MechE00

On another web board, the topic of brake pad knockback came up, and I was rather ignorant of the phenomenon. (Explanation here: http://www.stoptech.com/whitepapers/knockback.htm )

I've never _noticed_ it autocrossing, and I haven't heard you ECHC/IT/other autocross/etc guys talking about it.. Anybody here had to deal with it? get shocked/scared/puckered by it the first time they dealt with it? Had a devil of a time identifying what it was?

Or is there something about the cars/tires we drive that means we're not going to encounter it? The stock braking systems aren't very susceptible to it? We just can't drive well enough to cause the problem?

edit: grammar

Crack Monkey

Fixed calipers and stock (small) rotors lessens the impact, so many of us don't really ever see it.

rice_classic

During my novice races in ITA I had a problem with my pedal going to the floor (later I discovered it was the result of the in-effective Hawk Blues). Another racer had said to me, "well, don't you tap the brake pedal with your left foot down the straight to re-settle your pads?"... Of course I said no because I didn't know any better. He said that with pads under extreme conditions such as racing will suffer from tapering and knockback. Basically the pads will be tapered and require the caliper piston to traver further to make the whole pad contact the rotor or the pad will be "pushed" back a bit into the caliper also increasing pedal travel at the time of braking.

Ever since then I have been running good pads so I haven't suffered from the "tapering" problem but experienced knockback yet... probably because my left foot spends some time on the brakes while cornering.

MattD@Stoptech

I only heard of cars with strut suspension doing this....

Outrun

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by rice_classic &raquo;</TD></TR><TR><TD CLASS="quote">Another racer had said to me, "well, don't you tap the brake pedal with your left foot down the straight to re-settle your pads.</TD></TR></TABLE>

So that's what those drivers are doing in those Japanese race videos...

Chris Sawatsky

if tap the brake pedal at odd times down the straight, the guy drafting you will be quite confused and might not be ready for an outbraking pass

rice_classic

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by Chris Sawatsky &raquo;</TD></TR><TR><TD CLASS="quote">if tap the brake pedal at odd times down the straight, the guy drafting you will be quite confused and might not be ready for an outbraking pass </TD></TR></TABLE>

Uh.. yea.. this might be where common sense sets in... in which I hope this exercised by ALL those who race cars.

Jaker

My 98 EK suffered from this quite badly. Long fast hard corner would lead to no brakes at the next corner (120 mph). I upgraded to FastBrakes stuff and never suffered the problem again. However, I was led to believe that the knock back was being caused by flexing of the knuckle. I don't understand. The brake upgrade didn't change the knuckles.

MattD@Stoptech

which means j00 didn't suffer from pad knockback

bb6h22a

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by MechE00 &raquo;</TD></TR><TR><TD CLASS="quote">On another web board, the topic of brake pad knockback came up, and I was rather ignorant of the phenomenon. (Explanation here: http://www.stoptech.com/whitepapers/knockback.htm )

I've never _noticed_ it autocrossing, and I haven't heard you ECHC/IT/other autocross/etc guys talking about it.. Anybody here had to deal with it? get shocked/scared/puckered by it the first time they dealt with it? Had a devil of a time identifying what it was?

Or is there something about the cars/tires we drive that means we're not going to encounter it? The stock braking systems aren't very susceptible to it? We just can't drive well enough to cause the problem?

edit: grammar</TD></TR></TABLE>


I had this happen, when using old, tired calipers with less than perfect piston seals; going through the long sweeper at Buttonwillow and the following left hander, I'd notice a longer than I remembered pedal travel braking for lost hill.
I replaced the calipers and the problem lessened, but never fully went away.
Full floating rotors and multi piston calipers would cure the problem outright.

.RJ

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by bb6h22a &raquo;</TD></TR><TR><TD CLASS="quote">Full floating rotors and multi piston calipers would cure the problem outright.</TD></TR></TABLE>

Or new wheel bearings, much cheaper

I had this on the lunchbox. T11 @ CMP was always interesting.

bb6h22a

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by .RJ &raquo;</TD></TR><TR><TD CLASS="quote">

Or new wheel bearings, much cheaper

I had this on the lunchbox. T11 @ CMP was always interesting.</TD></TR></TABLE>

That's right!

I forgot , I replaced the wheel bearings when I installed the new front calipers.

Although the problem persisted, it was reduced.

It just gets to a point where you're going slightly faster then the cumulitive sum of your car's parts was meant to go.

.RJ

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by bb6h22a &raquo;</TD></TR><TR><TD CLASS="quote">It just gets to a point where you're going slightly faster then the cumulitive sum of your car's parts was meant to go. </TD></TR></TABLE>

Thats a problem with most honda cars running stiff suspensions, sticky tires and having the chit driven out of them on track They were built as economy cars...

bb6h22a

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by .RJ &raquo;</TD></TR><TR><TD CLASS="quote">

Thats a problem with most honda cars running stiff suspensions, sticky tires and having the chit driven out of them on track They were built as economy cars... </TD></TR></TABLE>


But, but ... I thought TYPE RRRRRRRRRRR meant RACCCCCCCCCCCCCCINGGGGG?

