Official: Brake Pad Knock Back Thread
 

As some of you know I and a few others have fought brake pad knock back issues. With a fixed caliper like many of the large manufacturers use, any slack in the bearings under load cause the pads to recede the pistons into the caliper. This isn't so great the next time you hit the brake pedal since you experience more pedal travel than the previous stroke.

The first time I autocrossed my car I had to pump the brakes between the corners. I determined I had .020.021 play end play in each axle. I bought some different retainer plates and got it down to .005/.006. While that has improved it tremendously, I'm still not satisfied on the street or track. I know it's not the front because I have C6 bearing packs and they only net .001 play. At this point I called Wilwood and they don't offer a floating caliper so I would need to switch brakes if I go that route. My number two thought is to go to a tapered bearing. I called moser and they said most circle track and autocrossers run a tapered bearing but he couldn't guarantee that would fix my problem. He also said .005 was tight enough for a roller bearing.

I can't really get anybody to cough up a spec on how tight the axle end play should be. Wilwood is actually sending me some shims but how much tighter can I go and will .002-.003 really help that much. Has anyone changed from a roller to a tapered bearing and has it helped? I'm experiencing a change in brake pedal even on the street under normal drving conditions. While it's managable on the street and track. I want a consistant brake pedal. I had a brief conversation with Frank at Prodigy yesterday and it's happening with other brake manufacturers as well. He said he knows a guy that is running a 10psi residual valve in the front and back. While I'm sure that would fix my issue, I feel it will cause excess heat and pad wear on a cruise that could potentially cause brake failure.

Do you think 10 psi residual valves would put that much heat into the rotors? I don't know, but I kind of doubt it. I do think it would make for a lot more brake dust, though.

It would be an interesting test to put your car on jackstands, run it in gear for X amount of time then check the rotor face temp with an infrared thermometer, then install the residual valves and repeat the test (noting the ambient temps).

The amount of energy absorbed and dissapated through the rotors from regular daily driving just makes the 10 psi residual valves seem underwhelming. Again, I don't know, just sayin. :D

Sounds like a 12 pack experiement. :rofl: I'd want to run the car at highway speeds for a reasonable distance. I just feel like that's a bandaid covering up the actual problem. I put a 2lb in the rears but it's not enough to move the pistons back. I'll check with Frank and see if the individual he knows is running a tapered bearing.

lol... :D

I'm running tapered bearings in my 9"... I'd hate to say that there's no runout, but I can't feel any when I've messed with it.

Are you running a fixed caliper out back Scott?

Yep, they're fixed.

Thanks. I deleted my post because I think the PBR is technically a floating caliper and I didn't want to confuse people.

What rear end are you running. I would not think .005 would be noticeable unless the pistons are actually moving the axle under pressure farther than you can move by hand

It's a Moser 12 bolt, True Trac, 33 spline, big ford end with roller bearings. The .005 is definitely bearing clearance. It's not enough to cause a problem in my opinion. It's the side load on the bearing causing the issue.

I've noticed the pad knock back with my car too. It was far worst with the factory c clip 12bolt, better with the moser but still there.
It's manageable, but very annoying.

I've thought about the residual pressure valve. I was hoping 2# would be enough. I with you Todd, 10# seems excessive for pad wear and dust....

I'll be following this one.

I have to wonder how much pressure a brake booster and hydraboost puts on the system before any pressure is exerted on the pedal. There has to be a threshold amount. Maybe hydraboost will disclose it's specs in hope of a sale. They just might get it.

Spoke to Frank this weekend at RTTH
 

and he stated the same to me....the 10 lbs residual valve fixed the problem.

I know some of the high end calipers have springs behind the pistons to stop the problem.

I have a therory that I have not proven as of yet. I think the lower volume of fluild used because of the smaller bore size in a manual system makes the problem more pronounced. Seems like you do not hear about it as much with a power set-up.

I am seriously considering converting to a DSE power system soon. I will let you know if my therory works out.

I did the same with the Currie plates and still have a little knock back, but not as bad as it was.

I may have to give the 10 lb residual valve a try out back. What's the worst that can happen? Strand me out in the desert with no water? :rofl: I still think the factory has a solution for this in there rear disc setups on straight axles. All it can be is some residual pressure. Whether that's caused by power brakes or a factory residual valve I have no idea.

