From: bense@oasys.dt.navy.mil (Ron Bense)
Newsgroups: rec.autos
Subject: Re: front disc brake question
Date: 10 Sep 92 12:34:12 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 <24790@oasys.dt.navy.mil> bense@oasys.dt.navy.mil (Ronald Bense) wri
>tes:

>    the temperature at which the fluid boils.  The pressure in the
>    brake line should be high enough to prevent the brake fluid
>    from boiling.

Hadn't considered this part, makes sense, but isn't what I've been told
since a was a knee-biter. :) Could have been mis-information on   my
part, <sheepish grin> but am not going to go immediately over and embrace
your ideas here without some further thought.

>#2) If the brake fluid did boil, and there were no leaks in the
>    system, then the brake pedal would be pushed upward due to
>    the volume expansion of the system.  Has anyone ever seen
>    brake-fluid forced out of a brake system because it came to
>    a boil and the internal system pressure exceeded the strength
>    of fittings/lines/etc?

Haven't seen this, but you should know that as metal gets hot, it expands
quite a bit, esp versus volume at the smal dimensions we're talking
about wrt brake lines.

>I have been told that drum brake fade is due to heat-induced
>expansion of the outer edges of the drum, which increases the
>distance between that part of the drum and the brake shoe.
>This leads to a reduced braking surface area, and hence more pressure
>must be applied to the brake pedal to maintain the same degree of
>braking.  This is believable, and boiling fluid is not.

True, this is believable, although the T rods, etc also get hot, and,
being made of the same material, would negate the clearance effect.
SOmething I hadn't thought of, however, is that the radius of the drum
and shoe might change, as the drum would expand along a new circular
radius while the pad would not due to the pad's influence to maintain
it's old shape since it has a different coefficient of thermal expansion.
Seems a simple fix would be to have a segmented pad, allowing the two
circumferences to remain the same. I'm sure that this has been tried,
and wonder if the results were any different.

>#3) What about the bad points about disks?
>     -> exposure of the disk leads to trapped material between
>        disk and pad

Very unlikely. Hasn't happened in over 300,000 miles of driving, or
at least not to the point I would notice. :)

>     -> corrosion is easier

YOu would think so at first, but in actuality, drums have a more dangerous
corrosion pattern. Discs have most of their wear and corrosion occuring
in the same place, the outside faces of the disc. The constant wear
this face receives pretty much negates any deep pitting or weakening
of a local area. Drums, however, have their primary corrosion surface
on the outside while the primary wear surface is on the inside, allowing
for corrosion to create weak spots in the drum through pitting.

>     -> water puddles / wet roads / generally wet conditions
>        can interfere with braking

Generally (and this is a large amount of common sense extrapolation
of personal experience :) this is not really the case, at least no more
for discs than drums. Discs are gernerally warm, and quickly evaporate
any water that gets on them while driving. If you enter a puddle deep
enough to immerse them, then it is a very short time before the brakes
are fully functional again, as drying is very rapid. Drums, under the
same condition, with my limitted experience, appear to take much longer
to dry out, probably because of the restrictive nature of the drum itself
to free air flow.

>     -> comprimises and sacrifices are being made on the altar of
>        "unsprung weight" leading to weaker designs

There are no weaker designs, perhaps poor choice of materials, but overall
no inherently weaker design. There is as much metal actually transferring
the stopping power to the car as in drums. It's that bit of metal that
the lug bolts go through.

>     -> Saftey or parking brake setup is more complicated

True, on 4-wheel disc. It generally entails (at least on mine) a second
set of rear drum brakes, which are supposedly good for *one* stop at
speed, although I've never tried it. :) Not a real disadvantage, imo,
since the reason you're using it is because of some failure in the primary
system. Why would you want to try and activate it by some other means?
Just use a secondary brake system. Seems much safer to me.

>I have yet to be in a panic braking situation (ie 60 to zero), but
>I will try it out (safely) and I will report on this fading effect.
>I must say that, having driven front disk / rear drum cars, I
>much prefer the solid feel of 4 wheel drum to the mushy feeling
>of front disks...

If front discs feel mushy, you need to bleed the system. :) They are
quite solid on mine, although I have power assist (much prefered over
the no-feedback full power systems:) and the feedback I get most now
is the pulsing of a bent wheel. :(

BTW, I didn't say panic braking, just heavy hard braking for a continuous
time period. Driving on the beltway around here is probably similar
to slow-speed Gran Prix track driving, as far as braking goes, as generally
some moron going the bare minimum (or lower) speed limit zips 2 lanes
left at a right angle and forces heavy braking of an entire line of
cars going 80 at regular intervals. (OK, OK, so I exagerated, they were
only going 65:) If I lived in a normal area, my brakes would probably
last twice as long, between 55-60k.

Ron

Potassium Benzoate included as a preserver.
Newsgroups: rec.autos