From: wlsmith@valve.heart.rri.uwo.ca (Wayne Smith)
Subject: Re: front disc brake question
Organization: The John P. Robarts Research Institute, London, Ontario
Date: Thu, 10 Sep 1992 17:37:22 GMT

In article <24826@oasys.dt.navy.mil> bense@oasys.dt.navy.mil (Ronald Bense) writes:
>>    [the temperature at which a fluid boils depends on the external pressure]

>Hadn't considered this part, makes sense, but isn't what I've been told
>since a was a knee-biter. :) Could have been mis-information on   my
>part, <sheepish grin> but am not going to go immediately over and embrace
>your ideas here without some further thought.

Mountain climbers will tell you that tea dosn't taste the same when it's
made from water that boils at 160 or 170 degrees at 10 or 15K feet.

Hospitals and other institutions steralize instruments in an auto-clave
(a big oil-drum-sized stainless steel pressure vessel) where steam
is fed into it and water comes to a boil at 230 or 240 degrees.

>>    the volume expansion of the system.  Has anyone ever seen
>>    brake-fluid forced out of a brake system because it came to
>>    a boil and the internal system pressure exceeded the strength
>>    of fittings/lines/etc?
>
>Haven't seen this, but you should know that as metal gets hot, it expands
>quite a bit, esp versus volume at the smal dimensions we're talking
>about wrt brake lines.

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...

>>I have been told that drum brake fade is due to heat-induced
>>expansion of the outer edges of the drum, which increases the
>>distance between that part of the drum and the brake shoe.
>>This is believable, and boiling fluid is not.
>
>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.

>>#3) What about the bad points about disks?
>>     -> exposure of the disk leads to trapped material between
>>        disk and pad
>
>Very unlikely. Hasn't happened in over 300,000 miles of driving, or
>at least not to the point I would notice. :)
I've heard stories of small rocks'n stuff messing up peoples disk brakes.

>>     -> corrosion is easier
>
>YOu would think so at first, but in actuality, drums have a more dangerous
>corrosion pattern. Discs have most of their wear and corrosion occuring
>in the same place, the outside faces of the disc. The constant wear
>this face receives pretty much negates any deep pitting or weakening
>of a local area. Drums, however, have their primary corrosion surface
>on the outside while the primary wear surface is on the inside, allowing
>for corrosion to create weak spots in the drum through pitting.

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.

>>     -> water puddles / wet roads / generally wet conditions
>>        can interfere with braking
>
>Generally (and this is a large amount of common sense extrapolation
>of personal experience :) this is not really the case, at least no more
>for discs than drums. Discs are gernerally warm, and quickly evaporate
>any water that gets on them while driving. If you enter a puddle deep
>enough to immerse them, then it is a very short time before the brakes
>are fully functional again, as drying is very rapid. Drums, under the
>same condition, with my limitted experience, appear to take much longer
>to dry out, probably because of the restrictive nature of the drum itself
>to free air flow.

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.

>>     -> comprimises and sacrifices are being made on the altar of
>>        "unsprung weight" leading to weaker designs
>
>There are no weaker designs, perhaps poor choice of materials, but overall
>no inherently weaker design. There is as much metal actually transferring
>the stopping power to the car as in drums. It's that bit of metal that
>the lug bolts go through.

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 

a) does the job
b) weighs less
c) has fewer parts (is cheaper to build and install)
but
d) requires more (rotor) maintainance

Why are the car company engineers on the net so quite on this topic?

>>     -> Saftey or parking brake setup is more complicated
>
>True, on 4-wheel disc. It generally entails (at least on mine) a second
>set of rear drum brakes, which are supposedly good for *one* stop at
>speed, although I've never tried it. :) Not a real disadvantage, imo,
>since the reason you're using it is because of some failure in the primary
>system. Why would you want to try and activate it by some other means?
>Just use a secondary brake system. Seems much safer to me.

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.

>>I must say that, having driven front disk / rear drum cars, I
>>much prefer the solid feel of 4 wheel drum to the mushy feeling
>>of front disks...
>
>If front discs feel mushy, you need to bleed the system. :) They are
>quite solid on mine, although I have power assist (much prefered over
>the no-feedback full power systems:) and the feedback I get most now
>is the pulsing of a bent wheel. :(

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.

>BTW, I didn't say panic braking, just heavy hard braking for a continuous
>time period. Driving on the beltway around here is probably similar
>to slow-speed Gran Prix track driving, as far as braking goes, as generally
>some moron going the bare minimum (or lower) speed limit zips 2 lanes
>left at a right angle and forces heavy braking of an entire line of
>cars going 80 at regular intervals. (OK, OK, so I exagerated, they were
>only going 65:) If I lived in a normal area, my brakes would probably
>last twice as long, between 55-60k.

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!)
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