From: welty@cabot.balltown.cma.COM (richard welty)
Newsgroups: rec.autos
Subject: Re: disc brake question
Date: 12 Sep 92 22:56:57 GMT
Organization: New York State Institute for Sebastian Cabot Studies

In article <1992Sep11.175338.6606@julian.uwo.ca> wlsmith@valve.heart.rri.uwo.ca (Wayne Smith) writes:
>In article <1685FABC8.R1004@vmcms.csuohio.edu> R1004@vmcms.csuohio.edu writes:
>>The brakes are a HEAT SINK, if they can dissapate heat faster
>>they will work better.  If the brake works better there is more
>>stopping power.

>MORE ARM WAVING!
>How do you know that the brakes (pad/shoe material and/or drum/rotor
>contact surface) can't function above a certain temp?
>How do WE know what temp (short of melting or plasma) will cause
>some kind of effect on braking!

>Post facts, or discuss the theories of how the brake material
>changes with temp, and how those changes affect braking ability.

the maximum viable operating temperatures of various friction materials
are well documented; i don't have the stuff right in front of me at
the moment.  typical street compounds start to break down at maybe
400 degrees F; racing compounds may not start working right until
up over 300 degrees F.  there are many pad formulations which try and
split the difference, with varying degrees of success.  i can post a
listing of typical temperatures tomorrow, perhaps.

when overheated, the pads will start to break down, as the binding
materials inside give up the fight, as it were.  partially disintegrated
pads are not a pretty sight.

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

>>Since disks are more efficient the surface area need not be
>>as large.

>How about because they CAN't put larger pads on a disk system, unless
>they use larger diameter rotors!

according to Puhn, for a given weight of brake, disk brakes will
always have _more_ surface area than drums.

pad size is generally result of an engineering decision (hey hey hey --
just like shoe size.)  larger diameter rotors are not necessary, although
for many purposes, large rotors are good as increases in swept area
result. in fact, for a given diameter and weight, disk brakes will
_always_ have a greater swept area.

in estimating braking performance, Puhn suggests that swept area per
ton of gross vehicle weight is a good gauge; this leads to the inevitable
conclusion that for a given size and weight of disk unit, disk brakes are
_always_ _more_ _effective_ than drum brakes.

>>>why (are) disks are better than drums...

>>Because the brake system has a finite capacity to transfer heat.  If
>>the hardware cannot dissapate the heat as fast as it come in the
>>components will heat and eventually overheat.

>>As the components
>>heat up they loose effectivness.

>AGAIN more arm-waving.

but _true arm waving_; see the other posting i sent out under the
subject line of ``them's the brakes'' for a discussion of why heat
is your enemy.

but as a reminder:  heat causes pads to emit gas, which contributes
to fade (more so with drum brakes; again, this is addressed in my
other posting); heat may cause boiled brake fluid in high stress
environments; in drum systems, heat will eventually cause expansion
of the drum, leading to fade due to reduced effectiveness of the
servo action.

richard
-- 
richard welty                            518-393-7228
welty@cabot.balltown.cma.com
  ``if you can read this, mario, you're too close''
    -- bumper sticker seen on a CART safety truck
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