Bahnzii

So a while back I posted that I was having to double pump into many corners at some recent trackdays.

So at the latest trackday I deliberately made mental notes of my braking around the track (Thunderhill).

First I went back and did a bleed of the brakes again and focused on air in the lines as the culprit, so I even gave the calipers a good few whacks with a mallet and bled them once more to get that last little bubble out.
(all of this with Motul)

I'm running Cobalt CSR pads in ST60 fronts and ST40 rears on my Z06 with no mods except for Nitto NT-05's.

So here is what I noticed;
From the pit lane, I could go into turns 1,2 & 3 with a nice firm pedal. Turn 4 had a little longer pedal. Now Turn 6 was a dead give away to me that it was knock back. Turn 5 is a nice right hander (one of the few) and a little bumpy but with good g's. So by the time I got to turn 6, my pedal was very long, it needed a definite dbl pump

The rest of the track was fairly constant as far as pedal feel except turn 14 which required a dbl pump as well.

After talking to some coaches there with much more experience, they agreed what I was experiencing was knockback. One of them had a similar experience in a race season where they drove a 350z with Stoptech brakes but every corner was a dbl pump.

As a litmus test, I went the other day to a well known backroad that is very curvy and bumpy and ran a decent pace to see if I could experience knockback. Sure enough, if the previous corner was smooth and not high g (keep it relative this is a public road, so I am not a loon driving ballz out) the pedal was consistent but when the previous corner was bumpy and/or tighter and high g the next corner the pedal was a LOT longer.

This was consistent over the next 10 miles (nice long stretch of curves)... So I am pretty confident of my results at the track and on the back road.

So I have a few questions for everyone:

1. Does everyone else with Stoptech's experience this?

2. It seems to be that the non-floating (or is it semi-floating?) design contributes to the knockback. Is there a way to switch to full float for the hat/rotors?

3. How many of the Stoptech owners run knockback springs?? If not are you just used to dbl pumping now??

4. Would the Brembo full floating design help to alleviate this issue??

5. What about AP Racing brakes, do they have a similar issue??

Cheers...

rustyguns

ok i am stupid...what is knock back?

USAsOnlyWay

Couldn't you just run knockback springs? Track only car?

(I claim to know nothing, for the record)

EDIT:

Found this, I get why knockback springs could be unwanted.

http://www.stoptech.com/tech_info/wp_knockback.shtml

flink

I have st-40's on the front on the GTO. I get a little knockback. It's most noticeable in #5
at thunderhill - the car's just gone through #3 (right), #4 (left) and the entry to #5 (right) so
the pedal is longish. You appear to be braking for #4 (why??) so it's not biting you in #5.

The knockback is acceptable - for some incomprehensible reason the GTO seems to
have good front wheel bearings.

I suggest what you do is to teach yourself to left-foot tap before entering the braking zone.
This fixes the knockback but more importantly, prevents you from piling into the braking
zone only to find that your fluid boiled and you have no brakes.

I had a very lucky experience a while back - entered #8 and did a light brake only to
find that I had basically no brakes at all. I got through #8 and took it home. If I'd gone through
#8 without braking (as I was doing half the time that day), I wouldn't have discovered
my lack of brakes until I hit #9 at 110ish. I'd have joined the cows in the neighboring farm.

Since then, I left-foot tap! Practice it on the street for a few days..

longdaddy

you can reduce it somewhat by inserting shims (i.e. backing plates from old pads) behind the pads as your pads are wearing off.

Webz

Consider replacing your hubs/wheel bearings plus adding the knockback springs. In theory/testing the new SKF bearing provides improvement in this area. JMHO

UstaB-GS549

Rotor runout is the cause. You should probably be looking at rotor and wheel bearing condition.

To check knock back, pump up brakes, drive 2-3 miles on the highway without using the brakes and see if you have a long pedal at your first stop.

Just my .02

davidfarmer

I'm having the same issue with PFC's indycar based BBK on the C6Z06. Always at the same place, notably going into turn 10 at VIR. It is caused by high G left/right/left/right combinations, and is probably caused by play in the bearings. The caliper is mounted to the upright, and the rotor to the hub, and any play between them will allow the rotor to "open up" the pads during multi-turn (in different direction) combinations.

I never had this issue with the stock Z06 setup. I have a set of knockback springs, but haven't put them in yet. A lot of work for something that I can deal with by remembering to give an extra pump every lap. Will likely take care of it over the winter.

btw, I see no noticable play in the bearings. Also, I ran Stoptech brakes on several pro race cars and never ever had any knockback issues. I'm confident it is a problem with the hubs or other play in the suspension, not an issue with the braking hardware.

