Brake pad knockback
 

I just went to the 11.75 BBK and dynapro calipers and I have pad knock now. Dont know what is causing it but i just accepted it as how things are.The only way i thing to properly fix it is with pad shims like on a lot of oem cars.

Which axis do you think you knock is occurring? I was referring to the rotational forward/backward movement that would cause knock.

Disclaimer:
I am an idiot and know some things about a lot of things

Knockback refers to the rotor pushing the piston back in the caliper. Basically upright/hub flexes under cornering load, rotor moves in relation to caliper, pushed piston back in. Then the next time you hit the brakes there is extra pedal travel before the pad touches the rotor. This is why calipers are not mounted on the top or bottom of the rotor (they are near 90 or 270 degrees, or at least close) because that would magnify the issue. This is not a "knock" noise.

I don't have enough time on my 11.75" kit to comment on knockback. But, this is certainly an issue that's magnified by fixed rotors and fixed calipers. What tires do you run, and do you have any aero? Miata uprights aren't the stiffest out there, so it could just be a sticky tire, high load, flexy upright, some hub flex (normal, not saying bad hub), and fixed/fixed brakes. If no one else has this issue, then I'm not sure what's different about yours. It's good practice on long straights anyway to lightly tap your brakes before you let off and actually brake. Just enough to feel that the pads are there, and this would bring them back in contact. More of a bandaid fix for knockback, but if the root cause is a flexy upright/hub then there's not a whole lot you can do.

Copy that i read it as pad knock. Pad knock would be rotational knock correct? sorry if its a dumb question.

Pads can rattle axially, or knock radially. It's just completely different since you're talking about a knocking noise, whereas k24 is talking about the pads getting knocked back in the caliper. But yes, the pads are essentially floating, and if there's too much clearance radially between the "ears" of the pad and the caliper, then it can make a noise when you load up the pads.

Im all up to speed now. Thanks for the clarification. My derailment of the thread has now ended

A local V8 miata is having really bad problems with hub/upright flex right now. Running the V8R 11.75 Stoptech kit with 275 hoosiers on the front. He had the car out a week or two ago and the flex was so bad that the rotor was rubbing on the steel stoptech pad retainers during cornering and wore a groove into the rotor face.

I talked to the owner of a former trans-am series c5 vette with ridiculously wide slicks at Sebring. With the grip, tight springs, and uneven surface of that track he said he needed two full strokes of the brake pedal to get the pads to engage in most corners due to knock back from spindle flex. Sounded scary.

I've seen a device to help this issue on some Miata related vendor site recently.

:eek3:

I found the device.

brace

No reviews yet but if you are having problems it's not a huge amount of money to solve or help if it works.

On my spec miata I run all the factory springs(or the thickest aftermarket ones i can find) on my brake pads and gutted the emergency brake mechanism in the rear caliper to increase knockback. Its real bad after rumble strips but I will take the decreased rolling resistance all day. A quick brush of the pedal with my left foot before braking zones is enough to build the pedal back up, careful not to push hard enough to physically feel any loss of speed though.

Have you ever watched a video of a top sedan racer racing an older car where they had to use all the peddles all the time? Its amazing watching them left foot set pads before braking zone prior to beginning threshold braking with right foot.
At Sebring in my C6Z I had to learn, otherwise ran out of brake before turn 7. Huge Stoptechs fixed that issue.

I was thinking about these as well, but couldn't remember who was selling them. Curious if anyone at Goodwin has tested them out.

Those braces will do absolutely nothing to assist with knockback. Knockback is the rotor moving independently of the caliper, so the stub axle has to flex in relation to the caliper mounting tabs. Even if that does stiffen the spindle (I am highly doubtful that it does), it stiffens the spindle behind the caliper tabs, leaving the stub axle to flex.

This is my thought too. At the ball joints, the cornering forces are already reacted in tension/compression by the control arms, so the flex that is already there between the spindle and upright is still going to be there if the ball joints are constrained vertically. It may make a difference, but it's not addressing the main lack of stiffness that causes knockback, so I doubt it'd be night and day difference.

