Hey Larry - Brake Pad Size?

[Steve Simmons]

Since Larry is our local brake expert, maybe he (or anyone else) can answer this question. I've seen a lot of argument over whether or not surface area on a brake pad affects braking power. It seems to me that increasing brake pad area would make little difference because the force applied is dispersed over a larger area, which decreases the force per square inch applied to the disc, while the added area would certainly increase friction and perhaps make up for the loss of pressure.

Any thoughts or definitive answer on this debate?

[Larry Kluss]

Steve, you are correct that pad surface area has little (if any) affect on brake stopping power (torque). When dealing with brake torque calculations, the amount of caliper clamping force is expressed in pounds (lb), not pounds/sq. inch (PSI). So the pad area is not a consideration. It is assumed that the pad area is large enough so as not to yield from the compressive forces, and that area is surprisingly small. Brake pads are normally sized (volume wise) based on the energy load requirements for the intended vehicle so they do not wear out too quickly, nor last forever.

Without getting too much into really boring technical stuff, the effective radius of the pad to rotor interface is one primary factor in determining brake torque. The effective radius is roughly the middle of the brake pad when measured from the spindle center. Another, even bigger, factor is the coefficient of friction of the brake pad material itself.
As you know, I run the factory brake calipers and rotors on my (not exactly stock) MGB, with EBC Green-stuff pads. Nothing fancy. They work very well and I've haven't faded or boiled them, even after getting them quite hot during spirited canyon runs (chasing a certain MGC-GTS down Tuna Canyon comes to mind) and autocross. Of course there are a lot of other factors to consider in extreme applications, but I won't go into them here.

Always keep in mind that the single most important factor in stopping a vehicle is tire adhesion! Virtually every vehicle I know of has enough stock brake capacity to lock up its tires, even modern aftermarket tires.

This is probably way more info than needed (or wanted) to answer your question, but I thought I'd throw it out there anyway...

Larry

[Steve Simmons]

That's exactly what I was looking for, thanks! I agree that the brakes on an MGB are more than adequate for the job. Even with 195 tires on my '65, I can lock up all four wheels although I must admit it takes a lot more leg effort than with the old skinny tires.