From: rat@erg.sri.com (Ray Trent)
Subject: Re: disc brake question
Reply-To: rat@erg.sri.com (Ray Trent)
Organization: SRI International, Menlo Park, CA
Date: Fri, 11 Sep 92 18:23:56 GMT

In the referenced article, wlsmith@valve.heart.rri.uwo.ca (Wayne Smith) writes:
>>better brakeing action.
>
>All I ever see is this kind of arm-waving!
>No-one has shown that heat dissapation affects the braking action,

Plenty of people have done that. It is, however, rather difficult to
explain why the coefficient of kinetic friction generally goes down
with increasing temperature to a layman. The best simple explanation I
can think of is that the faster the molecules are bouncing around the
harder it is for them to get a grip (but that's a pretty pathetic way
of putting it).

Maybe this would be a helpful analogy: Every last bit of power that
your brakes exert to slow down your vehicle ends up as heat. If
there's no place to put that heat, it is intrinsically impossible for
the brakes to slow down your car, due to conservation of energy. You
*must* get rid of that heat somehow. Disk brakes simply do this much
faster and more efficiently than drum brakes. The fact that the better
disk brakes are *vented* (i.e. have holes bored through the disk)
should be a clue that simple area of brakes surface is not the most
important factor.

>Has it ever occurred to anyone that you have more friction surface on
>shoes than on disks?

True, but again, the important factor is not surface area.  However,
when calculating the swept area of disk brakes (which is the important
feature, not how much disk is in contact with pads at one time...the
energy dissipated is the integral of the force with respect to
distance), it seems like you have forgotten that the disk has 2 sides. 

>Again I ask what do planes/trains/big trucks use?

Well, I would guess (though I don't actually have real knowledge) that
planes use disk brakes, because they have had ABS for longer than cars
have, and it's considerably more complicated to make drum ABS
systemss. As for trains, railfans will have to answer that one...the
one's I've seen don't look like either disk or drum brakes. Once you
start getting to really large wheels, you start to have room to make
drum brakes that allow for more ventilation as well, so it may be that
disks lose much of their disadvantage.

>>come with four wheel drums?  Why don't the buyers insist on them?
>
>Why don't buyers insist on full safety glass (on all windows)?

Because the government does it for them? Unless you think there's
something inherently unsafe about tempered glass on side and rear
windows (where flying pellets aren't really likely to get into the
driver's eyes).

>>But I have had several drums turned in my time.  When something gets
>>caught in a drum there is no where for it to go.
>
>True (about the drum).  But I still can't figure why (it seems) so many
>people need their rotors turned...

Mostly because some people are stupid enough to let their brake pads wear
down to the point where the calipers start grinding the rotor.

>>1. The rears don't do that much of the work,
>That's how they design them.  Why?  I don't know.  No car engineers 
> (apparently) want to tell us what's going on.

Huh? Rear brakes don't do as much work because people normally drive
in the forward direction. When you apply the brakes a lot of weight
(and therefore gripping ability) transfers from the rear wheels to the
front wheels. If the rears weren't designed to do less work than the
fronts, they would lock up long before the front wheels did and you'd
enter a rather nasty spin at the slightest provocation.

>>and 2.  They are CHEAPER!
>Again more arm-waving (how can they be cheaper? They've got more parts.)
>you've forgotten about the mechanical saftey/parking brake setup too.

The reason they're cheaper is that a) they operate at lower pressure
(because the surface area of the shoes is larger), b) there're more
room to play around with, c) it's harder to design something that
reliably squeezes both sides of a disk than simply pushes out from a
braced central point, and d) as you've pointed out yourself they have
to put drums on the rear anyway for the parking brake. (Have you ever
wondered why this is? If it was so easy to design disk brakes, why
would it be so complicated to use them as a parking brake?)
-- 
"When you're down, it's a long way up
 When you're up, it's a long way down
 It's all the same thing
 And it's no new tale to tell"                      ../ray\..