From: bense@oasys.dt.navy.mil (Ron Bense)
Newsgroups: rec.autos
Subject: Re: front disc brake question
Date: 9 Sep 92 13:57:34 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 <1992Sep4.132408.213@webo.dg.com> dibb@jac.webo.dg.com writes:

>>Disks are easier to service and are far better as far as
>>heat dissipation

>When (or how) does heat become a problem?
>Does heat play a role in stopping performance during a particular
>braking event, or does the heat play a role in long-term material
>fatigue (warping) etc?

Stopping performance of drums pretty much sucks eggs. Try going 80-100
after some freeway driving, and then try a *hard* stop with drums. You'll
find that they fade away about halfway through the stop. Drums will
get so hot that the brake fluid will reach the boiling point in the
slave cylinder, effectively reducing hydraulic pressure to nil. This is
not something you want to have happen in traffic. As far as long term
fatigue etc, drums are admittedly better, but what difference does that
make if you fatigue the car into other objects? The reason drums are
better is because there is more metal in a drum, which makes them heavier,
and less desirable from a suspension design viewpoint, as there is more
free weight to deal with that needs to be countered by heavier springs,
dampers, and support bars. Overall, this causes a much heavier car to
result, which is why a lot of older cars only get in the 8-15 mpg range.

>If drums are more succeptible to heat problem, why does it seem that
>most (if not all) warping/distorting problems come from disk brakes?

Disks are lighter, and because of their greater heat dissaption coupled
with inherently cooler slave cylinders, (or pistons and calipers, if
you will:) they can actually get hotter than drums, because the brake
fluid doesn't get hot.

>>and fade resistance.  The more metal used, the heavier, and thus less
>>desirable they are for
>>performance cars.

>I'm talking about general/every-day cars.  Why can't they make a
>maintenance free disk?  The only crock bigger than getting rotors
>turned every 10 or 20K miles is getting your cylinders rebored...
>(maybe they'll think of that next!)

If you are getting you're rotors turned every 10-20k miles, and you
are an every day driver, the first thing I'd recommend is immediate
revocation of your driver's liscense, not to be returned to you until
you'd completed a few driving courses where they taught you proper driving
techniques. I drive a performance car, with most of the heavy braking
done by the front disks (the back are disks too:) and I only replace
the pads every 30k or so. I wouldn't call me a sunday driver either,
and most of my driving is city/city freeways, which, if you're familiar
with DC means lots of 70-0 braking on the freeway during rush hour.

Admittedly, there are some cars that through design flaws run through
brake pads rather quickly, but there aren't many of these around from
my experience. (a replacement pad to a semi-metallic or metallic generally
fixes those problems, although the rotors will wear faster)

>I replaced the (front) drums on my 65 Dodge because parts of the outer
>edge was getting to thin due to rust.  In all the years (and close to
>200K miles) that my family has put on 60's cars with 4 wheel drum brakes,
>we never ever had to turn the drums.  I can easily go 15 or 20 K city
>miles before I need new brake shoes.

My front brakes get changed around 30k, and after 78k, the back still
look fine. Like I said, about 80-20 braking, or so it appears. The car
weighs about 3200#, so it isn't one of the real lightweights, either,
although not one of the two ton monsters of the 60s. Oh, and I normally
don't turn the rotors, either, unless they are really grooved, or, as
in the last change, I goofed and had a slight metal on metal contact.:(
Otherwise, I just put in new pads, and even with the double spring set-up
and eight clips per caliper, it still only takes me about 25-30 minutes
to do both sets of front brakes.

Ron

Generally, a person breaks a law per mile. - Texas DDC
Newsgroups: rec.autos