Solving Pad Knockback

[silver69camaro]

As our R&D Camaro has progressed, we've been increasing front and rear grip by a fair amount. Late last year, I've been battling some pad knock-back that's been driving me nuts. This car uses modified C6 front suspension with a 9" axle and tapered bearings. Knock-back is more noticeable if you have manual brakes, because there is more pedal travel needed to overcome the pad-to-rotor gap. If you are experiencing a soft brake pedal after a hard corner, follow this to hopefully solve your problem.

First, find out which side is having the problem. Does the brake go to the floor immediately after a hard left turn, or right?

Second, find out if it's the front or rear. I do this by turning hard, then shutting the engine off while in gear, coasting to a stop. Jack the car up, remove the wheels, and you should see daylight between the pads on the offending corner. Typically you'll see .010" gap or so - it doesn't take much. Obviously this requires an area you can safely do this in. A guy could find a parking lot at night and run a tight circle to accomplish the same thing.

Now that you know if it's front or rear, you can start to diagnose the problem. If it's a front wheel, replace the hub assembly if you have C6 hubs. If you have a spindle pin & bearing, the bearings and races probably need to be replaced.
The rear is a bit more difficult. Possible causes are:

1. Axle flex: Happens typically if R-compound tires are used on stock axles with heavy cars. Aftermarket axles tend to have more material that negate this problem.

2. Improper bearing preload: If the bearing isn't held tight enough in the housing end (press-on bearings only), it can move around causing the rotor to kick the pads into the calipers. Try adding some 12-bolt carrier shims between the bearing and housing end.

3. Excessive c-clip clearance: For guys that run c-clips, you may need to run thicker clips to keep the axles walking laterally. I've heard of people making a thick clip, then shaving it down a couple thou at a time until it fits snug.

4. Axle retainer flex: For axles with press-on bearings, check the axle retainer plate. This was the problem I was having, which the retainer would flex enough to allow the axle to move. The Wilwood retainers are pretty thin and are prone to flexing. Mine was pretty bad, and replaced it with a 1/4" retainer plate. Problem solved! Currie makes a nice thick retainer for about $16, but is a full-circle type retainer so you have to put it on before the bearings are installed.

Here's the set that I made. It ended up working very well and I didn't have to remove my bearings.

Brakes in question


Old, Bent Retainer Plate


New Retainer




Hopefully this helps. It's nice to fix this problem without resorting to piston springs or residual valves.

[Bow Tie 67]

Nice work Matt.

Quick question: What is your axle axial play before and after the fix?

[silver69camaro]

Tough to say. After a hard corner, the pads would be moved back about .015", but I couldn't move the axle by hand at all...it felt pretty solid.

The new retainer eliminated the knockback entirely.

[silver69camaro]

Update: Talked to the folks at Wilwood and turns out they've been aware of the issue. As of about 10 months ago, all brake kits have an updated retainer that should take care of the problem in most cases. It increases the preload for better retention. If you have an older kit (like I did), just order one up.

Something else we talked about that I didn't mention on this post. If you have a heavy car with 335's with minimal backspace, it's possible that you're overloading the axle and bearing which could cause flex.

[Vegas69]

Nice Matt, but I caused Wilwood to re design that bracket. It's nearly as thick as the Currie now. I also tweaked mine a little. I had Currie plates on my old roller bearing axles but decided to use the updated thicker Wilwood retainer when I changed to the tapered bearing setup. I also can't feel any end play in either axle. I do have the shims needed to put behind the bearing race, it just feels tight now. I'll post back on the 1" bore master.

[Vegas69]

Here are two videos I did over a year ago. This was with the thin retainers that came with the kit and roller bearings. The problem was the retainers had no preload on the axle bearing. In fact there was a small gap between teh bearing race and retaining clip. That's when I contacted Wilwood and they started working on a fix. Unfortunately that meant I had to purchase Currie plates and pay to have my bearing pressed off and on.In fact I paid for new bearings as well. That change did cure the no pre load issue and retainer flex, but it didn't totally solve the problem. I'd say cut it in half.

Next I switched to tapered bearing axles from moser. At that time Wilwood had came out with the updated retainer so I used it and found that the pre load machined into the surface was identical to the Currie plates. So I used the wilwood plate to keep from needing to press bearings off and on. This didn't totally cure it either. It did cut it in half again.

I just switched to a 1" bore master to attemp to make up for some of the rest by providing more volume. I haven't driven the car but will this weekend. At this point I feel it's a combination of axle flange and bearing flex. I have 33 spline axles. Part of me wonders if the deep dish wheel and parking brake rotor offset effects it as well. Bottom line is I hope the 1" bore makes it decent until Wilwood comes out with a floating caliper or rotor. That's the real fix in my opinion.
http://www.youtube.com/watch?v=Ff0fkCmHNWk
http://www.youtube.com/watch?v=4d98pjx6sz8&NR=1

[silver69camaro]

Todd, what is your wheel size and backspace? I'm wondering if you're overloading the bearing...

