Residual valves for pad knock back [Archive]

Van B

09-03-2010, 11:54 AM

I am curious about what calipers are being used when installing a residual pressure valve on the rear. I have 6 piston fixed on front and PBR floating calipers on front and have a good amount of knockback that is more evident at track events than on the street.
Should I be using a residual pressure valve in front?

69X22

09-03-2010, 12:49 PM

They certainly help but do not cure all of it.

Frank,
10 lb is supposed to be for drum brakes, but I see people are using them for the disc. What lb are you installing your cars?

Steve68

09-03-2010, 07:01 PM

You could have taken one home 2 hours ago!

We have them in stock.

Don't worry I'll be back,

Good to see the JBI car in the shop,

The WidowMaker

09-03-2010, 07:13 PM

Finch, youre using them with the z06 rears? did you run without them before you installed them? what rear are you using and with what style bearings?

thanks, Tim

Finch

09-03-2010, 08:33 PM

Finch, youre using them with the z06 rears? did you run without them before you installed them? what rear are you using and with what style bearings?

thanks, Tim

I ran the Z06 rears without the valve and the knock back was very noticable. The 10lb valve helps a ton but does not cure it 100%...I have learned to drive with it as is so does not bother me anymore.
I am running a 9 inch in the back with the torino style ends.

The WidowMaker

09-04-2010, 07:22 AM

I ran the Z06 rears without the valve and the knock back was very noticable. The 10lb valve helps a ton but does not cure it 100%...I have learned to drive with it as is so does not bother me anymore.
I am running a 9 inch in the back with the torino style ends.

Thanks! thats what i was looking for.

Vegas69

09-04-2010, 07:56 AM

What master cylinder are you running?

Steve1968LS2

09-04-2010, 09:58 AM

RPV will not do anything to help with pad knockback..

Think about it.. even 10-lb valve is nothing compared to the force of pushback exerted on the pad. It might help a tiny bit driving around town, but if you think of the physics involved then how can it help in any real driving enviroment? Go get a bathroom scale and push on it with your finger to see how little force 10-lbs is..

I really can't see how a valve that only holds a tiny 10-lbs of pressure can do anything.. it think it's a placebo effect. lol

The only real way to fix knockback is to eliminate the flex and the movement that causes the pad to push the piston back into the caliper. Another way is to move to a floating rear caliper which can help mitigate the flex and movement that is still there.

The WidowMaker

09-04-2010, 07:03 PM

im probably way off, but in my head i see it as this; as the car is cornering and the forces are being exerted on the bearing and the rotor, the pads get pushed away on one side of the caliper as a result. after your straighten out the car and the forces are gone, the 10 lbs push the pads back a few thousandths. i could even see that with the residual pressure how one side of the caliper is knocked away, and the residual pressure slides the other side of the caliper closer to the rotor. once on the straight, the side that moved closer moves away when the rotor moves back and the side that was further now gets the extra pressure and slides closer.

i dont think the pads are acutally getting moved very much either with the cornering, or with the 10 lbs, but i "think" i can see how just the little residual pressure could help. i have a feeling its thousandths of an inch we're talking about and although its only 10lbs i again "think" that should be enough to move a piston.

Vegas69

09-04-2010, 07:26 PM

Brian clearly has experience with the 10 psi residual so I'm going to take his advice and say it does help. I can tell you a 2psi valve does nothing because it's in my car. Autocross is a tough enviroment for knockback. My cars had it severe since the day I built it. I pumped brakes between every turn up until my last autocross. In fact, my 7/8 Wilwood master was beat up and the front lines and hose had aluminum material when I swapped it. The brakes SUCKED on the race track. Atleast on the road course I only had to brake check it 10 times per lap. It was slowing me down since I'm a right foot braker. I swapped out to a 1" and didn't have to pump brakes at my last autocross. That's one habit I had no problem forgetting. I've yet to Road Race again since El Toro but I think it will be OK. I may take Brians advice if I'm not happy. My concern was always long distance driving with the 10psi residual. I'd say a road trip from TN to Sema debunks that myth.

CarlC

09-04-2010, 09:19 PM

So a 10psi residual valve is used. How much force can be applied to the face of the brake pad before the valve releases?

ProdigyCustoms

09-05-2010, 03:57 AM

Carl, your putting 1200 to 1800 PSI out, so touching the brake pedal is more then 10PSI.