I've done a lot of this.

The problem is flex in the upright/hub assembly causing the rotors to push against the pads and subsequently push the pistons back in to the calipers. Typically worst in high-G back and forth complex sections. Floating rotors eliminate the knock-back. It's a reactionary solution, I'd prefer to eliminate the flex that is the root cause, but at least at this point in the development of fast Miatas, we don't have that solution yet. Floating rotors are a solution. Regarding floating rotors, how much float you need depends on lateral grip and specific track. Stoptech has a Street and Race version of the floating hardware. The Street has solved 99% of the knockback in my car for all of the SoCal tracks I frequent. At VIR I had substantial knock-back, first time since changing to floating rotors. Possibly because that track has way more complexes with lots of load than most, including the "esses" which I was doing 135mph through. On my dry laps I was pre-loading the pistons with my left foot before every braking zone, and elsewhere just to reassure myself they were still there. Looks like I need the race version at that track.

As a result of my testing, we sell the floating version of the ST kit because that's what works. Street floating version by default, Race by request.

I haven't tested the Ken Auto brace, but I'm skeptical.

I too have been chasing pad knock back ever since I installed my TSE dynapro kit. Glad to hear its not just me. Anyone else having issues with slightly funky pad wear? I think the calipers need the bridge bolt that FM sells. https://www.flyinmiata.com/dynapro-bridge-bolt.html

I get tapered wear with the dynapros, but no knockback.

The bridge bolt might help a bit, but I wouldn't expect miracles from it.

--Ian

Its not that bad. Only slightly tapered but enough for me to care. FM says 10% increase in stiffness which should be enough to take care of most of the problem I think. Its only $30 for the kit so whatever. The flip side is with how much im tracking the car, im thinking of doing superlites on custom full float rotors. Or an AP racing caliper of some sort. This will allow more pad without the knocked issue. We are working on a potential kit now.
https://cimg6.ibsrv.net/gimg/www.mia...c2f30787d0.jpg
https://cimg7.ibsrv.net/gimg/www.mia...f530cbc578.jpg
https://cimg8.ibsrv.net/gimg/www.mia...d531f8d63c.jpg
https://cimg9.ibsrv.net/gimg/www.mia...246e51be14.jpg
https://cimg0.ibsrv.net/gimg/www.mia...bc33e95b27.jpg
https://cimg1.ibsrv.net/gimg/www.mia...85b1aba306.jpg
https://cimg2.ibsrv.net/gimg/www.mia...3b8bd02aba.jpg

I haven't worn out a set of pads with my setup working properly, but the set that ate shit in quick fashion were tapered/uneven as well. Hopefully i'll get enough driving in this year to wear the PFCs to see if they're tapering or not. I just added bridge bolts and went to the HD rotors in my car.

im runnig PFC 11s for what its worth. This is my 2nd set.

I have 01s, which i think are same, or close enough to 11s.

The set that ate shit were not PFCs.

I think im gonna go back to G-LOCs. Ive gone through 2 sets of PFCs in 9 months.

Stock brakes? Ducting?

With a good driver in a fast car, you can only expect to get 3 days or so from a set of 12mm pads. Maybe 4-5 days from 16mm pads. 6-7 days from 20mm Superlite pads is the norm.

I just saw a discussion about that valve on another forum yesterday. Apparently it's frequently used by honda road racers who have problems with knockback caused by flex from weak hubs and knuckles. Supposedly works pretty well. Should have basically the same effect as the knockback springs that AP Racing and some of the other high end calipers have built in. I'm trying to get my friend to try it on his V8 car.

TSE Dynapro kit, 3" Singular ducts

When is the last time the calipers were rebuilt? I fought knock back for a while on a different car and it went away when i rebuilt the calipers with new seals and pistons. Sticky pistons on old seals make the problem worse. I also added anti knock back springs behind the pistons.The springs are made by Brembo and I got them from Hoerr Racing Products.

