
> > > fluid would handle
> > > the heat better. regards tim colson
> > >
> > >
> >
> >
> > I don't question the added safety backup of a dual master
> cylinder. But how
> > does a master cylinder prevent brake fade? Isn't that an heat issue?
> >
> > Steve
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__________________________________________________________________________
Rad Davis: rad.davis@mindspring.com
Side Drummer
Co-keeper of Scottish Drumming Web-- http://drums.tico.com/
Corvairs--65, 66 Corsa coupes, '65 'brier Deluxe http://www.corvair.org/
"We did Nebraska in seven minutes today. I think that's probably the best
way to do Nebraska." --Brian Shul, _Sled Driver_