[200q20v] Re: Brake clamping forces

Bernie Benz b.m.benz at prodigy.net
Thu Jul 5 09:20:04 EDT 2001

Great comments on the subject.  Thanks.

> From: GSTREIN at aol.com
> Date: Wed, 4 Jul 2001 12:08:48 EDT
> To: 200q20v at audifans.com, audi20v at rennlist.org
> Subject: Re: [200q20v] Re: Brake clamping forces
> This topic is being confused by physics.  The clamping forces may be the same
> yet the impact the force has on the pad size (dimensions) may be quite
> different when more pistons are present.  An evenly applied force which
> covers more area of the brake backing plate will hold the brake pad against
> the surface of the rotor with very little distortion to the pad and thus
> greater contact area - leading to greater friction and more dependable
> stopping power.  Consistent brake pad contact surface area is what counts
> when racing.
> The high performance market has very good reasons for more pistons and
> monoblock caliper designs.  At high speeds it takes very little time to bring
> the brake rotors/pads surfaces up to a extreemly high temperatures (you will
> see the rotors/pads literally glowing).  When the brakes are applied in these
> conditions the pad, caliper and rotor will all tend to distort.  Using thick
> rotors and rigid calipers (monoblock being more rigid than most 2 piece
> units) you reduce this distortion/warpage.  The more pistons the caliper has
> will keep the pads pressed evenly from edge to edge (less distortion than one
> with fewer pistons) and thus will be less likely to distort as much and more
> likely to give more contact area and more friction.
Not a universally true statement.  This is true if the braking system design
employs long and narrow pad surfaces.
> Even with the UFO large piston if you were to race this application you would
> more than likely upgrade to a multi-piston setup.
Not and retain the UFO disk!  You would upgrade to a different brake system,
which might well employ multiple pistons and dual sided calipers.

> The large piston works
> fine for daily or even moderate brake use but if you were to measure the
> contact pressure at the ends (leading and trailing edges) of the pads you
> would see that it would not be less than the center which is directly under
> the large piston.
I understand the point that you are trying to make, but believe that you
picked a bad example.  In fact, the center of the UFO pad is further from
the direct support of the hollow piston than are the outer edges.  For the
unique UFO pad geometry, the single piston is optimum in maximizing piston
area to pad area ratio.

> As the temps and distortion increase the pad contact will
> continue to be reduced. In addition, the gases taking the least path of
> resistance out from under the pad will "float" the pad off the rotor surface
> in the areas of least contact pressure or the areas of greatest distortion.
> If you replaced these UFO Calipers with performance multi-piston units that
> covered the pad backing plate more completely you have better braking
> performance due to less distortion and more consistent contact of the pad to
> the rotor surface.
The points that you make are a commonly accepted, and IMO. a simplistic
theory of disk brake dynamics, which in actual conditions are tempered by
additional factors not therein considered.  Assuming that backing plate
distortions are a significant factor (?), these deflections will easily be
within the elastic limit of the backing plate material, i.e. elastically
proportional to the applied load.  Thus, maximum braking load, causing
maximum backing plate deflection, will quickly wear the pad contact surface
flat for that condition.  Lesser loads, causing lesser deflections, will
cause the now distorted, not flat pad contact surface, to contact the disk
with less than full contact area, which will result in a contact pressure
over this reduced contact area approximating that resulting from maximum
load.  A self adjusting system.  IMO, after the "break in" period, pad wear
makes backing plate distortions a trivial matter in a well proportioned
caliper/pad design.

If pad out gassing were a significant factor, IMO its not, because the total
pad surface is almost never in intiment contact with the rotor for the above
reasons, all pads would be supplied with a segmented surface, cross hatched
saw cuts.  Cheap, if of any advantage.  A simple experiment, try it.

Again, in the absence of a cost constraint, the number of pots per caliper
side is/should be based on maximizing the piston area to pad area ratio.
The absolute number is unimportant.

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