Koda

For 67-72 Olds A body disc brake, they went to single piston floating caliper in 69 across the board, other than Corvette, I think. Maintenance and longevity were better, but, is it known about performance and how it was affected by the change?

I suppose I could do math on piston area x 4 vs new piston x 1 and see which had more area to know.

joe_padavano

The four piston calipers used 2.058" diameter pistons. Since they oppose each other, only the area of two of them matter. Each is 3.326 in^2, or 6.652 in^2 total. The single piston is 2.953" diameter, for 6.849 in^2, so slightly more piston area, slightly more clamping force at a given line pressure. That wasn't the real reason for the change, however. The biggest problem with the four piston calipers was that since they were fixed, any brake rotor runout caused the pads to push the pistons back into their bores, resulting in unacceptably large amounts of pedal travel (and an OH $#!+ moment) when you pressed on the pedal - particularly in a panic situation. Things worked great when the parts were brand new, but as the brakes aged, rotors started to warp. The sliding calipers follow any runout and thus don't have this problem. Of course the other problem with the four piston calipers is that the seals were on the pistons and slid in the cast-iron bores. Any pitting from water in the brake fluid chews up the seals and causes leaks. Repair required replacing the caliper castings (sleeving wasn't really a thing in the late 60s). The single piston calipers flip this around, putting the seals in grooves in the bore to ride on pistons that were plated to resist rust. In addition, if the pistons DID start to get rust pits, it was much less expensive to just slide in a new piston.

JohnnyBs68S

I've often wondered this same question, thinking "all else being equal", dual piston calipers HAD to be better than single piston calipers. NOPE!, Thanks to Joe's seemingly infinite knowledge on this specific topic, all else is NOT equal, so the '69+ single piston FLOATING calipers were improved in both reliability and performance. 2 birds w/ one stone!

Koda

That confirms most of what I had thought. I neglected to remember that the rotor floats so it does not have reaction force of its own.

joe_padavano

You mean caliper, right?

Koda

No, I mean the rotor, within the bounds of the bearings, I suppose.

Reason I am thinking is, the fixed caliper, in a fixed system, is connected to the spindle rigidly. If the rotor is also connected rigidly, you don't even need a two sided caliper as the rotor won't move in and out, assuming a non-warping rotor. Even if there were a warping rotor, the other side of the caliper could just be a dumb block. I'm failing to get how the other side's pistons don't contribute more than just the reaction force that a static structure would.

joe_padavano

No, the rotor doesn't float. The four-piston calipers have a common pressure to all four pistons. Any lack of centering is made up by the two pistons with more clearance to the rotor moving further. It's this slight delay in clamping the rotor that leads to the pucker. Any runout in the rotor causes piston "knock back", which compounds this problem.

Once clamped on the rotor, the force on each brake pad has to be the same, or else you would bend the rotor. With a single piston caliper, the caliper body slides to equalize the clamping force in the two pads. With opposed piston calipers the pistons move to accomplish the same thing. The force applied to each pad is still proportional to the piston area on only one side of the rotor.

Vintage Chief

Following along because historical mechanical brake designing/engineering fascinates me. I have more knowledge witnessing interesting changes in motorcycle braking systems e.g. floating rotors.

Koda

Hrrrrm. That's why I said assuming a non-warping rotor. But, bending the rotor in mind, in an ideal system, there could be a machined block on the other side of the rotor, attached to the fixed caliper, that has nil clearance over a perfectly straight rotor, and a single piston would push against the rotor against this ideal block. However, since this can't be done, the "reaction block" is the other piston, which comes in, as you mention, through the knockback distance. Free bodies are counterintuitive sometimes.

Seems like, other than historical correctness, there is no point to the GM 67-68 system style of fixed caliper. I did read somewhere that a single piston, floating caliper, is larger per piston area. One might be able to pack superiorly performing, larger area fixed position, multi-piston aftermarket calipers under factory disc capable wheels than one would with aftermarket single piston sliding calipers of superior performance and larger physical size. Other than that and correctness, using factory disc to upgrade, one should use the floating caliper system.

With that settled, follow up question.

The disc master cylinder / valve system updated throughout 67-72. Could one use a 67 disc master cylinder with 69-72 calipers? One would need to use the appropriate proportioning/metering or rear cylinder smaller sized setup, of course.

joe_padavano

Your assessment is correct.

As for the follow-up, the only part of the M/C that matters to the calipers is the bore diameter (assuming sufficient stroke). The valves are independent of that. The calipers also don't really care about the valves, either.

For the metering valve in the front brake circuit, all that does is delay application of the front brakes for a fraction of a second to allow the pistons in the rear wheel cylinders to move enough to take up the slack in the links between the pistons and the shoes. I hadn't thought about this before, but one could argue that the four piston calipers might have required a slightly longer delay (due to the piston "knock back" problem) than is needed by the single piston calipers. There are three different part numbers for the metering valve on the Olds A-body cars, one for 1967, one for 1968-69 (so the same one for fixed and sliding calipers), and one for 1970. After 70, of course, it was built into the combo valve.

As I've written here previously, the 1967-70 A-body cars with disc brakes didn't use a prop valve (so this includes the 69-70 single piston calipers). The front/rear brake balance was controlled by using appropriately sized rear wheel cylinders - which were smaller on disc brake cars than on four wheel drum cars. The whole point of either this method or the prop valve is to regulate the rear wheel brake force to preclude premature rear wheel lockup. Presumably the greater braking force of the front discs results in more unloading of the rear wheels under hard braking than would front drums. Of course, the OEM brake balance calibration, whether through wheel cylinder sizing or prop valve, is only valid for stock cars on stock tires. Any changes to those parameters upends the balance calculation and potentially changes the balance requirement.