Brake Judder!!! What is it? Why does it happen?

Howard Jones

Supporter
I have had a ongoing vibration in the front of my SLC. I am beginning to believe that it is related to changing brake pad materials. More on that as I get into this but does anyone has any experience with this. Especially those of you with track experience?

My symptoms:

1. Car feels completely normal at low speeds around paddock, in the street in front of my house, and at very low brake effort stops.
2. On track, on warm-up lap none to very minor vibration in the steering wheel . I will rate it in severity at 1-2 on a scale of ten. Car is being driven at speeds that will heat tires up, primarily moderate corner speeds, engine revs held to 75% and brake effort held to less than 50%.
3. As lap times come down in subsequent laps, full power, hard cornering, and working up to max rate stops, vibration get worse until it is at 8-10 of scale. This is hard to describe but I have looked on the incar video and you can see it in the steering wheel/ hand contact just a bit. I feels much worse that it looks.
4. At no time is it felt in the brake peddle, No pulsation as would be present with a warped disk. Peddle is hard.
5, Disk run out = < .004 all four. Hub rubout = near 0 or < .001.
6. No other indicators in suspension. All rod ends on A arms are ok (took them off car and gave them the what for) , Wheel bearings are good (checked). Calipers are OK in that all the pistons (all four calipers) are free and retract normally when pressed back into the caliper by hand (all feel the same).
7. Steering tight and there is no pulling to one side. Car stops straight at all brake efforts.

More info:

Current configuration: Wilwood 6 piston calipers F Wilwood 4 piston calipers R. Wilwood two piece rotors all around (aluminum hats/ iron HD48 1.25/ 13.06 fronts and 1.24/12.88 R. Masters are both .625 on a Tiltion peddle box.

Car was first built with BP10 pads as a place holder to get the car going and see where I was with brake balance. Changed to Wilwood A pads on front and Wilwood B pads on the rear. The friction value of the A pad is nearly double that of the BP10 and the B pad is nearly so. I think that this issue may have come up at near the same time as the pad change.

Is this a compound interaction problem?

Info I found on Brake Judder:


Any good info you may have would be greatly appreciated especially track related in nature. Thanks in advance.
 

Howard Jones

Supporter
What has me stumped is the relationship to what appears to be brake temp. But to answer your questions, same tires (no flat spots). I would have to look and see if any wheels weights are missing. I tape over the wheel weights with duct tape so I'll have a closer look. The tape is still there I believe.

More on Brake Judder:


 
I had another set of rotors so I changed them, but now that I look at them it looks like pad material. I had just changed brake pad material also. I would take the rotors off clean sand and see.
 
Brake pad compound low temp rating means they melt and leave pick up on the rotor -- basically
I use performance friction rotors and Hawk DTC70 pads --works for me -- and probably better knowledge out there tho
 

Howard Jones

Supporter
Jac, Toe on front is 1/16" in per side. Camber is -2 degrees. Running R6 Hoosiers. Ed, that's what I'm thinking to. Gonna look for a place that will skim cut them.
 
1/16" per side? so is that 1/8" total? I wonder if you reduce that to Zero it might be better! With combo of wheel offset & scrub radius you could be in that zone where front wheels cant decide to track correctly and any slight wear in steering system allows that to happen.. Ive seen the pick-up mentioned by Chris & Ed when swapping compounds etc, devilish stuff to get off, coarse emery tape seemed to be best rather than brake lathe etc, as your not getting pedal pulsing I dont see it being your problem.
 

Allan

Supporter
What if any brake residual valves are you using? I've seen where the stock residual valves get stuck causing a vibration like that.
 

Howard Jones

Supporter
Allan, I have the standard kit residual valves on the car now but I have the wildwood valves in hand. I intend to switch them our maybe just empty out the original ones and put them back without any guts. In effect not using residual valves. My masters are good 10 inches in elevation above the masters. In any case I thing you are correct, The original ones are questionable and I think they are the source of my recent event.

Jac, 1/16" toe in per side is where I have settled. I have tried something like 3/64s but that is hard to measure. The car drives nice at high speed and even under hard braking it goes very straight and predictably. I will try near zero though because your ideas are usually very thoughtful and more importantly work.
 
Howard, Im thinking along the toe setting lines because you are feeling the judder thru the steering wheel, not the brake pedal and therefore the problem is more likely to be in alignment related. The Residual valves cause brake drag and usually a lot of brake dust build up on the calipers & wheels, but recent changes/advances in pad material might be reducing those symptoms.
 
This is what I do with disc rotors and flywheels
Rough cut till true and free from hard spots then a finish pass, the finish pass represents a scroll effect (like a thread) then with a disc "sander" to achieve a planetary ground effect --removes scrolling The planetary surface finish greatly helps with the bedding in of new pads or clutch plates
 

Dave Bilyk

Dave Bilyk
Howard, I have experienced this in the past due to steering rack mounting issues, so as well as what you are doing with the brakes themselves, do check carefully for any looseness, failure, cracking in the steering rack mounting.
Dave
 

Randy V

Admin
Lifetime Supporter
In my past experience, the judder has been from the transfer of pad material to the rotor. Specifically the deposition of material was not consistent. Alcon brake specialists advised our team to make sure drivers did not hold the brake pedal down hard after any stop as this causes some of the material transferred from the pad to the rotor to “de-bond” and fall off or transfer back to the pads themselves.
The method Chris outlines above is a good one to follow when renewing your brakes. However, part of the bedding process of new carbon metallic pads is to start that transfer process to season the rotors. So, this should only be done when installing new pads.

Sticking residual valve can also cause judder. 2# valve should be all you would use in a disc brake system, so ensure that you had not implemented a 10# valve by accident. There are various qualities of residual valves. Personally I have never seen one truly fail, but they can be fouled from debris in the fluid.
 

DaveM

Supporter
Agree with Randy. Had this same problem with Wilwood A or B pads after running on street pads. Had to get a separate set of rotors just for track use.
 

mark b.

Supporter
What is the caster, right and left? Positive or negative? I know someone who had their caster reversed and ran into similar problems.
 

Howard Jones

Supporter
Dave and Randy. Sounds like exactly my problem. The BP10 pad that was on the car (all 4 corners) is a street pad. They were on the car for about 80-100 shakedown and easy laps and I never had any Judder. Then I went to the A's on front and the Bs on the R and turned up the wick. That was when the Judder occurred. I will turn the rotors to clear them and re-bed the A and Bs. Thank you. I think you are on to the problem.

I do think the residual valves, or at least the front one was causing quite a bit more drag in comparison than the rear on my SLC and both F&R on my GT40. Both cars have 2 pound residuals. The SLC has the original "OEM" valves and the GT40 has Wilwood valves. I have never had any issues with the GT40.

I took the R valves off the SLC and re-bled the brakes. Then I had a look at the height of the reservoirs above the masters. It is at least 6 inches. I might try the car without them around the neighborhood "test track" and see how they feel. I would like to not need them, if just for reliability.

Mark, caster is 6 degrees pos.
 

mark b.

Supporter
Your rotor size is similar to what is on my JBL comp roadster. My master cylinders are 3/4" front and 5/8" rear and I'm sure that is dependent on the calipers also, Wilwood Superlite in the front with two 1.75 pistons and Dynalite in the rear with 1.38 pistons. Starting with the bias adjustment centered, the bias adjustment is toward the rear quite a bit, more than a person would think. Yours might be too much front biased. I also use A compound pads all around whereas yours are mixed. I've been on track a couple times, it is very easy to over brake:).
 
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