Wilwood Pad Knockback

[67Rally]

I posted this question on PT as well, but haven't gotten much feedback yet.

I had been dealing with some pad knockback on the street over the past couple years. 12 bolt (c-clip) rear with 12.19" 4 piston Wilwood DynaPro's out back. (6 piston DynaPro's in the front, 7/8" Wilwood Dual MC, Wilwood Proportioning valve, manual brakes). All suspension, steering and brake items have less than 3,000 miles on them.

I swapped out the 12 bolt for a brand new Strange S60 with the tapered bearings, and kept the same brake setup. I have had 1,000-1,200 trouble free miles this year. Not a hint of knockback, until I ran my first autocross on Sunday, and it was back with a vengeance. First push on the brake pedal went most of the way to the floor, and then the next push I had perfect brakes, all day long. I had no faith in the brakes, and was too focused on the learning course to try left foot braking on my first outing.

I've done a ton of searching, but most threads didn't seem to have a resolution here or over there. I don't want to switch to a floater rear.

-I have seen mention of the D154 calipers, has anyone had good success with this option? Just adapting the D154's to my current rear brakes, or go with the full D154 rear parking brake kit?
-Some people recommend a residual valve. 2lbs? 10lbs? Is this a bandaid fix?
-Same with knockback springs? 5lbs? 10lbs? Better or worse than a residual valve?
-Some people recommended a larger bore MC, maybe a 1" bore?


Open to any and all REAL LIFE experiences. I really thought the tapered bearings had solved my issues, but they only fixed them on the street.

[dontlifttoshift]

Originally Posted by 67Rally

I don't want to switch to a floater rear.

Knock back is caused by the rotor pushing the pistons back in the caliper. Eliminate the movement of the rotor or allow the caliper to move with the rotor.

Wilwood's bearing retainers aren't exactly superstrong pieces, they usually bend slightly on installation and after that it doesn't take much for them to flex resulting in bearing end play and consequently knockback. Stronger retainer plates can be fabricated and careful machine work to maintain the proper bearing preload can go a long way in minimizing knockback.

Floating hub eliminates the rotor movement. You don't like that option, I get it. It's not inexpensive or easy at this point. It is the only surefire way that I am aware of to eliminate rotor movement.

The floating caliper sounds like a perfect solution. Late model Mustangs use a single piston floating caliper with a rotor diameter as large as 13.8" and have no issues with knockback. This is on a clip rear end with ridiculous amounts of axle end play. The key to a floating caliper working is that the force required to move the piston back must be greater than the force required to move the caliper on it's slides. If the piston pushes back before the caliper slides, you still have knockback. Even with a floating caliper, a residual valve may still be required.

What size wheels and backspace are on your car? Front and rear please.

Have you confirmed that the knock back is definitely coming from the rear? Misadjusted front bearings or spindle flex can cause this as well.

[67Rally]

Originally Posted by dontlifttoshift

Knock back is caused by the rotor pushing the pistons back in the caliper. Eliminate the movement of the rotor or allow the caliper to move with the rotor.

Wilwood's bearing retainers aren't exactly superstrong pieces, they usually bend slightly on installation and after that it doesn't take much for them to flex resulting in bearing end play and consequently knockback. Stronger retainer plates can be fabricated and careful machine work to maintain the proper bearing preload can go a long way in minimizing knockback.

Floating hub eliminates the rotor movement. You don't like that option, I get it. It's not inexpensive or easy at this point. It is the only surefire way that I am aware of to eliminate rotor movement.

The floating caliper sounds like a perfect solution. Late model Mustangs use a single piston floating caliper with a rotor diameter as large as 13.8" and have no issues with knockback. This is on a clip rear end with ridiculous amounts of axle end play. The key to a floating caliper working is that the force required to move the piston back must be greater than the force required to move the caliper on it's slides. If the piston pushes back before the caliper slides, you still have knockback. Even with a floating caliper, a residual valve may still be required.

What size wheels and backspace are on your car? Front and rear please.

Have you confirmed that the knock back is definitely coming from the rear? Misadjusted front bearings or spindle flex can cause this as well.

Thanks Donny, you also replied over on PT. I had posted the following question about ruling out the front pad knockback:

Let's say I take it to a vacant parking lot and make some hard turns and can replicate the knockback. Then pump the brakes back up, crank the proportioning valve all the way to the front (so there are no rear brakes) and perform the same manuevers.

