Miatas, WGI Bus stop, Turtles and pad knock back

[WillM]

I do not think that the residual pressure valves are intended to help or eliminate knock back. I doubt that the 2 or 10 pound offerings would help the situation much, but would be interested to hear the results if anyone has tried 'em. It takes about 50 psi of line pressure for me to even notice the effect of dragging the brakes when the car is coasting and out of gear (I have pressure sensors front and rear).

As far as I know, the main purpose of those residual valves is two-fold; to mitigate the return spring force of drum brakes and to prevent fluid from draining back to the master cylinder in cars where the master is below the brake calipers.

I'm all over the curbs everywhere and have yet to experience any real problems with pad knock back on well maintained, good/great condition stock Miata brakes. Same for the Wilwoods. The slight amount of knock back I've noticed has been at Watkins Glen, and at times when the pads were getting low. I wonder if there is any correlation between the amount the caliper pistons are protruding from the caliper (thin pads) versus the severity of the knock-back effect. In general, I try to have as little of the caliper pistons sticking out as possible. I have some thin backing plates that help a little, but often ditch pads once they get to the 40-30% mark. I experience a drop in pedal feel by the time the pads hit 50%, so perhaps knock back is the culprit (and I avoid dealing with it by swapping out pads/shimming). On my shopping list is a set of thicker pad spacers/shims.

I suppose if you look at the extremes, it makes sense. On brand-new pads, knock back can't possibly push the pistons in very far. I can just barely squeeze a fresh set of pads between fully retracted caliper pistons and the rotor, so even if the conditions were right to cause knock-back, the pads wouldn't have very far to go.

[naschmitz]

Speaking of knock-back, Bert how do you like those anti-knock back springs in your AP calipers? Do you feel like it made a positive difference or is it too hard to tell given that the rears are stock?

The Corvette AP T1 kit is awesome. In combination with the SKF racing hubs, I probably have less knockback than a standard C5/C6 as the SKFs are a different design intended for handling higher lateral loads. You can really tell the anti-knockback springs are working because you can hear the pads brushing the rotors continuously -- definitely a degradation to vehicle NVH.

[naschmitz]

I do not think that the residual pressure valves are intended to help or eliminate knock back. I doubt that the 2 or 10 pound offerings would help the situation much, but would be interested to hear the results if anyone has tried 'em. It takes about 50 psi of line pressure for me to even notice the effect of dragging the brakes when the car is coasting and out of gear (I have pressure sensors front and rear).

As far as I know, the main purpose of those residual valves is two-fold; to mitigate the return spring force of drum brakes and to prevent fluid from draining back to the master cylinder in cars where the master is below the brake calipers.

I'm all over the curbs everywhere and have yet to experience any real problems with pad knock back on well maintained, good/great condition stock Miata brakes. Same for the Wilwoods. The slight amount of knock back I've noticed has been at Watkins Glen, and at times when the pads were getting low. I wonder if there is any correlation between the amount the caliper pistons are protruding from the caliper (thin pads) versus the severity of the knock-back effect. In general, I try to have as little of the caliper pistons sticking out as possible. I have some thin backing plates that help a little, but often ditch pads once they get to the 40-30% mark. I experience a drop in pedal feel by the time the pads hit 50%, so perhaps knock back is the culprit (and I avoid dealing with it by swapping out pads/shimming). On my shopping list is a set of thicker pad spacers/shims.

I suppose if you look at the extremes, it makes sense. On brand-new pads, knock back can't possibly push the pistons in very far. I can just barely squeeze a fresh set of pads between fully retracted caliper pistons and the rotor, so even if the conditions were right to cause knock-back, the pads wouldn't have very far to go.

I think what you might be describing is pad taper, which also results in a long pedal. With new pads there is no taper. WIth the Corvette stock sliderail calipers, a lot of guys flip pads once a day.

[Chrispy]

I'm all over the curbs everywhere and have yet to experience any real problems with pad knock back on well maintained, good/great condition stock Miata brakes. Same for the Wilwoods. The slight amount of knock back I've noticed has been at Watkins Glen, and at times when the pads were getting low. I wonder if there is any correlation between the amount the caliper pistons are protruding from the caliper (thin pads) versus the severity of the knock-back effect. In general, I try to have as little of the caliper pistons sticking out as possible. I have some thin backing plates that help a little, but often ditch pads once they get to the 40-30% mark. I experience a drop in pedal feel by the time the pads hit 50%, so perhaps knock back is the culprit (and I avoid dealing with it by swapping out pads/shimming). On my shopping list is a set of thicker pad spacers/shims.

I think the pistons slide a little more freely the further they protude, but I think the main thing is heat. The less pad material you have, the smaller the heat barrier, and hence more heat gets transferred to the caliper. You can buy titanium alloy or stainless steel heat shield shims of various thicknesses, but I like Bert's trick of using the backing plates from dead pads since it is free.

[CP]

Why does it matter how much heat is transferred to the caliper if boiling the fluid is a non-issue? Are you suggesting that the caliper tends to flex when hot? I've never had any boiling issues, even during 45 minute National races. Like Will, I typically pitch my pads when they're about 40% worn...when the gap in the middle is gone or the white residue extends to the backing plate. The older the pad is, the more moisture it absorbs and reduces its braking power because the moisture turns to steam/gas. I also clean/lube all of my caliper slide pins before every race weekend...takes about 45 minutes with the car up on stands and reset my rear caliper allen bolts.

[Chrispy]

For aluminum calipers the heat is certainly a potential issue. Depending on the aluminum alloy used by the manufacturer, temperatures above 450F may cause the caliper to have reduced tensile and fatigue strength. More modern aluminum alloys are designed for higher temperatures, but have only really come about in the last 10 years or so. For your OEM steel calipers those kinds of temperatures are a non-issue as far as metal fatigue goes.

Piston seals certainly seem to take more of a beating with higher temperatures and will wear out significantly faster.

I think the main thing for the single piston (Miata's etc) calipers though is the further the piston is extended, the more the piston can move in the bore. As a result the pressure exerted may be less evenly and increase the likelihood of pad taper. The multi-piston brake calipers don't seem to suffer measurably from pad taper as the piston sizes are staggered.

Of course, non of this has anything to do with pad knock-back

[6PAK72]

This is a great thread, and I'm distilling some things from it.

Will, thanks for your comment, it puts a lot in perspective. Chris, great data and I never really connected the unsupported cylinder protrusion with a shear load...duh! I agree that once the pads hit 40%, the feel really changes and I don't care for it. I probably change pads too often, but I like a hard brake feel.

Cy, never thought about liquid and thin pads. And yes, I lube my slides before every weekend. Also check torque on the hubs front and rear.