Date:         Thu, 19 Aug 2004 11:35:41 -0700
Reply-To:     Al Knoll <al_knoll@PACBELL.NET>
Sender:       Vanagon Mailing List <vanagon@gerry.vanagon.com>
From:         Al Knoll <al_knoll@PACBELL.NET>
Subject:      Brake stuff
In-Reply-To:  <200408190834.i7J8YwSA028876@mtac3.prodigy.net>
Content-Type: text/plain; charset="iso-8859-1"

Disk brakes work something like this:

The pad is a thermal insulator with a reasonably high coefficient of
friction when matched with a 'smooth' surface.

The disk is usually a thermal conductor designed to rapidly dissipate energy
in the form of heat.

The MV^2 energy of your car is converted to heat where the pads pinch the
disk.  This heat 'flows' into the disk material (Cast iron is pretty good)
where it is transferred to the surrounding air.

If the braking demands are severe enough the disk can't pass enough heat to
the air and heats up severely.  If it gets hot enough it can change the
composition of the pad at the pad/disk surface such that the stopping
friction is severely reduced ("glazing" is a term sometimes used to describe
this change)  Note the overuse of 'severely'.  If the braking force
decreases the driver usually applies more pressure to the pads to try to
regain the braking.

If the system cannot produce more retarding force in the form of friction
the braking power is 'severely' reduced and you exit into the crumblies or
worse.

The pad isn't supposed to transmit heat to the pad carrier but under overuse
conditions will do so and raise the temperature of the caliper to such a
degree that the water in the brake fluid turns to steam and braking force is
reduced even further.

Brake fade is when any of the above mentioned phenomena reduce stopping
force per unit pressure on the brake pedal.

The key to all this stuff is moving heat away from the pad/disk interface.
Ventilated disks create a radial airflow between the surfaces greatly
enhancing braking efficiency.  Crossdrilling and any other surface
modifications do not increase dry braking performance assignificantly as the
airflow in the axial direction is much less and the swept area surface is
continually heated by the friction of the pads.

To improve the braking, ventilate the disks, use larger disks, use pad
material with a higher friction coefficient at high temperatures, use a
higher temperature brake fluid kept scrupulously clean and fresh, or throw
out those extra goodies from under the seat to reduce the M part of the MV^2
energy.

Some braking systems are designed for short term, high braking force but
fail under sustained braking such as descending Townes Pass on a 110F day
with kids, dogs, kayaks, dutch ovens, bicycles, extra water, four sets of
silverware, a full cooler, hiking, camping, photographing, fishing, skiing,
and other necessaries in the van.

It's fizzix, it's always just fizzix...Feynmann