Cory M

I have been having some issues with pad knock back. Typically it happens in one or two places per lap (depending on track), usually following a series of turns/ banking/ curbs. It's very predictable and I use the common method of tapping the brake with my left foot on the straight to close up the gap between the pads and the rotor to deal with it. This is not ideal, obviously, but I haven't been able to fix the problem and I've tried several things.

Recently I did an ~1 hour enduro where we shared my car. I have driven the other driver's car and his (stock Carrera) brakes are rock solid at all times. I didn't want him to be surprised with the pad knock back my car has so I warned him about it multiple times and instructed him on the two points on the track it may occur and what to do. He did the first stint and had no issues. I was on the radio with him and spoke to him after the race and he said he never experienced any knock back. Our average lap times were close. I drove the car for the second stint and experienced the same knock back I had all weekend.

Is it possible that my driving style is contributing to, or the cause, of the pad knock back?? I think my style is more smooth and methodical than the other driver. Neither of us bounce off curbs a lot. I tend to incrementally increase my cornering speed to find the limit, while the other driver is more likely to exceed it and then dial it back, if that makes sense. He may be hitting the brakes harder than me on initial application and not doing much trail braking. Different driving styles but as mentioned, our lap times were similar. Maybe I'm getting the brakes hotter? But due to the predictable nature of where the long pedal will occur I'm fairly certain it is knock back and not overheated fluid. This is the first time someone else has driven my car so not a lot of data points to compare. My AIM Solo has braking G's and I plan to check that out, but I don't have any sensors for brake pressure. Has anyone experienced anything similar? Could my driving be the cause of my knock back issues? Any advice?

Car info: Carrera with 993 Turbo brakes all around, floating Coleman front rotors with custom hats and factory hardware, factory rear rotors, dual Tilton master cylinders with adjustable bias. Cooling ducts and deflectors on front (probably overkill). New pads, rotors, fluid. Repacked and check front bearing preload. Adjusted bias at track and settled on best balance. I had the same knock back issues with the old worn out rotors, 2 different pad compounds, single 23mm master cylinder.

Veloce Raptor

Was the other driver tapping on the straightaway as you advised? Perhaps this is why he did not experience knockback.

Cory M

No tapping. I asked him about it and he said he never tapped and still didn't have any issues with a long pedal. Weird...

He's not the greatest at providing feedback on the cars handling but he would have noticed the knock back for sure if it was anything like what I was experiencing.

Veloce Raptor

When you look at your data, compare how long each of you is on the brakes in each corner.

ProCoach

And the longitudinal g vs distance...

sbelles

Maybe he's in the habit of checking to see if they are there before he gets on the brakes hard.

Musche

Pad knock back can be the result of a couple of reasons. IMHO most common is a rotor that is wobbling. Could be warped rotor, misalignment on the hub, piece of debris between rotor and hub, something like that. It doesn't take much. Put a runout gauge on the spindle and rotate the rotor to find out which one it is. Check all four. If they are all true, it could be a wheel bearing issue. Either not tightend to spec or a race loose in the seat (both can change with heat) . Also look for pull back with piston seals. When you change pads make sure to only push the pistons back far enough to slide pads in with a tight fit. Pushing them back too far can lead to this but it usually goes away once they get hot. Other than this you could have a seal that won't let fluid leak out but will draw in air when you change pads. Bleeding will tell you if this is the case. Then there is always the chance of a leaking master cylinder. Good luck in your diagnosis.

claykos

I've had some form of knockback in every race car I've ever had. The gt3r is the best, probably due to the very stiff uprights and brembo anti knockback seals, but I still habitually left foot tap before brake zones. My e36 with stoptechs is the worst. Wish I could find a cure....

Cory M

A quick look at the data reveals some interesting things. I looked at two average laps that were within 0.05 seconds of each other (and about 1.5sec slower than qualifying times). In all of the braking zones I'm on the brakes a lot harder, by an average of about 0.3G. I'm also on the brakes for a shorter period of time, by as much as ~0.5 sec and as far as 100 feet. I'm probably slowing the car too much, and I saw a couple of spots where I could carry a few more MPH through the turns. The other driver may not have been as heavy on the brakes because it was his first time in the car and he didn't know the limits before lock-up. My lap should have been faster but I lost all of my margin in one tricky high speed turn. So it looks like he was spending a lot more time on the brakes and not braking as hard.

Cory M

Thanks for the info. The rotors are all new, as are the pads. Fluid is fresh and bled, no master cylinder leaks. I repacked and checked the bearings and didn't see anything unusual. I have thought about rebuilding the calipers but the seals don't leak and don't appear to be overheated, the rebuild kits are really pricey for these brakes since they sell them individually for each piston. I'll throw a runout gauge on there and see what it shows. I have considered replacing the floating hardware with fixed hardware on the front hats to see if it changes anything.

In the GrandAM COTA race coverage they mentioned one of the teams having an issue with pad knock back, and said that they installed stiffer springs behind the pistons. Unfortunately I don't think that's an option for my brakes.

One experienced Porsche mechanic told me that all track cars with the 993 Turbo brakes have this problem. He thought the calipers required too much fluid volume to push them.

dizzyj

Not that this helps any, but I've been fighting really bad pad knockback. I just got my car back on the track after some major maintenance. Part of that was a full caliper rebuild with the addition of knockback springs.

Took the car out this weekend and was the first time I had zero knockback. I still left foot tap out of habit, but its nice to know the pedal is there.

Cory M

I'll have to do some research. If they make anti knock back springs that work with my calipers I'd love to try them out.

winders

Cory,

I found these:

http://www.hrpworld.com/index.cfm?fo...action=product

Scott

jakermc

What do these look like and how to they install? I have the issue in my '83 bodied 911 with 930 calipers. Anyone know the piston size for those so I can buy the right springs?

Matt Romanowski

Could be the other driver didin't notice or doesn't know what knockback feels like. I know what it feels like, but I'm not real sensitive about bad brakes. Having driven lots of older cars with fade and average brakes, I don't get too excited about it.