[200q20v] RE: Brake clamping forces

[QSHIPQ at aol.com]

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.  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.  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).  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.   It is common and accepted practice that more pistons provide 
higher pad clamping forces AT the rotor.

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.  And as a general rule, the larger the pad area, the more 
likely you'll see 4pots to evenly squeeze them.  G60's seem to be another 
outstanding exception. 

Alas, as I posted, incomplete, I guess I should have added "at the rotor".  
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