Also, you probably don't want to hear this, but some people have had to actually mix and match bearings and races...meaning, you may have to have a handful of races and find out which one allows the tightest fit. It's hard to imagine, but it's true.

[Vegas69]

Todd, what is your wheel size and backspace? I'm wondering if you're overloading the bearing...

Also, you probably don't want to hear this, but some people have had to actually mix and match bearings and races...meaning, you may have to have a handful of races and find out which one allows the tightest fit. It's hard to imagine, but it's true.

It's an 18x12 with a 5.25 BS. Mine will do it on the street in what I wouldn't consider close to race loads. With the offset of the wheel putting more stress on the bearing and the rotor being recessed more due to the parking brake, I think we may be onto something. I was sitting in the garage a few weeks ago just looking at the setup and it got my wheels spinning. Any axle flange deflection would be amplified the farther the rotor is from the axle flange. I'm interested to hear what you have to say on the bearing load.

[silver69camaro]

This drawing shows your 18x12 with a 5.25" B.S. using the Wilwood 13" Pro Street brake. You have a 4.088" lever-arm between wheel C/L and bearing C/L, which I know doesn't sound like much but it's about 2.25 times more than what it should be. I think you're in full-floater territory there...

As you know, the bigger the rotor, the bigger the knockback...

[Vegas69]

Good stuff Matt! I appreciate you taking the time. I'm going to limp through with my 1" master until they come out with a floating caliper or rotor.(Supposed to be in the works) The problem with a floater rear end is my 4 link mounting points are to close to the axle flange. I'm glad my hunch was correct.

[parsonsj]

Hey Matt,

Did our phone conversation a couple of months ago lead you down this path? I want credit, dammit!!!

Seriously, glad you found the problem. When I replaced my front wheel bearing and fixed the knockback on II Much it was such a great improvement.

jp

[parsonsj]

Lever arm, eh? My 7 3/8 BS (11") wheel would have approximately half that, I think. That's gotta be why the original Wilwood bearing retainers didn't cause me any problems.

Very nice analysis.

jp

[Ron.in.SoCal]

Nice job you guys. I read about Todd's issues/videos/resolutions over on Lat-G and it got me thinking about how to avoid it. This just adds to the body of knowledge.

Matt - nice diagram, as usual. Now i have two from you...

Todd- give me your take on the 1" master when you get a chance?

[Vegas69]

The 1" master totally transformed my car. I beat it as hard as I could on the street with not a hint of knockback. I can't believe the difference. I like the feel better as well. It firmer but with a much higher pedal and it gives me way more confidence. I don't know if the 7/8 was never right or this one made that much difference. There was no fluid behind the bore and the seals looked fine. The bore was scored and the front fluid was nasty with some fine particles. I think it was from the excessive pumping I did in my 8-10 races and street driving. Bottom line is, I'm happy !!!

[Payton King]

for the above write up. That will help a lot of people. I had sized my masters down too small trying to get the car to stop. Changed brands of pads and went to a .70 for the front and .75 on the rear per Tobin's recs and my car has never stopped better. Not getting the knock back I was...I changed my Wilwoods to the Currie retainer last year. Before I changed the retainer I would knock the pads back pulling out of the driveway.

Todd, glad your problem is fixed. If you are going on that long trip and will just be autocrossing, switch the AX6's into the front. It will stop way better than the XP10's on the street and autocross.

[Vegas69]

I still haven't put them out back. LOL Been busy.

[Skip Fix]

3. Excessive c-clip clearance: For guys that run c-clips, you may need to run thicker clips to keep the axles walking laterally. I've heard of people making a thick clip, then shaving it down a couple thou at a time until it fits snug.

My TA when new had so much free play the drum would wear on the backing plate! The dealer never figured it out under warranty where the rubbing sound came from , rebuilt the posi, put new sway bar bushings etc. Found it when I added semi metallic shoes a few years later.

[Payton King]

about you pulling the 10's out of the front and putting the AX6's in there for the weekend. Leave whatever you are running in the rear. Easy swap.

[a67]

for the above write up. That will help a lot of people. I had sized my masters down too small trying to get the car to stop. Changed brands of pads and went to a .70 for the front and .75 on the rear per Tobin's recs and my car has never stopped better. Not getting the knock back I was...I changed my Wilwoods to the Currie retainer last year. Before I changed the retainer I would knock the pads back pulling out of the driveway.

Todd, glad your problem is fixed. If you are going on that long trip and will just be autocrossing, switch the AX6's into the front. It will stop way better than the XP10's on the street and autocross.

I am interesting in knowing which pads you used for the .7 and .75 Mu. From my searches Wilwood has decent graphs of pad performance vs temperature. And EBC has performance graphs for their Bluestuff pads.

But for others such as Hawk's, CarboTech, Porterfield, and others, there isn't any real information on the friction levels.

Bob.

[Vegas69]

about you pulling the 10's out of the front and putting the AX6's in there for the weekend. Leave whatever you are running in the rear. Easy swap.

Ya, I already have the wheels back on. haha