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View Full Version : Pad knockback - vid inside



GlennCMC70
01-18-2017, 01:55 PM
Thoughts?
https://www.youtube.com/watch?v=eNDjVobCnGk

Sook
01-18-2017, 02:06 PM
The movement seems excessive, may have to stick a camera in the wheel well during a warm-up for more data.

Too bad the OP didn't give us more of a description. What triggered him to do the test? Was he having knock back problems all of a sudden, or was it always an issue on that car?

EDIT: Found another one of his videos https://www.youtube.com/watch?v=XbOmKYdnYhE

Says the problem arose after installing a new differential, probably a bearing or bearing preload issue.

- Josh

GlennCMC70
01-18-2017, 03:53 PM
I saw a note about this being axle flex either @ the flang (not likely) or flex in the shaft between the diff and the outter bearing on the tube.

Rob Liebbe
01-20-2017, 05:55 PM
There is some amount of axle flex in all of our cars. Flex is not a bad thing structurally. You certainly don't want a too stiff and brittle axle. Some brake calipers are more susceptible to pad knockback than others. The Camaro/Corvette PBR rear caliper is particularly bad on both stock Camaros and Mustangs that use that caliper in some kits.

I wonder if the axles in the video are original or cheap aftermarket replacements.

GlennCMC70
01-20-2017, 07:56 PM
Knock back can be solved with a spring behind the piston.
Not legal in CMC.

Al Fernandez
01-25-2017, 05:56 AM
A little bit of pressure on the pedal with your left foot will solve all your knockback woes.

ShadowBolt
01-25-2017, 08:51 AM
A little bit of pressure on the pedal with your left foot will solve all your knockback woes.

And keep the guy behind you guessing.


JJ

RichardP
01-25-2017, 09:43 AM
Knock back can be solved with a spring behind the piston.
Not legal in CMC.

Clarification is probably in order. Rear calipers have to be factory style per 7.32.8.6 of the CMC rules. Since the factory calipers didn't come with springs behind the pistons, they would not be legal in CMC. Good luck policing that...

Front calipers are open within some restrictions per 7.32.8.5. Those calipers could come with a spring behind the piston or you could add springs, as long as you keep under the cost limit. Legal for CMC in the front.


Pad knock back can come from many sources. There are different fixes depending on where it is coming from. Another cheap fix, beyond making sure everything you have is functioning properly, would be a residual pressure valve. Based on the wording in 7.10, I wouldn't think that would be ruled legal in CMC. If you have exhausted all other legal options, floating rotors/hats would be the way to go (legal front and rear per 7.32.8).

CMC summary: cheap fixes that would have a very slight performance hit - not legal. Stupid expensive fixes that would have other performance benefits for unsprung and rotational inertia - go right ahead...


Richard P.

mach1
01-25-2017, 10:45 AM
CMC summary: cheap fixes that would have a very slight performance hit - not legal. Stupid expensive fixes that would have other performance benefits for unsprung and rotational inertia - go right ahead...


Richard P.

I was just talking to Paul about this last night, hmm

Pranav
01-25-2017, 11:24 AM
CMC summary: cheap fixes that would have a very slight performance hit - not legal. Stupid expensive fixes that would have other performance benefits for unsprung and rotational inertia - go right ahead...


Richard P.

So then let's do some research on the viability and necessity of pad knockback springs in stock rear calipers, maybe have someone in our class test them and submit an RCR for 2018?

I am overdue to freshen up a spare pair of PBRs and use them in place of my current ones that have been run for four seasons. Wouldn't mind testing out some springs with approval.

GlennCMC70
01-25-2017, 10:00 PM
I tried in 2006.

michaelmosty
02-03-2017, 09:39 AM
In my 11+ years of racing I have never heard anyone mention a rear pad knockback issue. Is this a bigger issue than I am aware of?

nasawill
02-03-2017, 11:07 AM
In my 11+ years of racing I have never heard anyone mention a rear pad knockback issue. Is this a bigger issue than I am aware of?