94eg!

iTrader: (2)

Here is a good article on pad knockback...

http://www.stoptech.com/whitepapers/knockback.htm

Enjoy.

MechE00

Dude.. that article is so good, the original poster should have included it in the first post! Oh wait..

smokin rubber

I seem to encounter this problem when I rally the car at the H-C races. It first happened at VIR with my off at turn 5, I had gotten the car straightend out and back on track, but when I went to slow for the crossover the pedal required several pumps before I had pressure. Then during qualifing at RA I got wide coming out of 10b and rode the rumble strips up the hill, turn one was a wild ride, again I had to pump the brakes a couple times before I had good pedal pressure.
The first time I had no idea what happened, but after the second time I am fairly certain this was the culprit. I now check/tap the brakes after EVERY off, one wheel or all four. And everything has been fine since.

phat-S

Karl, I'm surprised that harsh vibrations would kick back a caliper piston if every was working correctly. I have seen it on well-well worn pads but not w/ meat on them. Have you checked your hubs and wheel bearings?

smokin rubber

the front wheel bearings and hubs were new about a year ago, and I only ran seven events on them. The slider pins are a couple years old, and the rear end is allmost all origional, but those will be replaced very soon.

cobraracer

There's another solution that I'm thinking of trying, a pressure valve in the brake line to hold 3psi on the system. This is instead of the spring behind the caliper piston. I guess it's a hydraulic solution instead of a mechanical one. I was talking with a Rolex Cup crew chief that eliminated the knockback and lost no top speed or lap time at Daytona.

RexRacer19

That is interesting. Traditionally Residual Pressure Valves are used in brake systems where the MC is at a level below the level of the caliper (formula cars, some street rods, etc.) Since in a fluid system the liquid will seek to level itself out the RPV allows all the line pressure to relieve itself enough to stop acting on the pistons but then acts as a check valve to keep the fluid from draining back out of the calipers.

IMHO I think that it might work if the problem was small but might not work completely if there was a more severe problem like a worn wheel bearing.

94eg!

iTrader: (2)

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by MechE00 &raquo;</TD></TR><TR><TD CLASS="quote">Dude.. that article is so good, the original poster should have included it in the first post! Oh wait..

</TD></TR></TABLE>

Jaker

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by El Pollo Diablo &raquo;</TD></TR><TR><TD CLASS="quote">which means j00 didn't suffer from pad knockback</TD></TR></TABLE>

You sure? Maybe the flexing for pad knock back wasn't occuring in my knuckles, but in the rotor hat/hub area, and my change to different calipers and rotors gave me a rotor that didn't flex anymore.

John

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by bb6h22a &raquo;</TD></TR><TR><TD CLASS="quote">
Full floating rotors and multi piston calipers would cure the problem outright.</TD></TR></TABLE>

Not entirely. If you get into the 13"+ rotor range, you're still prone to getting knockback with a multi-piston caliper and floating rotor. Even new wheels bearings have a bit of play in them (obviously no more than a trace amount), so with a larger rotor, you're going to be more prone to knockback.

SCoupe 5sp

Bringing this back to life after 10 years (never done this before, plz give a break) as it is quite relevant and might save someone from totalling their car or taking someone out.

So NOT KNOWING wtf Brake Knockback is nearly put me into a wall at Sonoma this past weekend. The NASCAR track layout has a wicked berm that I've never driven before. The has the potential to affect any free floating calipers, but particularly affects my ITR set-up. (google Brake Knockback and see the issue affects many other cars).

You will see and hear the vibration transmitted into the chassis. It seems to cause or allow the pads to be kicked off the rotors. The first application of the brakes puts the pads in contact with the rotors. But next to nothing by way of pressure. I nearly smash the wall of Sonoma NASCAR layout's T7 as I come in hot at 3rd gear redline, no brakes in Group C Race June 14, 2014.

As I confirm later, the solution is simple. A couple of quick nudges of the left foot snugs the pads after a berm and then the brakes work great. So pulsing the pedal before using the brakes fixes gets you back to normal. Solved.

fyi, My front bearings are new as of a few months ago, so I have minimal wheel wiggle. Yes, as mentioned above, Stop-Tech has a great artcle on this issue: http://www.stoptech.com/technical-su.../pad-knockback

This feels like the first press of the pedal after installing new brake pads. Except you are going 85 MPH and need to slow down right tf now.

(don't understand why hot link starts at beginning of race and not at 2:22 as cut and pasted, bump the video to 2:22 and you'll see it.).


1998 Civic EK Coupe, ITR Calipers, New Raybestos ST43 pads, Mini Cooper rotors, new wheel bearings, Hankook C51 Tires, 600# Front, 800# Rear. Fast berms cause it. If any braking done on berm, their is no knockback. (As seen in corner prior to T10 through out later sections of same video).