Miatas, WGI Bus stop, Turtles and pad knock back

[6PAK72]

FWIW:

I drove two days with Audi this week at WGI. 100 degree heat, track was very, very slippery, with some interesting damage at T9 that will be patched by the time CAM gets there.

First couple of runs I took it fairly easy, third run of the day turned on the timer and was driving pretty hard. Working a line through the bus stop that puts the car deeper over the entry turn turtles.

Third lap I lost brakes just like Wasilko and I did last year with COM. Pushed the pedal, went to the floor and only the slightest braking power at the very bottom of travel, and on the in-lap, the brakes came back.. Came in, checked everything.

Bled it out so I felt like I was doing something useful, checked the torque on the new hubs, all OK. The car has 4 new calipers, 2 new front rotors, new front brake lines , recent and full brake master reservoir, brand new Hawk front pads, half-worn rear Hawk pads.

Second day, I ran a different line only brushing the entry turtles, throwing away the second turn and straight lined through turtles 3 & 4 without more than a brush against the edges. Also did the left-foot pump going down the hill, and all day I never lost the brakes.

Thanks to Cy fo rthe left foot brake suggestion...

[dradernh]

I was taught by a professional to tap-tap coming out of the Bus Stop to ensure any pad knockback was handled. Same question every damn lap, even if he'd been watching: "Did you remember to tap the brakes?" Same thing with testing brakes in the hot pits prior to going out if he wasn't in the car: "Did you remember to test the brakes?" Geez, I got tired of hearing those questions. OTOH, I now do both without thinking about it.

[Chrispy]

Jeff, if the pedal came back quickly after an extra pump of the pedal then it could be pad knockback (hub flex and all that other good stuff).

Was there any sign of air when you bled the brakes?

[Mick]

Are your wheel bearings in good shape?

[paultg]

Jeff, I had a similar issue when my car was a 1.6L; last year down at SP. What I found was that the car was idling a bit weird after I came in; Chris P and I did bleed things but found no air in the fluid. I ended up tightening the vacuum hose connection on the valve cover (that runs to the master cylinder); and that seemed to helped.

So, if the motor is moving around a bit might want to also check those connections. - Paul G.

[6PAK72]

Thanks Paul, never thought of that.

Mick, the hubs are new that weekend, and I re-checked torque and they were to spec. I also re-checked every line fitting on the car, because I had had that line burst at NHMS the weekend before and replaced two fronts. Everything seemed very tight, no weeping.

I don't believe there was air, I bled just to be sure.

Everything was so friggin hot to start with that it wouldn't have been a stretch to think I overheated the fluid, though it's Wilwood 570.

Checked the master, sat there for 20 minutes in the Cool Shirt with my foot on the pedal and it never moved.

[turtlevette]

Floating calipers should not suffer from piston knock back. It's definitely a problem for fixed calipers.

Wilwood makes a 2lb (blue) resiual pressure valve that helps with this. It's a bit like holding your foot on the brake. It holds 2 pounds pressure on the caliper at all times so the piston resists knock back.

[CP]

Left foot tap for T3 NHMS, Big Bend at LRP and just about every corner at WG. Though I've never had a problem at the Glen and I run all over the curbing of the Bus Stop if I'm doing it right:

[Mick]

Floating calipers should not suffer from piston knock back.

Why not? Are you saying that it should be less of an issue, or that it's not possible?

[CP]

I've personally experienced pad knockback in my Miata with stock brakes at LRP (frequently) and the Glen. Road America is the worst from what I hear.

[turtlevette]

Floating calipers should not suffer from piston knock back.

Why not? Are you saying that it should be less of an issue, or that it's not possible?

In theory, the caliper should move side to side so it wouldn't be as bad. If you guys say it still happens i believe you. I wonder if greasing the bolt the caliper slides on would help?

[Chrispy]

Floating calipers should not suffer from piston knock back.

Why not? Are you saying that it should be less of an issue, or that it's not possible?

In theory, the caliper should move side to side so it wouldn't be as bad. If you guys say it still happens i believe you. I wonder if greasing the bolt the caliper slides on would help?

It can definitely still happen since the caliper is mounted to the spindle and the rotor is mounted to the outer hub. The caliper is free to move back and forth however any movement is going to apply pressure back on the pad and in-turn to the piston, so without positive pressure on the piston you will get knock-back. I would assume that you get approximately 50% less knock-back on a floating caliper due to the design, but it is by no means immune.

[joncowen]

Floating calipers should not suffer from piston knock back.

Why not? Are you saying that it should be less of an issue, or that it's not possible?

In theory, the caliper should move side to side so it wouldn't be as bad. If you guys say it still happens i believe you. I wonder if greasing the bolt the caliper slides on would help?

I think that theory doesn't work because the rotor isn't moving in a pure side to side motion. It's rotating about the center of the hub, so the rotor is prying on the calipers, like a wedge. I guess if you have enough play in the rubber bushings in the sliders, it would work, but i know my floating calipers sometimes cause pad knock back.

[naschmitz]

What's the difference between knocking the piston into the caliper or knocking the caliper back on the slide rails? Either way you have knockback and a long pedal.

[Chrispy]

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?