Thoughts?
https://www.youtube.com/watch?v=eNDjVobCnGk
Printable View
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
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.
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.
Knock back can be solved with a spring behind the piston.
Not legal in CMC.
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.Quote:
Originally Posted by Al Fernandez
JJ
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...Quote:
Originally Posted by GlennCMC70
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.
I was just talking to Paul about this last night, hmmQuote:
Originally Posted by RichardP
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?Quote:
Originally Posted by RichardP
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.
I tried in 2006.
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.Quote:
Originally Posted by michaelmosty
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/p...o-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! :)
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!Quote:
Originally Posted by nasawill
I routinely wear a dodge shirt while driving my ford truck towing my chevy race car.Quote:
Originally Posted by mach1
Good way to piss off a redneck :)Quote:
Originally Posted by Sook
I tow the Chevy to the track with a Toyota so not sure where that puts me.
I guess we haven't spoken much in 11+ years.Quote:
Originally Posted by michaelmosty
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.
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?
Only if you build completely out of Camaro parts.Quote:
Originally Posted by dtanker65
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.
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.Quote:
Originally Posted by michaelmosty
Michael needs some diversity in his racing life.
The Mustangs never really exhibit that behavior... part of the superior platform. lol.Quote:
Originally Posted by michaelmosty
Superior? I don't think that word means what you think it means.Quote:
Originally Posted by marshall_mosty