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 20:38:31 GMT

In article <24871@oasys.dt.navy.mil> bense@oasys.dt.navy.mil (Ronald Bense) writes:
>
>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.

Yes, that's *fit*, but how does that change the internal VOLUME of the
brake system (ie volume of the wheel cylinder)?!  Hardly any, I say.

>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

I said that the outside edge of the drum can expand (more than the inside)
since the inside part can't move or expand:

        (these are side view cross sections of the drum) 
                      ___
     _______     ___-- 
     |           |
     |           |
     |           |
     |           |
     |           |
     -------     ---__
                      ---
     
     (cold)       (hot)

If the above does happen, then the shoe couldn't compensate for the
change in geometry, so you have a reduced braking contact surface...

>>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.

By removing "corrosion products", you are removing the latest layer of
rusted material.  Little by little, the rotors are rusting away from
both sides.  Pitting is one thing, but constant surface rust is another...

>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.

!!! Like, are your pads always in (slight) contact with the rotors?
Bad design...
My brake shoes are less than 1/16 (estimated) from the drum, but the
springs always keep them away from the drum (when not in use).
But that's right, how are disk pads kept away from the disk? 

>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.

I guess it depends on if you ride your brakes all day.
Although if your pads are always in contact with the disk, that
wouldn't help matters.

>Total heat removed is infinite (within the wear
>limit, anyway). Structural strength is not really an issue for the braking

When I say structual strength, I mean the ability to absorb heat and 
stress without a permenant change of shape. 
It seems that rotors (of many makes) fall short in that catagory.
Don't you constantly see the postings of
a) my brake pedal pulses .....
b) how often should I get my rotors turned
c) should I get new rotors or get them turned ...

This is all bull crap!  If the rotor is such a good design, then
why can't it take the heat!!!!!!!!

>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

I don't buy "higher tech".  Ingnoring abs, the rotor/pad setup has
fewer parts, and installs faster at the factory.

>>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

I said rotor maintainence.  It seems most people can depend on turning
the rotor as part of a brake job.  (try that at home).
Changing my brake shoes is easier than getting the car off the ground
and the wheel off!  You should get the $8 tool that makes compressing
the springs easier.
 
>>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.)

Well then I guess that's modern technology (or skimpy make-it-good-enough
technology).  Yea, I'll buy that scenario in today's cars...

>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 would rather have a dual circuit, which came a year later (1966?) on
my car.

>opinion of seat belts are no belts, or lap belts at best?

I just have lap belts in front, no belts in the back (original).
I wouldn't mind a shoulder belt...
But I'll take the full saftey glass in my car (with plastic in every
window) to the cheap-stressed glass they put in the side and back
windows (like, if I get into a roll-over, the plastic in the side glass
will hold the window together and keep me in the car, or will keep
outside stuff out like BB's or rocks thrown from lawnmowers)...

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

Look at these race cars...
They are not designed for longevity, cost-effectiveness, easy maintainability,
and they will go to great lengths to reduce weight and improve on
handling.  They cut corners all the time, and are lucky to have a
motor/transmission that can go the whole race, and they have computers
that fail all the time.  Again I ask what is their reason for disk
brakes!??!?!?
(just saying "look what they use" is not good enough)
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