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Old 05-17-2018, 10:01 AM   #1  
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Default Brake pad knockback

Pad knockback is something I fight. I can’t seem to get my arms wrapped around the cause. Initialy it seemed to appear as the front wheel bearings were getting tired. New hubs solved the problem. Lately I just can’t see to make it go away. I am driving faster and more aggressively so curbs likely play a role now.

Is this just the way that it is? What is everyone else experiencing? I suspect the solid mounted 11.75” rotors aren’t making thinks better. Didn’t seem as bad with my 11” rotors.

I long for the days days of my 911 pedal. Rock solid and consistent all the time.
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Old 05-17-2018, 10:32 AM   #2  
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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.
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Old 05-17-2018, 10:48 AM   #3  
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Shims would help as the pads wear. New pads not so much. I expect knockback to be worse as pads wear and pistons come out of the bore. I’ll try shimming mid pad cycle. Last outing knockback was there and pads were 65%-75%+ so I am not sure how much that will help. Thanks for the idea though.
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Old 05-17-2018, 11:04 AM   #4  
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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
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Old 05-17-2018, 11:46 AM   #5  
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Quote:
Originally Posted by matrussell122 View Post
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.
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Old 05-17-2018, 12:10 PM   #6  
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Copy that i read it as pad knock. Pad knock would be rotational knock correct? sorry if its a dumb question.
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Old 05-17-2018, 01:30 PM   #7  
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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.
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Old 05-17-2018, 01:31 PM   #8  
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Just got word VR8 is working on floating rotor solution. Between that and shims it should solve most of the knockback issues. I see keisler uprights in my future too.
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Old 05-17-2018, 02:06 PM   #9  
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Im all up to speed now. Thanks for the clarification. My derailment of the thread has now ended
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Old 05-17-2018, 02:20 PM   #10  
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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.
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Old 05-17-2018, 03:25 PM   #11  
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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.
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Old 05-17-2018, 03:52 PM   #12  
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Quote:
Originally Posted by sixshooter View Post
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.
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Old 05-17-2018, 05:58 PM   #13  
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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.
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Old 05-17-2018, 06:42 PM   #14  
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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.
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Old 05-17-2018, 08:07 PM   #15  
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Quote:
Originally Posted by sixshooter View Post
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.
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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.
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Old 05-17-2018, 11:25 PM   #16  
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Quote:
Originally Posted by jpreston View Post
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.
Thatís pretty extreme. I canít imagine he has more grip that I do with the 245 slicks so deflection should be the same. He may need to check and see if the calipers are properly centered. I bet theyíre not.

I found new hubs really helped knockback. Not the cheap ones. Maybe thatís an issue as well.
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Old 05-18-2018, 08:29 AM   #17  
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Quote:
Originally Posted by sixshooter View Post
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.
I was thinking about these as well, but couldn't remember who was selling them. Curious if anyone at Goodwin has tested them out.
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Old 05-18-2018, 11:08 AM   #18  
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It’s hard to believe that brace would do anything. It’s does seem to box in the spindle but I just can’t see how that would strengthen the spindle in the area I’d expect it to flex. Would love to hear Ryan’s thoughts on it.




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Brake pad knockback-f32c0408-c9ec-4bcc-a404-d560e2fa1490.png   Brake pad knockback-7b39eca8-160b-4179-831f-c516b1291231.png  
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Old 05-18-2018, 09:18 PM   #19  
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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.
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Old 05-18-2018, 10:31 PM   #20  
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Originally Posted by Savington View Post
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.
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