Caliper/Pad Knockback in OEM Brakes

[Axxe]

Hey guys, has anyone experienced knockback in an OEM ATE braking system? Just experienced some this past weekend at Watkins Glen, would happen after going hard over the bus stop. First time it happened to me it locked up the rears and I put it into the tire wall in T6 :(. Thought it was cooked brakes (PFC08's unducted) so I made some super quick walmart special brake vents out of flexible dryer tubing, same thing happened on the 1st lap of the race so I knew it wasn't heat related.

Pretty weird issue, I suspect the bearings have too much play under load, will check the bearings this week when the car comes off the trailer but would like to know if anyone else has experienced the same.

Brake specs:

ATE calipers
Brass guides/bushings
PFC08 Pads
Ducted and Unducted
12+14+24+14+20=84 hrs of race time on front bearings
~12-15hrs of testing
245 width Hankook RS-3

Not running slow either, 1:40's at Mosport and 2:20's at WGI

[Wschnitz]

You still have the ABS?

Ever rebuild the calipers?

Your not running out of fluid or anything?

[Axxe]

Nope.

Every year.

Fresh bleed before every weekend, rebled Saturday night, same symptoms Sunday.

It only happens when going hard over turtles.

[FredK]

You say full ATE system; does that mean you're running the stock ATE 22.2 mm master cylinder?

I could definitely see having too much rear bias if you were running the 22/19 Girling MC. It would be around 60/40 if you have a Girling MC,48mm ATE front 33mm ATE rear calipers, and 260mm F 258mm R E30 rotors, and a deleted brake pressure limiting valve (the cylinder that's on the driver's side firewall, right next to where the fuel hoses mate up to the hardlines).

I ran a similarly badly proportioned system at the Glen a few years ago and I was a much slower driver back then, and the only time that I felt things were hairy was braking for T1. There's a bit of uneven pavement and I felt the rear of the car getting very squirmy when I started braking at where I needed to, so I backed off big time.

I've heard of people going off at T6 for sure. Do you have any in-car video? Is the car OK? I assume you're OK, right?

[Axxe]

We have the Girling 22/19 and no pressure limiting valve. All 4 wheels lock up at approximately the same time with our car the way it's setup. It really works well for us.

The issue is that the pedal went straight to the floor. Like after you do a brake job and the first pedal motion if you've retracted the caliper piston. Imagine that feeling, I couldn't trust the pedal after that, even doing the double pump or left foot drag didn't feel right.

I have in car footage, it basically looks like I braked way too little and then locked up the rears, which is what happened. Car is fine, just some crumpled sheet metal but I drove it back to the pits where we confirmed alignment, bled the brakes, pulled the left rear quarter out to miss the tire and did a thorough inspection of all safety equipment. Finished in 8th position on the day (was in the lead when the accident occurred). Me, well, I feel like I got rear ended again, sore neck and shoulders but otherwise OK. I drove the next morning as well and put us in first again, fighting through the braking issue (we thought incorrectly that it was a temp/fluid related problem so we didn't check bearing play).

The weekend was more fun before that too, losing 4th gear on Friday and still finishing 14th out of 83 cars.

[Jb325is]

That's a pretty common place for knock-back. If you were leading, I'm assuming you've raced the Glen before and that has never happened? If so, I would definitely check hubs.

I worked with Spec Miata's for a while and we had some knockback issues there, but there was a lot done to those cars to free them up and the drivers were prepared to deal with it. Road America was another bad track for knockback

[Axxe]

I've raced the Glen before without the same issue yes. First day was in the rain so no where near as fast on the concrete, and the next day only had 5 green flag laps to run hard, didn't feel the braking issue then. That was a lot of race time ago (28hrs on the car vs 84).

[B1mmer]

I too would be looking at wheel bearings. Here is an interesting white paper on knock back:

http://stoptech.com/technical-support/technical-white-papers/pad-knockback

Were you running a Chump race? - AB

[Axxe]

Yes, ChumpCar is the series we run it in.

[B1mmer]

Yes, ChumpCar is the series we run it in.

Glad the car wasn't destroyed by the situation and glad for the most part you are ok. I run in both Lemons and Chump series too, but have yet to make it to Watkins Glen. - AB

[Axxe]

Only photo that is online and showing damage:



Not horrible. Thanks for the well wishes. You need to make it out to the Glen, it's definitely got it's reputation for a reason.

[TobyB]

I bought some non- European front hubs in a pinch- they had enough play that
I could have seen them doing that. Sent them back for some SKF's and never had
a problem.

So maybe check your front hubs to see if they're tight?

t

[Axxe]

Ok checked the hubs Tuesday night. No play at all by hand. But I can hardly exert the same force as the car can through the bus stop and carousel. We are going to try new bearings at the next race, but we won't be back to WGI until next year so no idea if it's fixed or not, and by then we will have the same hours on the new bearings again. We will see how it goes, might have to just adjust my braking and use my left foot to drag them up wherever I can without losing time or upsetting the car. I tried it at WGI and lets just say I need a lot of practice to get it right.

[TobyB]

The ones I had developed a pretty noticeable 'klunk' and maybe 3-4mm
of play pretty quickly after I installed them. They also spat out their grease.

fwiw,
t

[Massive Lee]

Do this test to verify your four hubs are still strong : when driving in straight line, after pumping the brakes once or twice, your pedal remains firm, but it goes to the floor after the first corner (when G-load applies its forces against the hubs), then chances are your problem is one of the four hubs/bearings.