Pad knock back

I get this question occasionally from potential customers and I offer my own experience to them (possibly to dis belief) but its an honest reply, its not an issue.  Years of past customers with mono block converted c-clip axles, you would think at least one would have said by this point..Hey! I got pad knock back, what gives?  But no, its not happened…I always advise to check your axle end play if its c-clip and be reasonable, if its wasted- yes, your going to be asking for problems, replace the carrier or axles whatever is worn or weld on housing ends for pressed on bearings.  I ran across this article from Paul from Hydratech brake systems and I had to share it here since it reflects all of the same things I have preached for years. enjoy

Paul M. Clark says..

  Chevrolet solved pad knockback issues WAY back in 1965 with the introduction of disc brakes on the C2 Vette’s, yet everybody has either forgotten about it, died, or doesn’t know this. Flash forward all the way to 2018 (!) and we’re still talking about it like it’s one of modern man’s most mysterious mystery – go figure that one out… What did multimillion dollar General Motors engineering do to fix knockback problems with fixed calipers? (read that as stationary / solidly mounted calipers) Very simple – they simply installed springs inside of the calipers to keep the pads in contact with the brake rotors at all times! No matter what knock back occurred in extreme duty racing or even just simple nasty Detroit potholes in the road (and the classic dumb blonde beaning the curb)(DOH zoiks!), the springs would instantly put the pads right back in place, completely eliminating the issue = problem solved. What would GM do in court facing the aforementioned beaning of the curb, or an individual that had to make “harsh accident avoidment maneuvers” and then having no brakes afterwards? Things that make you go hmmm – sounds strikingly similar to what we purposely do to our cars as “enthusiasts”… no? You would think this would cause drag, but nope! The exact same thing is accomplished by running a residual pressure valve on the brakes. We have coached thousands of customers over the course of 18 years regarding this, and it has resulted in an improvement / solution every single time. Not once has anybody ever come back stating any ill effects. In my own A/B/A (back to back, then back again) test sessions, laser temp gun tests on the brake rotors show no difference in rotor temps on a 30 mile straight highway run before and after RPV installation = no measurable drag (friction) is occurring despite what conventional wisdoms would have you believe. You can spin the wheels freely while up on a hoist while watching the pressure gauge holding 9-10 PSI on the calipers. I prefer the spring loaded caliper pistons versus the RPV, only because the immediately active mechanical effect of the springs is dead consistent (and also tunable through different spring rates), whereas the RPV’s can be slightly~ less consistent in my testing (because the pads can knock back and blow off the residual pressure held by the RPV). After all of my years, it is amazing to me how the issue has been solved long before some readers where even born, yet everybody is still befuddled by it all of these years later.

BS you say? Contact Baer – their rear mega full floater kits include guess what – drum roll now please, yep, an RPV… Can I get a what what? and a who who?

Flash forward to the next gen C4 Vette’s, and voila! The next genesis of improvement – full floating calipers, special square cut seals (that hold the pistons in place much more rigidly)(articulating the floating caliper before actually pushing the pistons inward) – again, problem solved through further engineering advancements. Have you ever done a “brake job” and had some effort in pushing the caliper pistons back in? Surprise – this is not necessarily a partially seized / tight piston, instead being the (engineering) fight against potential brake pad knockback induced by daily driver potholes. In our world of automotive “enthusiasm’, we beat these machines to the absolute limits bending and moving parts around that are “rigid”, uncannily just like blasting an unforeseen pothole in daily commute. Does your daily driver have a shockingly low pedal after blazing that pothole – nope!

As far as turning the rotors? Damn right! Especially if it is a multi part “hat” design. A carbide tip replacement in the lathe is pennies on the dollar to insure that your rotors are true (especially in a seasoned rotor). Setting the cutter tips at the optimum angle is also an art in and of itself. If the lathe used also has a follow up “multidirectional polish” function, then you have really hit the mark, as the brake pads will bed in vastly faster (and much more evenly combatting T/V). I recall my days back in a previous life working with the GM Service Technology Group where it was argued that a brand new GM service replacement rotor shall not be turned before installation. Well, guess what? No matter what multimillion dollar mega certified down to ten thousandth’s of an inch accuracy machine was used, drum roll again please, it was always a jaw dropper for the lab coat wearing GM engineers to see out of the box runout and T/V (thickness variation) in 25+ back to back rotors cut, literally with their own eyes and gasp! even their own hands – yeah, go ahead and push me out of the way and show me how it’s supposed to be done… (it was actually quite funny to see a bunch of lab coats arguing amongst themselves about how the rotors must not be chucking properly in the lathes, as these are the same rotors being installed on the assembly lines). They would then have the lathe machinations laser certified in disbelief and disgust. Never mind then doing further testing using “on car” brake lathes (which is the rarest yet most optimum method even discussed). Then the ensuing arguments that a “turned rotor” is then not legally considered brand new / within factory spec item (!?)

And now there is the discussion of manual versus power brakes. What in the world does that possibly have to do with a cup of tea in China when discussing pad knock back gremlins? Everything… Whooo Whaaaat???? Why? How? Blasphemy you say? Yet again, it IS actually quite simple, but how? Power brakes allow for the use of a larger bore master cylinder sizing as compared to manual brakes. Yeah, and, ok, so what? The larger bore MC produces more volume per stroke, taking up any pad knockback vastly faster, most times even imperceivably – get it? Got it? Good !

I sincerely hope this information provided helps many out there find their “White Whale”

One thought on “Pad knock back

  1. Related to ‘knock back’, but not quite the same thing, is the (intentional) pad retraction created by square section rubber piston seals (and the geometry of the groove in which they are fitted). This is of course a design ‘feature’ intended to eliminate pad drag, and pretty much ubiquitously used in most (if not all?) calipers, whether ‘solid’ opposed piston or single piston sliding pin calipers. But it has a dark side, being increased pedal travel before the pads firmly contact the rotor.

    It occurs to me that a residual pressure valve has the potential to overcome the hysterisis in the rubber seal, and keep the piston lightly pressed against the pad, and therefore to eliminate the unwanted free pedal motion. Companies such as Willwood claim a 2 psi RPV won’t cause pad drag, so I assume won’t prevent the seal from retracting the piston slightly. But, a 10 psi RPV might?

    Considering the findings above (from Mr Paul M Clark), i.e. that a 10 psi RPV doesn’t appear to create excessive pad drag, then such a valve may be useful in helping to reduce unwanted pedal travel caused by pad the retraction designed into most calipers? Opinions?

Leave a Reply