I thought SA vans used 215/65-15 Tires... these would be
within 1% of the rolling diameter of 185/82-14 tires that are
stock wouldn't they?

Thanks,
Chris

---- Original message ----
>Date: Wed, 6 Nov 2002 14:52:16 -0800
>From: pensioner <al_knoll@PACBELL.NET>
>Subject: Re: Drilled Rotors vs. Big Brake Kit
>To: vanagon@GERRY.VANAGON.COM
>
>The engineering reason for the SA large brakes with the 15"
wheels and
>larger tires has to do with the "effective" braking force.
Larger diameter
>tires require a proportionately larger retarding torque on
the wheel to
>generate the same stopping force.  Putting 215x75-15 tires
on a standard
>syncro westy produces a loss of stopping power as the
braking force has to
>work through a larger "moment arm" (distance from axle
center to road).  A
>few definitions help here:  Braking force for this instance
is the torque
>applied to the axle by the brakes multiplied by the distance
from the hub to
>the center of the pad.  Stopping force is the force
generated at the road to
>tire interface that actually declerates the van.
>
>For a given braking system increasing the tire diameter will
decrease the
>stopping force.
>
>To compensate, VWSA added more braking force via a larger
disk and caliper
>assembly.
>
>Drilling rotors actually has very little effect in terms of
increasing
>braking force.  It does allow slightly better cooling over
the solid rotors
>and slightly bettter wet performance.  The SA vented rotors
are far superior
>in terms of heat dissipation characteristics than the
smaller solid rotors,
>drilled or not.  Drilling can postpone brake fade but not
really increase
>the braking force by any significant amount.
>
>The pad to disk interface describes a cylindrical swept area
on the disk
>itself.  This interface generates heat and the function of
the pad is to
>continue to provide contact friction as the temperature of
the interface
>rises.  At some elevated temperature the pad material will
break down and
>exhibit a "glazing" or "carbonizing" characteristic that has
a poorer
>pressure to friction coefficient.  For the same pad pressure
the failed
>material produces significantly less friction and therefore
less heat.  The
>heat being dissipated by the thermal mass of the disk and
its configuration
>is how the energy of motion of the van goes from kinetic to
heat slowing the
>van.
>
>Are you listening, Dr. Feynmann?  How'd we do?
>
>"fizzix, it's always just fizzix"
>
>pensioner (start slow then taper off)