Pad Knockback

[schemauer]

Just an idea for those of you experiencing pad knockback. I know there was a thread here at some point last year on this topic.

I had this happen last year in my Talon. I found that my front axle nuts were a bit loose when going through the car. Tightening them cured the problem....

 

[Elvenhome21]

I was the one who posted how to fix that, I used 2lb brake pressure residule valves. THe axle nuts and bad wheel bearing are the main cause of knock back

 

[cioc]

I'm currently battling with knockback as well. I've never replaced the wheel bearings...120K on the clock...but I don't feel any play either. I figure they HAVE to be pretty worn given the type of driving I do and the fact that I live in the northeast where potholes are large and numerous. If the bearings don't do the trick, I'm going to try the residule valves. Elvenhome21, does the installation of these valves cause constant drag on the brakes?

 

[schemauer]

You definitely want to replace the wheel bearings - and torque the axle nuts to spec. I think it's 150 ft-lbs. Hopefully this cures the problem.

 

[Elvenhome21]

with the 2lb valves, no they don't cause drag. As for the wheel bearings when I switch over from my steet spindles and struts to my racing setup. I just impact the hell out of the axle nut. 200-300 ft lbs. It doesn't seem like it matters what a person does to fix pad knock back, its just going to happen. In a lot of ways pad knock back is a good thing for road america because it cools the brakes better by not dragging down the straights. Just give them 2 or 3 good stabs before you get to your braking zone and your good to go.

 

[cioc]

Well, I replaced all of my wheel bearings, torqued everything to spec and guess what...still have knockback. I just had my first track day at Watkins Glen on a set of R compounds and I experienced knockback around every turn!!! Pumping the pedal once or twice cured the problem each time but wore on my confidence rather quickly. So far I've dug further from the suggestions of many and I've found that I do have .008" runout on my front driver side rotor(Stoptech) and .020" on my driver rear rotor(Powerslot). Is that enough to cause this and wouldn't it happen under straight line conditions as well? I'm just searching for reasons why it can happen other than bearings. If it's inherant to the design of a 1g suspension so be it, I'll drive around it but I don't think everyone on here has experienced the same problems. Any suggestions?

 

[Elvenhome21]

The knock back is a inherent design with a 1g, the spindles/bearing werent designed for high side load. But its a good practice anyways to stab the pedal once before you start your braking to 1. make sure you have brakes, and 2. To push the fluid around and hopefully get some cooler fluid in the caliper.

 

[cioc]

I hear what you're saying about the design flaw but what excuses guys like Collier and Mueller from experiencing this phenomena?

 

[mavisky]

I know that Greg was on Stoptech's and replaced his front wheel bearings once a year, and sometimes mid-year. I'm guessing he probably used the residual valve as well on the setup.

 

[cioc]

The residual valve seems like a good idea with one, maybe two drawbacks. Firstly and most importantly, what happens when you come back from a 20 minute session of killing your brakes and come to a complete stop? Your rotors and pads are extremely hot and from everything I've read you absolutely do NOT want the pads to transfer a localized layer of material to the rotor. With a 2lb valve, when you come to a stop, the pads will be touching the rotor. The second drawback is with drag.

Elvenhome21,
Earlier in this thread you mentioned that the valves do not create drag. Why is this? If it's maintaining 2lbs of fluid pressure after the valve how can it not create drag? Am I missing something here?

 

[Elvenhome21]

The residual valve seems like a good idea with one, maybe two drawbacks. Firstly and most importantly, what happens when you come back from a 20 minute session of killing your brakes and come to a complete stop? Your rotors and pads are extremely hot and from everything I've read you absolutely do NOT want the pads to transfer a localized layer of material to the rotor. With a 2lb valve, when you come to a stop, the pads will be touching the rotor. The second drawback is with drag.

Elvenhome21,
Earlier in this thread you mentioned that the valves do not create drag. Why is this? If it's maintaining 2lbs of fluid pressure after the valve how can it not create drag? Am I missing something here?

the valve only can hold 2 psi of brake fluid pressure, Which for an example a human can create 4 psi with there mouth. It doesnt create any more realistic drag than the pads already do. It just keeps that tiny amount pressure in the line.

As for the cooling down, I always jack up the front on my GST and leave it in 1st or 2nd gear and leave the engine running to keep the rotors spinning for a minute or 2. With an awd theres nothing you can do except move the car a couple inchs every couple minutes.

