[geeks] Drum versus disk brakes

[Greg A. Woods]

[ On Tuesday, March 12, 2002 at 09:56:09 (-0800), Fogg, James wrote: ]
> Subject: RE: [geeks] Drum versus disk brakes (Was RE: car question   RESOLVE D)
>
> Debates of cost/effectiveness aside, having driven many vehicles with front
> drum brakes (incl trucks), I can say that the best feature of disks is
> cooling. Drum brakes hold heat and the shoes have no where to disapate heat
> to.

Disks have better heat dissipation, but drums certainly do have lots of
surface to dissipate heat on (and many have additional ribs on the outer
surface that provide even more area).  (more about this at the bottom)

Unfortunately the way some truck disks are designed there's no way to
prevent water and dirt from collecting in between the disk plates.  GMC
went through a recall of disks in at least Canada and the northern USA
for the 1989-? model years of the full-sized pickup because the disks
are actually two plates with fin-like vanes separating them so that air
can flow between them.  Unfortunately they were not stainless steel and
they corroded far faster than expected in places where salt is used on
the roads, esp. with constant heating and cooling in short-trip driving
typical of small trucks.

> Nothing compares to coming down a long hill and experiencing brake fade
> (been there). I've had drums fade on a pickup truck so bad that I could
> stand on the power brake pedal and it didn't give me all the stopage I
> wanted.

Complete loss of pressure in the master cylinder compares!  It's far
worse and far more surprising!  I had that happen on my truck last
summer when I was at the top of a long hill with a red light at the
bottom (and lots of traffic!).  I got stopped safely though because:

At least in most vehicles you still have a way to mechanically activate
the rear brakes.

I suppose if the rear drums were oily or something (not a condition
you'll notice immediately in summer conditions with city driving), then
I would really have been in a pickle.  There was no way to run off the
road safely -- I would either have had to cross oncoming traffic, mow
down pedestrians (and probably crash into a house), or rear-end the cars
at the red light.  A very bad situation to find your breaks have failed,
even if you realise it near the top of the hill (as I did) and you're
not going over 30mph and you have a manual transmission and good
compression!  That's the best adrenalin rush I've had in a long time!
(and the closest call to a real accident I've had in my truck I think)

> Drum shoes are enclosed in the drum, so no airflow. The cylinder is enclosed
> in that space too. Shoes don't touch much metal, so there is not much heat
> transfer by contact. There is airflow around the drum, but not as much as a
> disk (drums nearly fill the space inside the rim). There is also much less
> fluid in a drum cylinder, so it heats up quicker.

This is all true -- I just wanted to point out above that there is still
lots of heat dissipation from a brake drum.

The shoes don't heat too much -- unless maybe they're these new stupid
metal pads.  I seem to remember though the last time I got my brakes
done that there were still asbestos shoes for the drums -- they only put
those stupid metal pads in the front disk brakes.  (Asbestos dust is a
nasty thing, but the general fear about it is absolutely insane!)

-- 
								Greg A. Woods

+1 416 218-0098;  <gwoods at acm.org>;  <g.a.woods at ieee.org>;  <woods at robohack.ca>
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