JAG IRS brake shudder

[ollykiwi3]

I have a JAG rear end with inboard brakes.
i have a really bad shudder/chatter when under light brake loads like slowing down to stop at an itersection or just generlly slowng right down, usually around 20-30mph and lower. it only appears to happen when brakes have warmed up a bit, and only during light braking.
If I push hard on the brakes its ok, i can push through it.
its not a warped disc, but it feels more like the pads gripping and letting go, or too much slop between the pads and the caliper body.
its like a high frequency "chatter" more than a shudder or out of balance vibration.
has anyone else experienced something like this?
Standard JAG XJ6 2 piston calipers. it also onlt seems to be coming from the Left side, but very hard to determine for sure.

any thoughts would be appreciated, other than the obvious of not slowing down
cheers

[joyridin']

Well, here are a couple of ideas if you haven't found anything yet. Check to make sure there is no oil or grease on the rotor. Inboard brakes have a habit of becoming contaminated if you have an engine oil leak, trans leak, or differential leak. If all is clean, then make sure everything is tight. Last, bleed the calipers. It is possible you have air in one of them for some reason.

One last thing...is your caliper in working order? If one of the pistons is not working, this might give you uneven braking also.

[Danr55]

Calipers on those brakes tend to vet fouled with all kinds of stuff. Make sure they are clean and the pistons aren't gummed up.

[CHANMADD]

Take the wheel off.........get all the 1/2 inch extensions you have ....put them together and tighten the four bolts that hold the sideshaft and rotor to the diff output flange........use an air gun or torque wrench....These slightly loose can cause a clicking in rear as well.......

[strictlypersonl]

Olly,

You say that the disk isn't warped. Have you checked with a dial indicator? Did you try to move the disk in and out when checking/rotating? I'm just trying to confirm that the differential bearings are holding the stub axle/rotor firmly.

[ollykiwi3]

gidday guys

will look into the cleanliness side,check tighten and also do a bleed.
its all new internals,pistons,seals ,pads etc as its only been on the road for a couple months so would be surprised if anything is sticking at this stage as ive done only 1000miles.
its definately not a warped disc type shudder or vibe. i will check it anyway, but im sure its not that. it feels like riding a push bike with old v brakes and rubber pads that squeek like its a very sticky pad thats not fully loaded an just floating on the surface and partially gripping.

anyway, will see what happens

thanks

[Thor maine]

It's a warped rotor.

[ollykiwi3]

time for an update:
i jacked up the car and ran it in gear to see what was going on. the LH disc was wobbling a little. I took the disc off and clocked it up in a lathe and it was only .001" out, so minimal. I found a burr on the stab axle flange and a corresponding imprint on the disc mount face. cleaned up the burr and remounted disc. problem solved.....no. did the run out check and it was still up to .015" out. still didnt think that was too bad.
pulled the stub axle off, took it and the disc to work and clocked them both up on lathe. nothing really to write about. checked the flange for run out, and was ok but skimmed it anyway just to be sure.
bolted them together on lathe and again checked run out. only couple thou. again minimal.
put it all back together and checked run out on car, again around .004" consider this minimal.
finally had some good weather yesterday and went for drive thining it was all good..not so. bloody shudder still there.
again, its only at light pedal pressure just as your rolling to a stop. wheels are barely turning.
my next step is to swap pads and pins over. Im going back to my original gut feeling that the pins are worn or the caliper slot for the pads is worn out. i cant hear it clunk as you apply brake,meaning the pads arent jumpimg forward, but maybe its not worn that bad yet, but bad enough to get a harmonic vibe going with a little pedal on and its gripping and letting go again in a high frequency.
will have a go at jacking up again and running it to see what the disc run out looks like also.
any more ideas?

[ollykiwi3]

Originally Posted by strictlypersonl

Olly,

You say that the disk isn't warped. Have you checked with a dial indicator? Did you try to move the disk in and out when checking/rotating? I'm just trying to confirm that the differential bearings are holding the stub axle/rotor firmly.

sorry, yes did check that, and ive set up the beariing with .002" float, so there will be a little movement there anyway. however, with weight on the wheels,all that load will be holding the stub axle firmly in.
whilst up on jacks, it will be unloaded with weight of wheel, so it has the same effect on DTI. its still got a little run out as stated, but not a lot

[joyridin']

Are you sure it is the rear? Not sure how your master cylinder is set-up, but if it is a single for the front and a single for the rear, can you unhook the rear to make sure it isn't the front causing the feel?