Steve, you will be shocked how much the 10LB can do. It can even cause excessive drag and brake pad wear, or rotor warping if we are not carefull, but if the knock back is enough that it needs help, it is not a issue.

Jeff, as for your front knock back, you need to check you bearings. If they have to much play, knock back. And front back knock back is really scary ****! JP fought this on II Much even with Corvette hubs.

Todd, the 10LB made a instant improvoiment on the Bull Run Racer but I am also changing to the thicker bearing retainers this week before RTTHs. But you can feel the difference. Try a 10LB, it will fit right in place of you 2LB

Yes Tim, I think you kind of have it. All these calipers have crossovers in them from side to side on the calipers. So with a RV, instead of pushing one side of the pads away and fluid lushed up line, with a RV, it Encouarges the fluid to push side to side in the caiper instead of up line. It kind of crosses over and the pads move togther.

Steve68, JBI is going to be cool as hell! And faster then ever. I am headed to the shop to work on it now. Don't know if Michael showed you the wheels, they are something one off we had built. I just giggle when I look at them! Pictures for everyone next week on that.

I think I will take it to Pigeon Forge next week.

parsonsj

09-05-2010, 08:47 AM

Jeff, as for your front knock back, you need to check you bearings. If they have to much play, knock back. And front back knock back is really scary ****! JP fought this on II Much even with Corvette hubs.That's true. I had a defective front hub. After I replaced it my knockback went completely away.

You guys probably saw it, but didn't Matt Jones post something about wheel offset and leverage, and better retainers than the Wilwood's?

jp

69X22

09-05-2010, 08:53 AM

I'm thinking about running a 10 psi in the rear with a better axle retainer and none up front. I am running a 1" Wilwood MC with a Hydraboost system. Do you guys think the 10 psi residual valve is going to be an issue on a 400 mile trip?

Vegas69

09-05-2010, 11:22 AM

That's true. I had a defective front hub. After I replaced it my knockback went completely away.

You guys probably saw it, but didn't Matt Jones post something about wheel offset and leverage, and better retainers than the Wilwood's?

jp

Matt did put together a rendering for me and the deep dish is a huge contributor due to excessive leverage on the bearing. I don't personally think the redesigned Wilwood retainer is a problem. The original retainer was thin and had the wrong pre load.

parsonsj

09-05-2010, 11:33 AM

redesigned Wilwood retainerI haven't seen the re-designed retainer, but the original one was pretty flimsy.

jp

a67

09-05-2010, 05:34 PM

www.centricparts.com/files/Centric%20White%20Paper%20B2-Knockback.pdf
www.g-machines.com/forum/showthread.php?t=68924

Bob

Vegas69

09-05-2010, 08:03 PM

I haven't seen the re-designed retainer, but the original one was pretty flimsy.

jp

If you would've been follwing my build thread, I got them to redesign the retainer and installed is quite some time ago. :fingersx:

parsonsj

09-05-2010, 08:48 PM

If you would've been follwing my build thread, I got them to redesign the retainer and installed is quite some time ago. Hangs head in shame.

Hey, it occurs to me there's another reason I may not have had as much rear pad knockback as some of you guys: I ran 35 spline axles. I'd bet they deflect less at the flange than 31 spline versions.

jp

Vegas69

09-05-2010, 09:21 PM

I have 33 spline so I doubt the 35's would fix it either. Wrong part for the application. How many factory cars have a 5.25 BS on a 12 inch wheel? NONE!

ProdigyCustoms

09-06-2010, 12:19 AM

What John is not saying is that we will NOT be running full floaters and are engineering another fix we think will solve the problem but we are a few months from testing and verfying our thought process.

I got to know II Much very well. I can attest to the fact that II Much showed zero knock back with a live axle. And I personally DO NOT think in his case BS mattered at all.

In the meantime, back to the original question, in A / B testing the the 10LB valve did make a big difference, Just did it this week.

Steve68

09-06-2010, 05:40 AM

Good to hear! I'll have to pick one up soon,

I've seen the wheels, they look good, it will look cool when put back together,

Steve1968LS2

09-06-2010, 09:15 AM

Matt did put together a rendering for me and the deep dish is a huge contributor due to excessive leverage on the bearing. I don't personally think the redesigned Wilwood retainer is a problem. The original retainer was thin and had the wrong pre load.