Nimo

I recently put ST60/ST40 on and don't have any issues, but your description matches what I have seen in the past trying to get my stock brakes to keep up.
- I did not have knock back, as it would all go away when bleeding.
So I don't believe its the rotors or calipers, but that your pads aren't up to the job and they are overheating your fluid - and possibly something else to.
- CSR is a autox pad?
- What's your fluid?
- Are the pads worn?

Bad wheel bearings could give knock back, perhaps other components?

What is the session after this has happened like?


/Nik

oldmansan

Thanks for posting. Good information. I wonder why you didn't see the same issue with the stock Z06 setup?

San

Bahnzii

I have been doing just that recently, though I admit, left foot braking is going to take some definite practice to build up some muscle memory

fatbillybob

I think a neat way to confirm knockback is if a light tap of the brakes reseat the pads and then the travel goes back to normal. If it fails this test there are some other valves and stuff that can fail in the ABS to give symptoms sort of like what the OP described. Also if the master is mismatched or marginal in size for the calipers that can make knockback worse. When we change parts out we often don't think about the "system" but think about pads or ducting or calipers or brake fluid etc.

Bahnzii

Thanks Nik...

The Cobalt CSR pads are listed as: "The new Cobalt Club Sport R compound sets new levels of performance for the competitive autocross and/or HPDE participant." Temp rang of 50-1000°F (10-538°C).

I'm running Motul RBF, I bleed after every event (speedbleeders).

Pads are practically brand new as I only have 3 track days on them.

Not sure its a hub issue as I don't notice a long pedal on freeways and then slowing for traffic or an off ramp. Plus its still so new, but that doesn't mean they don't have some extra play in them

The last session I was in there were only 11 cars (wooohooo) so I had a lot of open space to play, my pace was very consistent, I dbl pumped where I needed and track temps were in the high 90's, oil temps were in the low 280's and the tire pressure in the NT05's were running at 39 front/38 rear - from a cold of 27 all around (I pulled that from the dic and my tire gauge).

I'm thinking about knock back springs, but since I also drive this on the street (its still only an '09 Z06 with 7k miles) I'm not sure I want to go that route.

Bahnzii

I have tried exactly what you described both on track and on the backroads and from what I feel, that perfectly describes my current resolve. A light tap, say about 1/3 pedal travel, a slight lift and then back on the pedal and the feel and travel is MUCH firmer.

Bahnzii

So I am curious, has anyone tried the Doug Rippie master cylinder upgrade?? I wonder how that would work in this situation (assuming of course its not a hub issue).

What about going to some sort of full floating rotor setup?
Is there such an option for us Stoptech users?

Or, just throwing it out there, swapping to the Brembo setup. Aren't they supposed to be a full floating rotor?

By the way, I appreciate all the great feedback, keep it coming

davidfarmer

assuming you are running the correct Stoptech caliper sizes (piston size that is), I can assure you it is perfectly matched to your master cylinder. That is Stoptech's strongest point, their almost infinite ability to match piston sizes. Going to a larger master is only needed if you are running calipers with pistons that are larger than your OEM caliper was designed for. This was common back in the day before companies made custom calipers for each application.

Companies used to buy Brembo calipers and just stick them on any old car with brackets. They never measured piston sizes or how it would effect bias, they just did it. Things are much better today, and you normally would not need to worry about your master.

back to my situation, I can only guess that I have a bearing that is just starting to get a little play, and the timing just happened to match the brake swap. Two seasons on the stock brakes before swapping, so I'm due for a bearing failure.

MySR71

I have StopTech ST60's on the front and ST40's on the rear of my C5Z06. I've never had any knockpack issues or any of the other symptoms you describe. My only suggestion besides what others have said is that you might want to contact StopTech for their thoughts. I've always had terrific service from them when I have called with questions.

kentz06

I have ST60 & ST40 with DTC/70 pads and was having knock-back problems in 50% of the corners at Road America and Miller. Both tracks are large and fast and have usable curbs (and I was using them). A couple of light taps on the pedal would solve the problem.

The fix for me has been anti-knock-back springs and mounting the hat to the rotor to float. At low speeds my car rattles but is quiet at anything over 10 mph. StopTech can tell you how to float the rotors.

When Andy Pilgrim was getting ready to drive my car I warned him about the brakes. His response was "no big deal". When racing the Caddys he had to do several pumps for EVERY corner.

The stickier the tire, the rougher the surface and the harder you corner will produce more G forces that flex the system. Check everything out to make sure for safety reasons, but try these two things. Some time to install but low cost.

Dean

USAsOnlyWay

flink

Yes, that's just dangerous.

it's OK with a planned braking (ie a turn). But sometimes one wants the brakes to work when you weren't planning on doing any braking..