What factory cars run fixed calipers? I thought most use floating.

I have tapered bearings ( SET20 ) in my Moser, end-play is .015

I'm using Baer floaters with a C5 power booster, so I cant get any input other than the above end-play.

Thanks for the info. You are absolutely right, most factory setups are floating.

Ran a fast autocross this weekend and the brakes were useless. Unfortunately I'm going to have to go with a hydraboost unit since power brakes are not an option. My axles and bearing packs can't get any tighter. It's got to be fluid volume........ Wilwood really should come out with a fix for this but I'm not holding my breath.

Why is it Wilwood's problem? Many people run Wilwood's with sticky tires and 9" axles without any problem at all...including myself (manual brakes or not). When I had a knockback issue, it was actually due to the front bearings.

Many floating caliper designs can suffer from this problem as well.

Did anybody try the 10lb residual with success? The 2lb valve is what most people use.

You're right, it's my problem. I just assumed spending 3k on brakes wouldn't have me chasing my tail for a year. First it started with a mis communication between Wilwood and Moser. Guess who got Wilwood to design an updated plate to fix the excessive bearing slide? I have two of them sitting on my bench for free directly from the manager at Wilwood. And they are twice as thick since one of the originals was tweaked from side force. Oh yeah, that's after I paid for a set of Currie plates and spent the money to have the bearing pressed on and off.

I'm experiencing it on the street as well. When I say street, I mean regular driving. I go around two corners and my pedal easily changes about an inch. On the autocross is goes almost to the floor. If it was a track only deal then I'd be thinking front hubs. I still think it's out back.

The 10lb residual will probably fix it but I like to take it on some long cruises. Maybe I should try some new bearing out back before I drop a fortune on a new hydraboost setup and master cylinder.

Just got off the phone with Moser. They have changed to a non rubber O ring type bearing from napa. Hmmmmmm........ No good reason was given.

And this is a direct replacement for the set20 type bearing? Is it a roller or taper? Part number, pretty please?

Todd heres an idea, can you adjust your rear brake bias to complete soft and drive the car to see if there is a difference. That maybe one way to confirm your problem is in the rear.

Actually thought about that. I just don't see C6 hubs having excessive deflection putting around town. I can feel no play in the front bearings. The napa number is 88128 but it's a roller.

They shouldn't, and if they did, you'd feel it with the car up in the air. Mine only do it on a very hard corner, but I can still manage it on the street.

One way to tell what the heck is going on is to find a parking lot large enough for you to do a "skidpad test". Run the car hard enough to push the pads back, then coast to a stop (still turning in a circle). Jack the car up and you'll see the pad that is pushed away from the rotor. It's a very small gap, but it will be obvious. Trouble is finding a location to do this.

Nice. That's the attitude that makes me happy to help people out.

Thanks Matt. I should verify it first. I'm pretty sure I used the parking brake since it's internal on Wilwood to come to a stop and jacked up both ends. I seem to remember the front pads having drag and the rears not.... That's what got me focused out back. I'm ordering the updated bearings out back. I have a suspicion of what may be going on here. Hell I can drive it around the block and wiggle the car around and the knockback will be there. I'd bet a large sum it's out back still....

I really hate to point the blame at Wilwood. They have also been good to me. The whole being the guinea pig for both companies and having all that axle end play just pissed me off. I'll figure it out....on my dime. LOL

I have chased this problem for years...

List of what I have learned

• Roller bearing on a Ford 9 inch won't work for road racing.
  
  GM went to a stiffer front wheel bearing on the ZR1, STS-V and CTS-V to fix brake knock back.
  
  The bigger the rotors the bigger the problem.
  
  2 psi valves won't do it
  

The test I use is to do a figure eight in a parking lot at max lat then measure the brake pedel travel.

If you can't fix the problem learn how to left foot brake on the straights to pump up the brakes. I have done this for years. :yes:

Mark

Thanks Mark....they aren't a problem on the road course. One quick pump down the straight or between corners and they work great. The autocross is the real problem! 15-20 turns in 45 seconds. :yes: I do have the roller bearing and pulled them to reinspect last night and they feel like new. I have decided to try a 10 psi residual valve. Bret at Air Ride is using one on the rear of velocity with good results. If that doesn't prove to be reliable and make me happy then it's on to a tapered bearing and probably hydraboost.

