From: hugo@bluehau.bubble.org (Hugo Bueno)
Newsgroups: rec.autos
Subject: Re: disc brake question
Keywords: brakes
Date: 12 Sep 92 23:07:26 GMT
Organization: The Blue House

wlsmith@valve.heart.rri.uwo.ca (Wayne Smith) says:
>In article <1685F800D.R1004@vmcms.csuohio.edu> R1004@vmcms.csuohio.edu writes:
>>Heat.  The heat is then dissapated into the surrounding air by the
>>components of the brakes.
>> 
>>    Disk brakes have a higher rate of heat transfer, the exposed
>>rotor surface not in contact with the pads acts as a radiator.
>>For a given area of braking surface disks are more efficient =
>>better brakeing action.
>
>All I ever see is this kind of arm-waving!
>No-one has shown that heat dissapation affects the braking action,
>but sure, high heat can either dammage/deform parts and/or boil the
>fluid (both of which I really really doubt happens in drum systems
>for the average driver).
>(Don't send me e-mail, like I've already gotten, that you drive on
>race tracks and get your brakes hotter than spit and all the rest..)


Wayne, you are obviously ignorant of things mechanical
and cars in particular.  Brakes work by converting the kinetic
energy of the car into heat.  How else do you think the energy
can be dissipated???  Magic?  The more heat the brake system can
dissispate, the stronger the braking action.

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

On drum brakes, only one of the shoes does any real work.  This is
due to an effect called self-locking.  Get a machine design
book and do some reading.  Anyway, I'd wager that the contact
area of the one active shoe is equivalent to the contact area
of 2 pads on a disk brake.

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

>>    In most production cars the pads ride on the rotors ALL the
>>time.
>Because they don't want to put hold-back springs on them.
>(A case where "if it works without them, we won't put them in")

Why bother with springs?  If you're not pressing the brake
pedal, the pads aren't exerting any force on the disk.

>>The drag is minimal, prevents buildup of corrosion on the
>>rotor, prevents crap from getting stuck between the pad and the
>>rotor.
>
>And it also partially negates the cooling effect.
>Who knows, I could probably say that, because of the constant contact,
>they are no better off than the concealed brake surface of drums?!
>(If you can CONJECTURE, so can I)
>
>>   If the ONLY reason disks are put on cars is because they are
>>cheaper and easier to assemble why are four wheel disks an expensive
>>option on some cars
>
>Because they have to add a second set of rear drums for the safety brake,
>or add a complicated mechanism to the rear disks for the saftey/parking
>brake.
>
>>and standard on the VERY expensive sports
>>cars?  If the drums are superior why doesn't the Corvette ZR-1
>>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)?
>Why don't buyers insist on stronger seat-backs (so they don't end up
>crushing someone in the back seat when they get hit from behind)?
>(I'm not, at this point, saying that every one should go out and
>ask for drum brakes.  The (above) point is that there are a lot of
>short-comings on cars that people don't know about, and the only way to
>get them changed is if *A LOT* of buyers insist on it).


HAHAHA!  Disk brakes are a shortcoming????  HAHAHAHAHAHA!!
Oh man, you're not making a good case for yourself here.


>All I'm doing here is asking for more than arm-waving about heat
>dissapation as to why disks are better than drums...
>
>>  I've worked on about five cars with disk brakes, Never turned a
>>rotor.  In fact my old Chevette in the 145,000 miles I had it never
>>had a rotor turned (It had several calipers because the design sucked).
>>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...
>
>>  Disks are better.  Period.
>
>I guess the above sentence is the "politically correct" way to argue
>the point?!
>
>>The only reasons most cars still have
>>drums on the back are
>
>>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.

The majority of the braking is done by the front brakes.
The dynamic weight transfer that occurs during braking has
the effect of concentrating more of the car's weight up front.
That is, the front tires and brakes HAVE to do most of the
work.  The normal force on the rears (weight) is reduced to
the point that really good brakes don't help that much more.
In my opinion, rear disks are a nice extra, but not
absolutely necessary.


>>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.

I would tend to agree that disks are slightly more expensive. But less
effective? No way.
-- 
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Hugo Bueno  |Delphi  : MRGOOD  CIS: 71211,3662
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