<VV> power brakes

[AngryCorvair@aol.com]

In a message dated 9/6/2004 2:38:04 PM Eastern Daylight Time, 
charliemorgan@comcast.net writes:
Nick, boosters are often necessary when disc brakes are used.  Duo-servo 
drums brakes (like those used on Corvairs) are self-energizing and require 
much less pedal force to create the same braking torque created by disc 
brakes.

Charlie Morgan
Beaver State Corvairs
Ding Ding Ding we have a winnah!   a disc brake is not self-energizing, so it 
requires higher apply force than a similarly-sized duo-servo drum brake to 
generate the same brake torque.   force equals pressure times area, so the disc 
needs higher pressure, more area, or a combination of the two, to get the 
required apply force.   so the caliper piston is larger diameter than the wheel 
cylinder, which drives up the volume requirement of the master cylinder.   
master cylinder volume can be increased by larger bore or larger stroke.   larger 
stroke means longer pedal travel, so we don't like to go that way.   so, we 
increase the bore of the master cylinder, which means that we have LOWERED the 
output pressure of the master cylinder for a given pedal force.   the only ways 
to get the master cylinder output pressure back up are to either increase the 
mechanical advantage of the pedal (again at the cost of increased pedal 
travel), or to install some kind of force multiplier between the pedal and the 
master cylinder.   *that's* why disc-brake cars have boosters.

please remember, if your brakes are sufficient to lock up your tires, you 
don't need any more brake output!

patrick aka AngryCorvair@AOL.com
brake systems engineer for about a decade now...