Brake Pad Knockback

[Hydratech®]

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"

The cure for your pad knock back issues has been now clearly identified and explained - now get busy!

Hmmm – it looks like I just wrote my latest tech article after work on a Friday night! (I do suppose I should civilize it, "PC" and polish it a tad bit before publishing on the website though)

Then again, what do I know - just some guy sitting out here in the hills and woods of Tennessee..., right?

[greeze]

Really enjoyed your write up, thanks! I`m going to go one more step.. I get this question on a regular basis and have explained to the look of dis belief that this is not a problem with a modern brake system. For several reason like you have explained the modern cars do not exhibit this issue, larger master bore size, more residual back pressure, reactive pistons etc.. have all but made this problem about extinct. There is always a slight amount of pad knock back on a mono block caliper and even a pin guided caliper at times (even ones with piston springs..they do not stop it from happening, they just fix it). Newton`s third law: every action has a reaction. When pistons on a bank are pressed the opposite bank reacts by extending, this was observed on a 4 channel F body set up with Brembo 4 piston rear brakes on a c-clip axle with approx .020 of end play...no springs, no residual valve. No pedal drop, no recovery pump. I`m sure I could measure the residual pressure for argument sake but the fact is there is no problem driving\tracking this set-up and for as many rears Ive converted, (alot!) no complaints logged to date. Thanks !

[Coursey]

I am getting ready to order a new frame for a c10, probably Roadster Shop or No Limit. Both of which run a Ford 9". I have been told by a few people that when running big wilwood (Aero or Superlite) brakes that i will experieence "pad knock back".

I was told that the fix is to install a full floater rear. But this comes with an added cost of $3000, so not really wanting to do that.

Thoughts? Just run an RPV?

[Vegas69]

I'm not sure if this guy is trying to help others or tell them how stupid they are and how smart he is. LOL It may not be his intention, but I'd want a different tone for my business. You know what they say about opinions.

[Hydratech®]

Coursey, I / we need to know more about your build before accurately commenting on whether you would really benefit from a full floater rear axle. If this is going to be a street / strip / occasional wild ride, then I would say you can save the 3k by staying with a conventional arrangement. Now IF you are indeed going to be autocross / road racing this build, there is more merit to further expenditure of the full floater. Larger truck axles are typically full floaters, for the sake of having larger bearing areas to handle heavier loads. The spacing between the bearings can take a few thousandths of play (in the bearing adjustment) without allowing the hub to particularly register movement, but the bearings still have some movement in them unless cranked down tight, but then you risk overheating them if too tight. In a custom full floater arrangement, you are crossbreeding this heavy truck twin tapered bearing hub design into the mix to provide increased rotor stability. Looking at certain full floater aftermarket designs, some are provided with a residual pressure valve also, which means that the full floater upgrade dramatically helps control rotor stability but still may experience some pad / caliper knockback. Today's tires are capable of providing much more grip than before, so a wide rear tire combined with radical maneuvers will move things around much more than expected, increasing the possible deflection of the caliper in relation to the rotors. The larger the diameter on the rotors, the more knock back may occur, as the distance from the centerline from the hub to the friction area increases. A 12" rotor compared to a 14" rotor will make a difference, but it isn't like a 14" will create pad / caliper knockback while a 12" rotor will not. A 14" rotor will simply create more...

Unless you are building a vehicle that is really going to push things to the extreme limits, you are likely going to be just fine without an expensive full floater arrangement. Most any real build is more than just bolting a list of parts together, requiring specific fine tuning after the initial build. If you shake it down after the initial build, finding that it all behaves, then great! Reality dictates that you will have to optimize / tweak / and tune many things for a while after its maiden voyage. Considering that you can add anti knockback springs behind the pistons in the calipers and also install RPV's after it is built as a tuning mod / correction, you can rationalize this as a tuning process after the fact. If you want to cover any and all bases during the build phase to minimize any after build tuning, then overkill it with the full floater.

Again, tell us more about your build and its intended usage, as this goes a long way towards providing an informed opinion as to whether you should go with or without the 3k full floater upgrade.

[Hydratech®]

One member states that he really enjoyed my write up, then another (stuffy old bird) wants to detract. I left Detroit in 2008, but Detroit hasn't left me. If some think I am being abrasive, well, here's to you

You do you, and I'll do me

I know a thing or two about brakes, wanting to help people where and when I can. Am I smart? I don't think so. Experienced? Yes. Smooth and politically correct? No. Can I be? If it try, but what fun is that LOL.

[Vegas69]

Hey, it's all good here. I always say that if they love you or hate you, great.