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
Last edited by Sook; 01-18-2017 at 02:20 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.
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.
Rob Liebbe - Texas Region
Camaro, Mustang, doesn't matter to me, I'll race it.
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.
Al Fernandez
And keep the guy behind you guessing.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...
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, hmm
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.
Last edited by Pranav; 01-25-2017 at 11:32 AM.
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