[200q20v] Re: Brake clamping forces

Bernie Benz b.m.benz at prodigy.net
Tue Jul 3 12:23:25 EDT 2001

Hi Scott,

I thought this thread would get you up, out of the lurking position.

> From: QSHIPQ at aol.com
> Date: Tue, 3 Jul 2001 08:09:17 EDT
> To: b.m.benz at prodigy.net
> Cc: audi20v at rennlist.org (audi-20v), 200q20v at audifans.com (200q20V mailing
> list)
> Subject: RE:  Brake clamping forces
> Bernie:
> Agree totally with your Newton's Law example, however, in practice I doubt
> you will ever find a dual piston caliper exerting more clamping force than a
> 4 piston caliper.
I don't give a clamping force advantage to either single or double sided
calipers.  They are identical!

> There are many reasons for this.  One of the biggest is
> the larger and/or more even distribution of the clamping forces on the brake
> pad backing.
You have limited this comparison (above) to basic differences between 2 vs.
4 pot calipers, but its really about single sided vs. dual sided.  2 vs.4,
or 1 vs 2, is not a controling criteria on force distribution in the design
of either style caliper.
> Another is control of the "back" pad in terms of clamping it to
> the rotor vs clamping it to the outside of the caliper

> (remember, G60's, BR and many other calipers have 2 piston sizes, think of
>what that means when you are pushing against a flat surface).
A "huh?" to this point also.  The leading piston is of smaller size because
the pad reaction pivot point, on the backing plate, is behind the pad
contact line. Thus, this reaction force increases the pad contact force on
the leading edge.  Hence, the smaller leading piston tends to equalize pad
wear, front to back.

> And so is distortion of the caliper
> itself.  Even within the 4piston calipers (of the same mold design) this is a
> problem.  The main reason Porsche wouldn't use the Brembo Red caliper "off
> the shelf", and has since adopted the monoblock design towards brake
> calipers.
All calipers, having to bridge the rotor, are subject to cantilevered loads,
and thus deflections tending to "open" the caliper.  As always, some designs
are better than others, in this and other respects.  These caliper
deflections force the clamping force to be concentrated toward the outer
edge of the pad (UFOs the inner), which is not a sugnificant detriment to
braking torque, only to uneven pad wear, inside to outside.

> It is common and accepted practice that more pistons provide
> higher pad clamping forces AT the rotor.
Now you're counting pistons, not comparing single sided vs. double sided
calipers.  Would you have good thoughts about a 4 pot, single sided caliper?
I believe that your statement is a common and accepted belief among DIY and
Pro wrenches and some racers, but not an engineering fact.
> The reasons you stated for 4 piston calipers doesn't seem complete to me.  4
> piston calipers are commonly used on high performance (or severe duty)
> applications.  
There are pros and cons to single vs. dual sided calipers in addition to
your stated, IMO, trivial and untrue inherent difference in clamping force.
In racing applications, where cost and longevity between overhauls are not
factors, the dual sided caliper system can be/is made of lighter materials
and can/must be rigidly bolted to the axle housing.  Its 4 vs. 2 high
pressure seals and dust boots are a negative for minimum long term
maintaince relative to the addition of caliper centering slides on the
single sided caliper.

> And as a general rule, the larger the pad area, the more
> likely you'll see 4pots to evenly squeeze them.
With either caliper type, the rotor is a great isolator between sides, and
you are only talking two pots per side. "to evenly squeeze them" is trivial,
not the engineering design challange.

> G60's seem to be another outstanding exception.
Not IMO.  G60s are just a typical, low cost single sided caliper design.
Two pots are good, but only marginally better than the single pot, single
sided UFO caliper, which has an identical piston area.

> Alas, as I posted, incomplete, I guess I should have added "at the rotor".
"at the rotor" would have only added another point of disagreement, which
I've already covered.

Regards, Bernie

> Thanks for the note.
> SJ 
> Bernie writes:
> Scott lectures as follows:
> "Remember, if you put 800psi on 2 pistons vs 4, you have more clamping force
> on 4 piston calipers."
> Not so, gerneraly speaking Scott.  You're not being specific on your caliper
> design detail.  A two piston caliper generally implies two pistons on one
> side of a single sided caliper.  Likewise, a four piston caliper generally
> implies two pistons on each side of a dual sided caliper.
> If you are imbracing this convention, your statement is wrong!  A single
> sided caliper of given piston area will produce an identical clamping force
> per unit applied pressure as will a dual sided caliper having the same
> piston area per side.  If you don't believe this, consider the following
> simplification.
> What is the difference in contact force if you were to push on a rigid wall
> with 10 lbs. force, vs, your pushing against my hand, which is actively also
> pushing back with an equal and opposite 10 lbs. force?  None!  The contact
> patch doesn't know/see any difference,  Conclusion:  The advantage of dual
> sided calipers over single sided calipers is trivial, being only a function
> of caliper mounting and centering differences, and possibably weight.
> Regards,  Bernie

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