Porterfield Brake Pads

[jhoefer]

Time, experience, and heavy cars have resulted in multi-piston calipers and much more pad surface. There is no doubt a 4-piston Wildwood caliper with almost three times the pad area can exert more braking force than a dual piston ATE.

Pad area has no effect on braking force. It's piston area (and caliper mounting radius/rotor diameter) that effects braking force. Generally speaking, friction is independent of area. E.g., it takes the same force to pull a sheet of plywood across a floor whether it is face down or on edge. The larger pad gives you better heat distribution and less wear.

Wilwood makes a superlite 4-piston caliper that has nearly the same piston area as the ATE one, so you wouldn't notice any more braking force from using it, but less heat and wear would make it a better choice for racing. But they also offer that same caliper with piston areas up to 160% larger if you needed to rebalance your brakes that way.

[ONEBADBUG]

Ummm, that doesn't sound right.

[FJCamper]

Hi Jhoefer,

I think we're talking apples and oranges here. For instance, pure texbook physics tell us a tire that supports 500 lbs. can withstand a 500 lb. side load, but 501 lbs will make it break loose.

At one time, most engineers believed lateral 1g forces were all a car could handle. We now know we can do better than that, because the pure physics are modified by friction materials, i.e better tires.

Put the plywood edge on roller bearings and the textbook answer of equal effort flat or on edge is moot.

Hydraulic engineers have two basic ways of exerting disk brake clamping forces, small pads and high pressure and large pads and lower pressures, both modified by piston size.

Given the same hydraulic pressure, and equal piston sizes, a larger pad might not exert more clamping pressure, but the modifier is pad material ... just like tires were the modifier in lateral G forces.

When we say a larger pad can exert more braking effort, we're presuming it has sufficient pressure behind it. Cranking up the pressure on a smaller pad only leads to faster pad failure once the pad material thermal limits are reached.

FJC

[jhoefer]

Put the plywood edge on roller bearings and the textbook answer of equal effort flat or on edge is moot.

Lay the plywood flat and it will take the same force to pull over the roller bearings as it does on edge. What's your point?

Hydraulic engineers have two basic ways of exerting disk brake clamping forces, small pads and high pressure and large pads and lower pressures, both modified by piston size.

No, there's only one way of applying force and that's hydraulic pressure times piston area (F = PSI x A), the area units cancel: (lbs-f = (lbs-f / in^2) x in^2). Yes, a small pad will have higher PSI against the rotor than a large pad, but they will apply the same total force (given equal line pressure, piston size). It's that total force (Fn), not pad PSI that you use when calculating the force from friction (Ff <= u x Fn).

Given the same hydraulic pressure, and equal piston sizes, a larger pad might not exert more clamping pressure, but the modifier is pad material ... just like tires were the modifier in lateral G forces.

Well of course if you use pads with a greater coefficient of friction they will have greater braking force given the same clamping force. But the coefficient of friction has nothing to do with pad area, it's a dimensionless quantity.

You'll notice I never mentioned anything about pad material, I was assuming they were the same. Since you never mentioned anything about differences in pad material in your post, I'm not sure why you think I should have taken that into consideration in my reply.

Cranking up the pressure on a smaller pad only leads to faster pad failure once the pad material thermal limits are reached.

Yes. More pressure equates to more heat. And because the small pad has a higher PSI against the rotor than a large pad, it generates more heat per square inch even with the same pressure. Make the pad area really small and you've got yourself a chop saw.

[ONEBADBUG]

You're still in college, right?

[jhoefer]

Bit more than a decade since those days.

[PhillipM]

Well of course if you use pads with a greater coefficient of friction they will have greater braking force given the same clamping force. But the coefficient of friction has nothing to do with pad area, it's a dimensionless quantity.

It's only dimensionless if you're still in school and dealing with perfect physics.

Back in the real world, it varies with temperature, surface speed and area/pressure.

[jhoefer]

Well of course if you use pads with a greater coefficient of friction they will have greater braking force given the same clamping force. But the coefficient of friction has nothing to do with pad area, it's a dimensionless quantity.

It's only dimensionless if you're still in school and dealing with perfect physics.

Back in the real world, it varies with temperature, surface speed and area/pressure.

That doesn't make it dimensional (it never has a unit of measure, it's just a number). And I never said it didn't vary with conditions. It's a strictly empirical measurement after all. I don't know why you all keep thinking I should have addressed all these tangents that make no difference to the original point.

[yodogg]

I think the issue is more with the real world relevance of your original point. You may be correct in a very strict and abstract sense, but in real world scenarios the supreme relevance of such absolutes becomes somewhat more murky.

[Piledriver]

A modern set of calipers with huge pad area will also typically have much more piston area.
The pad area>piston area ratio also tends to be higher, so they are less loaded.

I think FJCamper was trying to make that point, at least.

[jhoefer]

A modern set of calipers with huge pad area will also typically have much more piston area.
The pad area>piston area ratio also tends to be higher, so they are less loaded.

I think FJCamper was trying to make that point, at least.

Quite right. And I know he was, I was just pointing out a small technical correction to his statement.

And thank you for replying without insulting me.

I think the issue is more with the real world relevance of your original point. You may be correct in a very strict and abstract sense, but in real world scenarios the supreme relevance of such absolutes becomes somewhat more murky.

The real world relevance is you pick your caliper by piston size, not pad size. That pad size generally goes up with multipiston calipers is a secondary benefit. Going back to my original Wilwood example, the 4-pot caliper with the same piston area as the ATE, all the way up to the 4-pot that has 1.6x larger piston area use the same pad.

[yodogg]

The real world relevance is you pick your caliper by piston size, not pad size. That pad size generally goes up with multipiston calipers is a secondary benefit. Going back to my original Wilwood example, the 4-pot caliper with the same piston area as the ATE, all the way up to the 4-pot that has 1.6x larger piston area use the same pad.

Jhoefer,
I didn't get the impression that anyone has been promoting the notion that calipers should be selected by the size of the pad they accommodate. I realize you were making a technical clarification, but from some of the responses it seems that intention may not have come across.

Peace

[Piledriver]

I think "the biggest pad that still fits on the rotor" is the right size, which is more or less inline with what has been said.
(assuming one can find a caliper that takes it with the right range of piston area)

One of the biggest issues hunting down suitable aftermarket (or junkyard) calipers for retrofit is getting the piston sizes is often hard work, either of the research or wrenching kind.

It often takes a lot of Googling to dig out the caliper/rotor specs of $random victim vehicle.
(and even then it can be a multiple choice question)

[ONEBADBUG]

Hey Mr. Hoefer, I wasn't being mean. You reminded me of when I tried to have conversations on the FSAE forum. Those kids were so sure something they just learned was all there was to it. They would say crap like "you can't go fast enough in an autocross for wings to matter". Check my avatar, what would I know, only been doing it for decades and beat the fastest guy in the world.

Anyway, you got me thinking about something. This event it was quite cool. We used up ~25% of our brake pad. So does that mean that if I reduced the pad area by cutting a groove in it, it would improve the brake performance?