2MCHPWR

hi - yesterday first time this season at track.
i noticed a little dead spot in the brakes near initial application of brake pedal. But if i would tap the brake before the real brake zone, it was much better.
what is that from? what will fix it?
i used last seasons hawk blues and fluid was fresh.

thanks!

MNCrystalRed2011

You are experiencing knock-back. The pads get pushed off the rotor a small amount and that is what you are feeling in the pedal. I've been doing that for years. I started back when I used to drive my old Taurus SHO. Here is a link to a description of knock-back.
http://www.trackpedia.com/wiki/Brakes#Pad_Knock-Back

2MCHPWR

thanks man.
but damn i just checked the level in the MC and its lower than what i started with. that stinks. where's it going!?!?!?!?

Aardwolf

Yep knock back. It is normal for fluid level to go down as the brake pads wear away.

MNCrystalRed2011

froggy47

You can shim the pads as they wear down to reduce it.

Racingswh

Replace your wheel bearings. That's typically where the deflection is coming from. They may be in the early stages of wearing out. I found knock-back starts to become noticeable in brake zones after a longer higher speed corner like turn 10 at Summit Point Main when I go to the brakes for turn 1. Now I am just in the habit of tapping the brakes before the brake zones to make sure the pads are snug against the rotor before I go to the brakes in earnest.

AU N EGL

cruzin2

...and if better bearings don't totally solve the issue floating rotors will get rid of the rest. (the bearings fix most of the issue but if you are running slicks, then floating rotors will help after stiffer bearings are used)

BEZ06

I don't have the problem, but just curious how you guys do the brake tap before the brake zone where you're still on the accelerator.

I assume you do the tap with the left foot - is that correct???

TIA,

Bob

Bill Dearborn

Best way is to reach over with the left foot and tap them. Here is a Stop Tech White Paper about pad knock back written by Jim Walker.
http://www.stoptech.com/technical-su.../pad-knockback

Here is a video taken from my car 8 years ago showing Jim tapping the brakes before a corner. When I first saw the brake lights flashing like that I was a little unnerved because I wasn't sure what he was doing. Afterwards we talked and he told me how he was solving the knock back and pad tapering issues of his 04Z.


Bill

vms4evr

That brake tap is not such a big hit with instructors at HPDE. I have had some tell me in the past to knock it off. It upsets and screws with the driver behind you because they don't what you're doing. They assume you just brake too early then they do for no good reason. Then they expect you do it all the time when they follow you. Not everyone owns a car that has brake issues. I know the P-car drivers don't do it.

CGZO6

09ZO6 - Had real bad knock back last november at Road Atlanta. Cause was Titanium heat shields between padlets and caliper pistons. The padlets were not wearing evenly, when off the brakes the heat shield would straiten out and create alot of space between caliper piston and padlet. Required a couple of pedal strokes to get the pedal firm. So naturally I called it a day, as it was unsafe.

vms4evr

To help solve this problem so you feel more confident in your brakes and don't annoy the hell out of every other driver.

I took a leftover set of pads I had. Sent them off to the machine shop to be squared and cut down to the size I wanted. They are shims for my front brakes. They work well. When the pads get half worn I shim them. Now the knock back problem is reduced significantly.

froggy47

Was that because the ti shims were too thin? Did you try other shims or just stop using?

Bill Dearborn

I wouldn't necessarily think driving a car with pad knock back is all that unsafe. If you don't tap the pedal most of the time you just have a low pedal that stops at a hard point. Some times the pedal travel gets bad enough you hit the floor without getting solid braking. Then you pump the brake. This tends to slow you down too much approaching corners since you are anticipating a double tap in the braking zone. Doing the double tap before the braking zone whether it is needed or not makes sure you have a high solid pedal when you get to the braking zone and also reassures the driver the brakes have no other issues. Once drivers around you get used to what you are doing it really isn't any more misleading than all the other stuff that happens on the track.

Bill

AU N EGL

Graham is correct here. In the beginner to intermediate classes, this will upset the driver behind you. however, they should also be looking UP the track, not at the brake lights in front of them

That said for the advance groups, to left foot tap the brake too SEE if you EVEN have brakes is a good idea.

JRitt@essex

2MCH,
The most proven solution for knockback is using anti-knockback springs. A spring is placed inside the piston, which counteracts the forces levering it back into the bore. You can see them in our AP Racing calipers...scroll down a little bit on this page for a closeup pic. So if you ever decide to upgrade your brakes, it's something to keep in mind and address.

CGZO6

The 1 piece shims with the 3 padlet (per side of rotor) combination was the issue. When the padlets wore un-evenly, the shim would flex to conform to the padlets. Then when off brakes the shim would straighten out creating lots of room between piston and padlet. I had to pump the brakes about two 3/4 strokes to get any pedal.

So, I won't be running shims with uneven pads again. Going to 1 piece pads.

froggy47

Thank you.