The best retainer is one I got from Currie that had to be installed before the bearings on the axle.. it clamped all the way around and was WAY thicker than the ones that come in the brake kits.

A little more of a pain, but the right way to do it (isn't that always the case?)

You can see them in this story:
http://www.camaroperformers.com/camaro-tech/wheels-and-tires/camp-0812-1968-chevy-camaro-third-member/index.html

https://static1.pt-content.com/images/pt/2010/09/camp0812041968chevycamarothirdmemberaxle-1.jpg

parsonsj

09-06-2010, 10:20 AM

I like the looks of those bigger retainers, for sure! How do you get the Wilwood backing plates on with that 360* design?

jp

Vegas69

09-06-2010, 10:26 AM

Been there, done that. The new Wilwood retainers are nearly as thick and have the proper preload.(Same as Curries) When I changed to a tapered bearing it would've required pressing off the old bearings to send Moser the Currie plates. I still think the new design is fine. I've had no axle leak yet with the tapered bearings and Wilwood updaed retainers. That was one of the main reasons Currie designed the plate. Updated plates and Currie retainers pictured. The only tow things that have made a difference for me was changing to a tapered bearing and going from a 7/8 to 1" bore master.
https://static1.pt-content.com/images/pt/2010/09/0064-1.jpg

silver69camaro

09-14-2010, 09:48 AM

On another note, if some of you have tried everything to eliminate knockback and haven't solved the problem, you may want to check rotor runout with a dial indicator. Runout is a problem with fixed calipers, and excessive amounts will make knockback worse.

If you have excessive runout, you could probably use shims used on C2/C3 Corvettes for the same issue.

silver69camaro

09-14-2010, 09:57 AM

Think about it.. even 10-lb valve is nothing compared to the force of pushback exerted on the pad. It might help a tiny bit driving around town, but if you think of the physics involved then how can it help in any real driving enviroment? Go get a bathroom scale and push on it with your finger to see how little force 10-lbs is...

Remember the 10-figure is pressure, not force. If you have a rear caliper with 5 square inches of piston area (typical for rear calipers), that's about 50lbs of force. That's enough to hide knock back. Also enough to cause drag as well.

The only real way to fix knockback is to eliminate the flex and the movement that causes the pad to push the piston back into the caliper. Another way is to move to a floating rear caliper which can help mitigate the flex and movement that is still there.

This is true.

I may have mentioned this before, but some have resorted to matching bearing cones to races with sucess. I'm not sure what is done to "match" a set, but people have found that not all cones and races are the same, and some are a better fit for others.

sik68

09-14-2010, 10:45 AM

In addition to deflection, I think axial play in the shafts is a real killer.

I was watching a NASCAR race at Infineon; they had a footwell camera for some of the drivers, and they were all tapping the brake pedal on the straights. I suppose that's the most tried and true way to eliminate knock back with a solid axle. Don't we all need something else to do with our feet anyways? :)

The GMR

09-14-2010, 07:57 PM

The best retainer is one I got from Currie that had to be installed before the bearings on the axle.. it clamped all the way around and was WAY thicker than the ones that come in the brake kits.

A little more of a pain, but the right way to do it (isn't that always the case?)

You can see them in this story:
http://www.camaroperformers.com/camaro-tech/wheels-and-tires/camp-0812-1968-chevy-camaro-third-member/index.html

https://static1.pt-content.com/images/pt/2010/09/camp0812041968chevycamarothirdmemberaxle-1.jpg

Dam, did not even think of this but I made several sets of those about a year ago for some of the off-road guys running BBF 35 spline ends with 1/2 T bolts.

basically we cut them from 3/8 cold rolled steel. Still on the trucks to this day after several long races.

jason

John Wright

09-15-2010, 03:50 AM

I was watching a NASCAR race at Infineon; they had a footwell camera for some of the drivers, and they were all tapping the brake pedal on the straights. I think some of that is to get the front shocks to settle back down after accelerating off the previous corner...so they can get some weight transfered back up on the front tires to turn at the next corner.

You should see them during the qualifying runs at Daytona and Talledaga...LOL..the crew chief reminds them about pumping the brakes back up when they are slowing back down to enter pit road after the run. I think they run very little, if any, RVPressure trying to get the most speed out of the car....they don't want any drag at all at the pads.