Brakes G60's vs UFO's and other combos............

[Alan Cordeiro]

1) Heat energy and temperature rise:
The heat energy from the converted kinetic energy (1/2.MVsquared) of the
vehicle
will end up in the brakes. Small seconds of delay will not appreciably
affect the final
temperature. What really will, is the THERMAL CAPACITY of the rotor + pads,
since the materials are similar enough (not like carbon fiber) it boils down
to the differences in the MASS of the two rotors.

A small rotor will run hotter, and be more likely to fade under repeated
hard
braking e.g. under track conditions. Older brake fluid (with extra moisture)
will tend to get hotter, and may require a quick bleed to get them back to
normal.

2) Concerns with moving away from carefully tuned/calibrated factory
systems:
  (the real reason I wanted to speak up)
All these discussions have not addressed the concern of BRAKE FORCE
DISTRIBUTION, critical to shorter braking distances (tires, Mu, etc being
equal).
brake system designers spend tons of design and test time, with
sophisticated
rear brake force distribution modulators, to ensure better distribution of
the
brake forces under the unusual weight distribution that occurs when a car
is in hard braking mode. (Weight transfers from the rear to the front
wheels,
and hence the force on the rear rotors needs to be reduced).

If the front wheel calipers and rotors are changed, the BRAKE FORCE
distribution may not match the weight distribution (or the rear modulator's
adjustments) and hence provide inferior braking results even in ONE STOP.

ABS sytems will begin to release calipers that begin to lock rotors, to try
and
compensate, but their effect is a two edged sword. A chattering ABS braking
stop is not doing as good a job as it could do if it were controlling a well
balanced
front/rear brake system, which would give higher braking force before it
slips

Putting a huge diameter piston caliper and larger diameter rotor will give
better
braking force up front for the same pedal force, but the rear braking force
will be
reduced (since the pedal force is reduced), and hence the overall brake
distance
is longer, since the ABS will have to take over earlier if the force is
sufficient
to cause it to activate.......(front wheels lock up... remember, same tires)
Putting an undersized front caliper-rotor in front will do the opposite, but
the overall
result is the same, "poorer" braking.

The best braking will be obtained if all wheels are held to just under 100%
of the
point where the tires will begin to slip. the engineers at the factory in
Ingolstadt
spend many months trying to get it to be the best....understand exactly what
you
are changing, and try and do it in such a way that the front/rear braking
force
distribution remains close to what it was (unless you think you can do it
better..YMMV)

Sorry for the LONG burst....I didn't expect it to take so long...or so many
words

Alan
(yes, you guessed it, original UFOs..)

----- Original Message -----
From: "Bernie Benz" <b.benz at charter.net>
To: "Dan Cordon" <cord4530 at uidaho.edu>
Cc: "200q20V mailing list" <200q20v at audifans.com>
Sent: Tuesday, February 17, 2004 11:49 AM
Subject: Re: Brakes G60's vs UFO's and other combos............


> Think harder, Dan.
>
> Your lesson in "physics phacts" is irrelevant to the point of the
> discussion, that being the relative temperature rise of the braking
> components.  Neglecting the very slight additional heat loss during an
> extended breaking time, the temp rise is independent of time, in the
> practical sense.  Your erroneous initial point, and one that you have been
> backing away from since, was that the temperature rise is not a function
of
> the kinetic energy absorbed, but rather the rate of absorbption.
>
> Bernie
>