Hi Mark,

Do you know the GM part number for the stiffer ZR1 front wheel bearing?
I want to make sure I have the right ones...

Let me see if I can help here..

1st I agree with Mark on the roller bearing, they are not the best for road racing.. tapper is the way to go..

2nd I would see about rotor run out on all 4 rotors, when setting up a car for racing I always have the rotors turned on a brake lathe.. when you have rotors that are assemble to hat, hub, & rotor you are going to have a stack up tolarance of pcs that have been machined probably flat & parallel within .002 to .004 & you added it up & you have an excessive amount of rotor run out which will knock back the pistons & have pulsation in your brake pedal.. run an indicator on the faces of your axles & hubs & see what your run out is..

3rd Caliper flexing on brackets.. The Wilwood parking brake assembly/caliper mount has some flex on it.. remove your pads & grab the caliper with your hands & you can move it.. For racing I make new brackets out of steel or aluminum with gussets machined into them to make them stiffer & it makes a difference on how the pedal feels & when you are going into corner.. look @ it this way.. large diameter rotors, hydraboost, high friction brake pads & racing .. got to have good stiff caliper mounts.. attached is a rear mounting caliper bracket I made for racing.. for asphalt racing steel brackets are welded on rear axle tubes & front brackets are welded on a fabricated spindle/upright also have floating rear ends with large bearings.. Take into consideration that in some forms of racing they want piston knock back to free up rotational weight.. can be done with o-rings in calipers instead of square cut rings

I'm starting with some tapered bearings. Moser should be sending them out today. Thanks for the info on the rear brackets being flimsy. I'll definitely look into that if I'm unhappy after my new tapered bearings. :thumbsup:

Steel caliper mounting bracket
 

Here is an example of a caplier bracket I made for racing that I forgot to attach..

Todd the tapered bearing will not eliminate the knock-back, but they will survive large lateral loads, unlike a ball bearing.

Also for future reference the moser uses a common tapered bearing used on trucks the SET20 bearing. Napa, autozone, ect.. should carry these

It should eliminate some of it. I don't expect perfection but feel the roller bearing is causing at least 1/2 my problem. Moser said I won't have any end play with these new axles and bearings. I'll report back next week!

update?
 

Todd-
any updates? I had heard that Wilwood now includes the improved bracket...

Gonzo-
Have those steel brackets proven to work or cure the issue, and do you sell them? How about thos pedals you made for the Wilwood base as well? :)

Kinda, I got the tapered bearing axles a few weeks ago but they were machined wrong. I took some measurments and sent them back to moser. They paid for shipping each way and the repair. I just got them back last Friday and proceeded to put them in but noticed one of the wheel studs got boogered up in there lathe and now I'm waiting on a stud. I'll be racing Sunday and that will be the true test. There is definitley no end play now in either side. I'll update you guys one I drive and race Sunday.

The new bracket from Wilwood helps but will not cure the problem.. If Vegas 69 gives me all the components that he has for the brake system I might be able to help.. If he was closer we would get this problem solved.. Maybe he will want to take a 5 hour drive!?!??

I have been thing of making these brackets for production..hmmmm :unibrow:

The machined pedals for the Wilwood base will see in the next couple of months in production.. tooling for the forgings will be ordered next week.. You didnt hear that from me though :_paranoid

knock-back
 

Gonzo-
Let me know if you need another test vehicle, as my Camaro will have the Wilwood W4As on the rear with a 14" rotor. I'd say if there is any runout left, we'll be able to find it (but I hope not).

As for the pedals- that's some nice stuff. They'd be perfect anodized black. Like I said before, let me know if you need me to do some beta testing, cuz I have the same base going on my ride.

Todd- Good luck this weekend

I will let you know if I make another set of caliper brackets out of steel.. nice rear brake set up!

The pedals will be coming soon for the Wilwood bases.. from what I have heard the pads will be stainless & offered with different coatings on the pedals :unibrow:

How did the weekend go? I am waiting with baited breath and a poised pen and checkbook.

It was better but still not fixed. For normal street driving it's fine and an improvement of a roller bearing. Once race time comes around I lose about 2-3 inches of pedal after a couple turns. It's more managable but not what I hoped for when I bought the brakes. If you are getting ready to buy and rear end, make sure it's a tapered setup or even a full floater if you plan to race a bunch. If I knew then what I know now......