Update - back on Rivals at WSIR and therefore less lateral load (about the same as most non-aero cars on A7s), only a very minor hint of knockback after T2. This is still with the "street" floating hardware.

We made some big changes to our brake system recently with (amongst other changes) going to the Trackspeed 11.75" SUperlight kit and suddenly have to pump the pedal 2-4 times to get the brake pedal up to full position.

We never had this proble EVER with the 11.75" Dynalite kit.

Would you (and others with this issue) mind posting the following?

We are trying to figure out where our problem is coming from.....im being a bit vague on purpose to start with. Will explain more after a few responses.

- M/C size?
- Booster or not?
- Caliper
- Rotor size
- Pedal system / leverage factory or modded?

One pump is knockback. 3-4 pumps is a major hydraulic system issue. The Superlite calipers we use are 3.00" piston area, same as the Dynalites they replaced, so hydraulically they are identical. If you changed nothing else, it's either a caliper issue (not specific to the Superlite) or a bleed issue.

Thank you. I appreciate all that. We think we have something else going on we can't quite resolve. Please note, in no way do we believe it is the Trackspeed Superlight kit at fault.

We can get it to happen in first gear in the car park just turning around. Without turning a corner the brakes work 100% and stay that way so we don't believe it is a hydraulic issue (of course we stand to be corrected). Although we have tried repeated bleeds with no sign of air resulting.

It ONLY Happens after cornering, using the brakes on the highway on straight roads in normal traffic leaves pedal perfect.

I would blame flex in spindle or hub (as mentioned in this thread) but it happens in first gear at sub 20 km/h as well, to me this rules that out so intersted to hear if this happens to any of you guys also?

We have made some changes to the system (things most of you haven't) so if any of you could provide the following I would appreciate it.

- M/C size?
- Booster or not?
- Caliper
- Rotor size
- Pedal to MC pickup point factory or modded?

Sweet thank you.

We were sure it was failing or bad bearings but that just doesn't seem to be the case. We fekt the same about air/beeding, but that also doesn't seem to be the case.

So strange the problem only happened after we changed our system (Superlight brake kit and removed Booster, retaining OEM M/C, mechainc did a nifty trick to retain OEM pedal feel)...im thinking for our next race meeting to go back to the Dynalight to try and rule in or out parts.

and this low speed version of it has us beat too. I just can't see that being a flex issue unless the thing is nearly torn in half.

Superlight Kit has only done 1.5 race meetings, so pads are barely touched at this point (plus it did it from new so doubt pad wear is the issue ).

We have oem hubs still. What do you look for with failing wheel hubs?

I have seen weird caliper piston pushback with certain pads (not sure why..) but the pad cycles between convex/concave bending due to thermal expansion. Using IR temp sensors you can verify that the pad contact surface has fairly uniform and consistent heat traces, not hot edges alternating to hot centers.

While I dont think our underlying issue is resolved.

Further to suggestions above, we did fit the willwood 4psi residual pressure valves and that has made the issue a one press of the pedal solution. So it is now very driveable.

Given our problem was extreme I would think for anyone with a mild case these would bea great solution.

No idea what our issue is given it happens just in the car park IF you turn a corner but for the moment we will see if anything becomes apparent.

No. Stop it. You are adding unnecessary parts to bandaid a major hydraulic bleed issue. Residual valves are for drum brakes, and they have no place in a proper disc brake system.

Thank you for advise (seriously) and that is my concern, but we cant reslove how/why it stays pumped up without going round a corner..... "sometimes" even after a corner it stays up, but mostly turn the wheel and go round a corner even at carpark speeds, it will drop to half of what it is.

We have power bled many times, with no air.

Has it ever been necessary or known for Superlights to be bled any different to the other dynalights/dynapros ?.....i.e. from the underneath bleed nipples as well?

edit: doesn't seem heat affected either. I would have thought a bleed issue would be heat affected......?