[ollykiwi3]

definately in the rear.

another thought.. has anyone experienced smaller than advertised pistons when they got them? if the piston is undersized, and its moving in the bore, couldthat cause shatter?the pads appear to have anti chatter tape on them, so relative movement between pad and piston shold be eliminated, but im not sure if the piston diameter is too small for the bore.
cant remember size but its series 2 2 piston caliper.

Im getting the disc trued up this week, and although it is slightly warped, im not convinced its enough to cause this problem.

its a high frequency chatter, not a shudder. sounds a bit like running a damp finger down a sheet of glass.grips, then lets go dozens of times a second.
not sure how else to describe it really.

[ollykiwi3]

right oh, time for the guys who work in the braking industry to chime in..
I have skimmed the stub axle, skimmed the disc, skimmed the disc mount flange face and put it all together. went for a cruise, still have the same problem. bled the hell out of the system, but still no improvement.
put it up on stands and ran it while underneath.
slight movement of rotors, but both about the same.
the only thing i can see thats different, is AFTER applying brakes, one caliper piston(one piston on one caliper) seems to retract slightly further than the opposing one after releasing the pedal. if i grab the pad and move it, its easier to move than the opposing one, meaning it is further away from the disc than the other.
If i grab the pads on the other caliper they are harder to move(havent retracted very far)
I think whats happening is with a little trailing brake(approaching stop sign etc), one pad is not doing quite as much as the other, which tends to make it skim on the rotor a little, causing the vibration.

If i push on the pedal, i can go through the vibe.

If a piston has air in it, would it retract more or less??
What about a rolled seal??

thanks
All the pistons and seals are new. what could cause a piston to retract more.

[Tom Wells]

ollykiwi3,

Have you changed to a different brand of pad? I use EBC RedStuff or YellowStuff and they are quiet...

Tom

[Eljaro]

are the Universal Joints ok?

[legenmetals]

I installed a 2 lb. risidual pressure valve inline to stop back flow of brake fluid into my master cylinder. Is your master cylinder mounted high or low?

[CHANMADD]

Maybe try bleeding them again....

[ollykiwi3]

new type of pads is next on list.
Uj's are all good. all new
my master cyl is mounted high.

will try bleeding again later in week.
busy pulling the head off for now....ahhh the joys of motoring.

[ollykiwi3]

been a while.time for an update.
still cant figure this thing out.
have pulled the the diff out and checked caliper mounting alignment and disc runout. again, all seems ok. parallel and runout below .004"

down to last few theories.
1) change pad material and brand

2) pad to caliper free play is .016" in direction of rotation. not sure if this is too much. its possible the pad is doing what I suspected at first and gripping ,being driven against the lower stop, letting go bouncing back, gripping etc and so on several times a second until it gets more load on it to hold it solid.

3) have found the stub axles spline with a bit of wear. wondering if the are chattering inside the diff centre with certain load on it and pivoting off the stub axle bearing? have the early series 2 ball bearing stuba xles if that makes a difference.

still cant believe im the only one experiencing this.

[RICK LAKE]

ollykiwi3 Olly out of round for a rotor should be under .002" Balance of the rotor should be "0". Does your car have a parking brake. Go for a road test and at 30 mph stop the car with just the parking brake. Pull up on the handle lite. If you have a pulsation it's in the rearend. If you have no pulsation. I would look at the front rotors and wheel bearings. 70% of braking is done with the frontend. If you are intune with your car, it is possible a rear problem but few people can feel this.
Big question, where to you feel or hear this noise? Is the steering wheel wobbling when stopping? Feel the pulsation in your butt? Brake pedal also pulsating? Lets get back to basic checks first. Rick L. Ps I have blown an axle bearing like you said. I had grinding and oil leaking from the stub shaft seal. Spin wheel over by hand and check for noise or drag. Also check the color and condition of the fluid. Silver color in fluid major problem. I have been racing for 14 years with stock ball bearing on the stub shaft and no failures after first one at 3 miles, my bad.

[CHANMADD]

Have you tried grinding a champher on the leading edge of the brake pad friction material....