Bad bearings/hubs make it worse. Some cars get it worse than others. Curb hopping in the miata makes it bad enough to need to pump the brakes gently on straights with your left foot. Spec E30s like it too. The GTS BMW I used to race never had an issue. Most people don't know it's an issue and drive through it. So it's not really an issue, but a condition that does exist regularly where some people can make up tenths under braking correcting for it. The brakes will generally still work, but you'll get much better initial bite if the pads and caliper are put back in place before the next corner. SO if you're ever getting your butt kicked on the brakes, this could be one of the reasons. (Don't get me started on F/R bias and pad compound combinations. That's a whole different thread) This is why you'll see really fast folks tap the brake pedal with their left foot going down straights.

This guy isn't the smoothest at it, but it was the first clear example I could quickly find on youtube: https://www.youtube.com/watch?v=XHunY_BEuTs

Funny enough, looking for some AIM data on the interwebz this was the first article that pops up: https://nasaspeed.news/tech/brakes/pad-knock-back-what-it-is-how-it-happens-and-what-to-do-about-it/

PS Speed News has VERY good search engine optimization, hence why I want to get our weekend race reports in there

PPS I hope nobody minds me sharing my experience with the poodle cars and bimmers. I just don't have much AICMC experience since Al and Todd took the big CMC fields to Texas when I was just a California kid! :)

mach1
02-03-2017, 12:04 PM
PPS I hope nobody minds me sharing my experience with the poodle cars and bimmers. I just don't have much AICMC experience since Al and Todd took the big CMC fields to Texas when I was just a California kid! :)
I think we love all motorsports here and appreciate the input! I hope nobody had issues with me wearing Chevy pants last Sunday at the track!

Sook
02-03-2017, 12:49 PM
I think we love all motorsports here and appreciate the input! I hope nobody had issues with me wearing Chevy pants last Sunday at the track!

I routinely wear a dodge shirt while driving my ford truck towing my chevy race car.

mach1
02-03-2017, 12:53 PM
I routinely wear a dodge shirt while driving my ford truck towing my chevy race car.

Good way to piss off a redneck :)

Pranav
02-03-2017, 01:08 PM
I tow the Chevy to the track with a Toyota so not sure where that puts me.

MikeP99Z
02-03-2017, 02:09 PM
In my 11+ years of racing I have never heard anyone mention a rear pad knockback issue. Is this a bigger issue than I am aware of?

I guess we haven't spoken much in 11+ years.

michaelmosty
02-03-2017, 04:53 PM
I guess not about that, haha. I was meaning more specific to the CMC rules since this is the first I have heard of this issue being discussed.

dtanker65
02-03-2017, 08:26 PM
I got to watch the Spec E30 guys at Laguna Seca West Nationals last season. It was like SM without the contact, most impressive!

Will the E36 with an LS motor be AI legal in 18?

39PitCrew
02-04-2017, 11:38 AM
Will the E36 with an LS motor be AI legal in 18?

Only if you build completely out of Camaro parts.

liquidroam
02-10-2017, 12:01 AM
I can tell you first hand that never feeling knock and experiencing, all of a sudden, a brake pedal that sometimes seemed to work and sometimes not can freak you the hell out, or catch a car on fire..... Thanks Dan for giving me knock back 101 lesson post fire. Also Craig thanks for the Hub!! Josh identified the issue in about two seconds.

Rob Liebbe
02-10-2017, 04:48 PM
In my 11+ years of racing I have never heard anyone mention a rear pad knockback issue. Is this a bigger issue than I am aware of?

In CMC land, it's more of a Camaro thing. I had to do the on-the-straight brake pump on both Al's car and Randy's car. Not that those cars were really bad about it, but it was enough of a concern that I didn't want any real surprises.

Michael needs some diversity in his racing life.

marshall_mosty
02-10-2017, 09:15 PM
I guess not about that, haha. I was meaning more specific to the CMC rules since this is the first I have heard of this issue being discussed.

The Mustangs never really exhibit that behavior... part of the superior platform. lol.

Supercharged111
02-11-2017, 03:51 PM
The Mustangs never really exhibit that behavior... part of the superior platform. lol.

Superior? I don't think that word means what you think it means.