 

[schemauer]

FWIW torquing the axle nuts properly took care of 90% of the pad knockback I was experiencing. I was in a hurry and had torqued them with my impact gun to what I thought was adequate. Checking them with a torque wrench revealed that I needed another 30-40 ft lbs to get them to spec. I think the solution is partially that plus using quality OEM wheel bearings and replacing front hubs as wear items each year.

 

[Defiant]

Wheel bearings should last at least 125K miles. These ain't FIATs.

 

[stealthTT]

We tried the residual pressure valves on SlowOldPoop's car and still had knockback. We saw no abnormal wear, drag, deposits, or hotspots.

We have torqued the axle nuts several times....but maybe not enough?

The wheel bearings test good and have not been replaced. But maybe they're bad? It's something to look at.

Thanks to everyone that's sharing what worked and didn't for them.

 

[mavisky]

Wheel bearings should last at least 125K miles. These ain't FIATs.

Mine are 130k old, but once you start subjecting them to sideloads from r-compound wheels or wide offsets then their life is quickly shortened. Combine sticky tires, big offsets, and high speeds and you've got a very very short lifespan for these things.

 

[cioc]

My problems definitely worsened when I went to R-compounds. My next question would be how from a design standpoint can this be eliminated? Is it merely a materials issue with the bearings/spindles? Or is it something far more complicated than that. Does anyone know of any aftermarket bearing suppliers that are stronger than OEM?

 

[Elvenhome21]

its not the bearing, its the iron spindle thats bending, there is no realistic way to fix it, other than just trying to mask it as much as you can. But a good way how to help your entire suspension out is put poly bushings in, compensate with more caster and camber. If you really want to try and fix it and had the right connections you could try to find a NEW spindle, or go rumaging through a junkyard and find a dodge/mitzu vehicle with a spindle that looks beefier and has the same spline count.

 

[mavisky]

Sounds like someone's going to have to cnc some spindles then.

 

[underradar92]

C'mom guy's, just do floating rotors. That's one of the reasons they exist. Crikees!

 

[Elvenhome21]

C'mom guy's, just do floating rotors. That's one of the reasons they exist. Crikees!

Second that or just learn to deal with it.

 

[Zero Bar]

Well, reading all this explains the soft brakes I get when cornering. Where do I find these valves people are talking about and how hard are they to install?? I will definitely need these next year with what I have planned.

 

[schemauer]

Wheel bearing won't last over 100k in a race car. Or 50k or 20k. Maybe in your grandma's FIAT, but not in a race car. As Elven mentions you're going to see some flex in the stock suspension. This is compounded if you have worn bearings, improperly torqued axle nuts, stock brakes, etc... There have also been instances of hub failure in the 2g cars that see open track use. I'd replace them every season on a race car. I replace them every two years, along with the wheel bearings on my track car.

Use a torque wrench on the axle nuts and torque them to spec. I can't remember the torque spec offhand but it's ungodly high. I can tell you that my impact gun only got them to about 120. Which, when corrected, got rid of 90% of my knockback issues with no other changes to the front suspension.

I'd suggest going through the front suspension with new components and proper torques before going the route of the residual valves. They're a a great solution, but only after all the other variables are eliminated.

 

[Elvenhome21]

Well, reading all this explains the soft brakes I get when cornering. Where do I find these valves people are talking about and how hard are they to install?? I will definitely need these next year with what I have planned.

http://www.speedwaymotors.com/p/1377,13_Wilwood-Residual-Pressure-Valve-RPV-2-psi.html

http://www.speedwaymotors.com/Produ...SYSTEM-TECHTIP_L.GIF&deptId=13&displayId=1377

 

[Dave532]

Just my 2 cents:

I have knockback purty bad depending on how new the pads are. I firmly believe that road racing a fixed caliper will give you knockback. I agree with Elevenhome21...I just accept it and dab the brakes before each 'problem' corner. The one nice thing about this problem is that it is consistent...the same corners the whole weekend.
I went through my front end before this season started I still experienced it once my pads got a weekend of wear on them (Wilwood Poly H's).

Tapping brakes is a good habit to get into to.

-Dave532

 

[Zero Bar]

http://www.speedwaymotors.com/p/1377,13_Wilwood-Residual-Pressure-Valve-RPV-2-psi.html

http://www.speedwaymotors.com/Produ...SYSTEM-TECHTIP_L.GIF&deptId=13&displayId=1377

Thanx!!! The whole brake tapping thing might work easily for you road race guys but in autocross you can't always just pump the brakes when you want to, especially if you try to trail brake and the brakes aren't there the instant you need them.