Is it safe to assume that if I don't have knockback (with the rears blocked off), that the fronts are not the culprit, and if I do still have knockback that the fronts are part of the issue, but the rears may also be involved?

Fronts are 18x8 w/5" bs, rears are 18x9 w/5.25" bs. 245/40/18 and 275/40/18 Nitto 555's.

I the bearing retainers are thicker than standard. The came from Strange, when I had the new rear built.

[dontlifttoshift]

I don't think that will work. Matt Jones had a method that he outlined in one of the floater threads on PT that seemed legit. I don't recall what his method was and I can't seem to find the thread......sorry.

That wheel and tire package isn't terribly aggressive on the rear, I'm really surprised you are having an issue with knockback at all.

The prop valve really doesn't shut off the rear brakes, it only raises or lowers pressure to what ever end it is plumbed in after the knee point. I think the knee is about 300psi.

[Vegas69]

I went through this 4-5 years ago. I got moser and wilwood on the same page and wilwood redesigned their retainer. It's plenty thick these days. Besides, I had the full circumfrance retainer from currie and it didn't make a difference. Went to a tapered bearing, 2 psi residual, shimmed the bearing races, 1" master cylinder and who knows what else. A floating caliper was the only solution. I believe it was a d154. Never a problem after anywhere.

[67Rally]

Originally Posted by Vegas69

I went through this 4-5 years ago. I got moser and wilwood on the same page and wilwood redesigned their retainer. It's plenty thick these days. Besides, I had the full circumfrance retainer from currie and it didn't make a difference. Went to a tapered bearing, 2 psi residual, shimmed the bearing races, 1" master cylinder and who knows what else. A floating caliper was the only solution. I believe it was a d154. Never a problem after anywhere.

Thanks Todd. I found one of your old threads (can't remember if it was here or on pro-touring), but didn't see a resolution to the knockback.

Do you still have the 2psi residual valve on there? What about the 1" bore MC? Just curious if it was strictly the Wilwood floating calipers that resolved your problem, or a combination all the fixes.

[Vegas69]

The solution is definitely in my thread but good luck finding it... ha

Everything diminished the amount of knockback except for the 2 psi residual valve which did nothing. My opinion is that the D154 floating caliper is the best solution to knockback on anything except for a car that is going to be raced 10/10ths. If you look at a majority of rear wheel drive factory cars, they have a floating caliper. The only time you see a fixed caliper is in a bearing pack situatioin like a corvette. There is a reason for that. Even with a traditional front hub, it can cause problems if things aren't in perfect working order. Wilwood has a solution for you. They used to advertise it.

[67Rally]

Originally Posted by Vegas69

The solution is definitely in my thread but good luck finding it... ha

Everything diminished the amount of knockback except for the 2 psi residual valve which did nothing. My opinion is that the D154 floating caliper is the best solution to knockback on anything except for a car that is going to be raced 10/10ths. If you look at a majority of rear wheel drive factory cars, they have a floating caliper. The only time you see a fixed caliper is in a bearing pack situatioin like a corvette. There is a reason for that. Even with a traditional front hub, it can cause problems if things aren't in perfect working order. Wilwood has a solution for you. They used to advertise it.

Found it here, only 2000+ posts into your thread: https://lateral-g.net/forums/show...ing#post356326

Wilwood does sell the full d154 floating caliper kit with internal parking brake now. I'll probably buy the whole new assembly and then sell off what I have as a full rear brake assembly rather than buying new brackets and the floating calipers and trying to sell off the fixed Dynapros by themselves.

I still need to rule out front knockback first, just to be sure.

[sik68]

For what it's worth, I installed thicker c-clips that took out nearly ALL end play of my axles...barely able to slip into the axle groove. You can search for the corner carvers thread for the guy that sells em. Here is the email address fyi:

[email protected]

I can say that I put a residual valve AND return springs behind the caliper pistons and neither seemed to help knockback one bit.

My car is still being put together so I can't tell you whether the thicker c-clips have reduced knockback. I have C5 corvette floating calipers.

[Vegas69]

Originally Posted by 67Rally

Found it here, only 2000+ posts into your thread: https://lateral-g.net/forums/show...ing#post356326

Wilwood does sell the full d154 floating caliper kit with internal parking brake now. I'll probably buy the whole new assembly and then sell off what I have as a full rear brake assembly rather than buying new brackets and the floating calipers and trying to sell off the fixed Dynapros by themselves.

I still need to rule out front knockback first, just to be sure.

I agree, if you have an emergency brake, autocross it and coast to a stop and see which pads are separated from the rotors.