From: bense@oasys.dt.navy.mil (Ron Bense)
Newsgroups: rec.autos
Subject: Re: front disc brake question
Date: 10 Sep 92 18:57:58 GMT
Reply-To: bense@oasys.dt.navy.mil (Ronald Bense)
Organization: Carderock Division, NSWC, Bethesda, MD

In rec.autos, wlsmith@valve.heart.rri.uwo.ca (Wayne Smith) writes:
>In article <24826@oasys.dt.navy.mil> bense@oasys.dt.navy.mil (Ronald Bense) wri
>tes:
>But in that case, what part of the brake line actually gets hot?
>Certainly not the brake line, and at most the wheel cylinder,
>which is quite a hefty piece of work.
>Besides, you have to get metal almost red-hot for any *appreciable*
>expansion...

That entire structure gets quite hot, and quite uniformly. As for the
amount of heat, we're not talking a lot here to make differences. Less
than a hundred degrees can make the difference between a tight fit and
an intereference fit, for shop tolerances. A thousand degrees on an
eighty-inch metal structure has more than an inch expansion. It may
not sound like a lot, but remember that in this case, the tolerance
is under .1", and 1" causes big troubles.

>>True, this is believable, although the T rods, etc also get hot, and,
>>being made of the same material, would negate the clearance effect.
>I don't quite follow you here.

You were talking about the drum getting hot and expanding, correct?,
causing the clearance between drum and pad to increase. I was pointing
out that the rest of the structure is made of the same material, and
is also getting about as hot, therefore negating the differences in
clearance, as they would also expand. Exactly how much I would have
to calculate out, when I know actual temperatures.

>I've heard stories of small rocks'n stuff messing up peoples disk brakes.

Stories. I've never heard of this happenning, although something small
might get in there and cause a small gouge or something, as it would
probably penetrate into the pad. It seems reasonable, anyway.

>I can think of no worse situation than a freshly polished metal surface
>the is constantly exposed to the elements.  It takes drum brakes years
>(at least 10, maybe 15) in my kind of climate (salt and 3.9 ph acid rain)
>for the outer part of a drum to corrode to a bad state.
>The contact side of a drum is protected from the elements.

The point was that the corrosion area is constantly being polished,
removing any corrosion products, and preventing pitting.

>Granted, once inside, trapped water can take longer to leave a drum
>brake, but that also makes it harder for water to enter it in the first
>place.  What I'm saying is that you must physically immerse a drum brake
>in water for water to interfere with braking, but it is easy for mist and
>road spray to interfere with a disk brake.

As I stated, not true, as even under normal driving conditions with
no braking, the pads rub against the braking surface, keeping it warm.
Jack your car up sometime, and rotate the wheel back and forth and listen
to the pads move.

>Yes, but I'm talking about the amount of metal that is backing up the braking
>surface (ie structural strength / heat sinking ability, etc).
>It's my guess that the disk brake system

The breaking surface itself has never had problems in my experience.
As for a heat sink, wouldn't you rather remove heat? After all, the
heat sink has finite limits to hold heat, whereas removal only has a
heat/time limitation. Total heat removed is infinite (within the wear
limit, anyway). Structural strength is not really an issue for the braking
surface in any car I've owned, although I did have broken rotors in
the Fuego where the braking surface separated from the hub area. :(
Not fun.

>a) does the job
>b) weighs less
>c) has fewer parts (is cheaper to build and install)

If I remember correctly, disk brakes cost more, because it is a higher
tech system, and tolerances have to be tighter than for drums. a) and
b) I agree with totally, though. :)

>but
>d) requires more (rotor) maintainance

Not in my experience. One car I had had a 50-50 braking set-up, and
I had to do both front and rear at each 25k change (4 of them). Front
were disc, rear were drum. Front took about 30 minutes, rear between
1-2 hours, depending on how lucky I was that day to have an extra pair
of hands to hold all the gobbledy-gook in place while tightening the
adjusting screw.

>Remember-> the failure of the primary system is hydraulic failure
>(from a leak or blow-out of a line to a problem with the master
>cylinder).  There's nothing wrong with the idea of a mechanical
>backup that uses the same brake (shoes/drums) in the event of a
>hydraulic failure.

What if it is the shoe/pad failure that causes the brake failure? (Never
experienced this, but I suppose it could happen.)

>I think that there are a few people out there that would agree that
>*some* or even *a lot* of properly adjusted front disk/rear drum
>cars have a mushy feeling.

Mine are four wheel disc. No drums on this car, and that was one of
the criteria when I went shopping for a car last time.

>Well, I drive under those conditions from time to time, and I
>experience no problem with my braking system.
>(full power 4 wheel drum brakes, and a single (not dual) master cylinder!)

I wouldn't want to be in front of your car here (hmmm, maybe I would,
I need a new rear bumper, it's discolored after all this time, but I
wouldn't want the metal damaged.:) at least not on the freeways during
rush hour. I have a dual master cylinder, (One set can fail, the others
will still work) as I think failsafes are good business. I suppose your
opinion of seat belts are no belts, or lap belts at best?

And why are all race cars given disc brakes, if drums are so much better?

Ron

Potassium Benzoate included as a preserver.