JAG IRS brake shudder

[ollykiwi3]

hi guys
I have apretty good feel with my car and im sure its in the rear.
there is no brake pulsation back through the pedal or the steering. I know what a warped disc feels like and also a worn or loose wheel bearing and its not that kind of shudder.
I have had several people in the car and all say its from the rear end as it shakes your ass
rear bearings and seals were new few thousand km ago, but the chatter was there from day one. its not just come on recently.

again, its not an out of round or balance type chatter. its like if you park on a hill and release just enough brake to let the car CREEP and you get that squeaky drag sound like running fingers glass as they grip and slide.

one thing I forgot to mention is I measured the padsd. they are both slightly tapered at top by 1mm. they are both evenly worn though at the leading edge(and yes I have chamfered the edges). its like the leading edge is wearing quicker than the trailing so it makes the pads a slight wedge shape.i have measured all the gaps and the calipers sit square and even. I guess its possible with the wedge shape it could be dragging the leading edge in and then popping out again creating the chatter. again kind of like bicycle brakes.

I have twin cylinders and balance bar, and have tried more and less bias to rear to see if it changes, but no real distinction.

anyone know what the caliper to pad max clearance should be, from the load face to load face.(top to bottom) on the lands. is .016" too much between pad and caliper?

car is off the road for few months now for rebuild and paint so road test is not possible, but as stated, diff is out so now is a good time to come up with some ideas.

thanks

[ollykiwi3]

Does any one have any more ideas?
I'm down to my last options now.
I think I've done every thing people have suggested and more.

[DanEC]

I think I would try a change of pads and see what happens. Don't know about the .016 clearance - is this between the piston face and the pad?

[Eljaro]

If one of the caliper pistons stick slightly, even a runout of only 0.004" may be too much and cause chatter on light braking. When applying force the piston will move and chatter will disappear. Jaguar states maximum runout to be 0.006". I am working on front brakes of an E type V12 Jag which had bad front wheel pulsing and one disk had 0.006" the other .003" runout.
If caliper pistons move freely they can absorb that runout and you will not notice. Old calipers with slightly corroded pistons or brittle rubber seals will not move fully under light pressure and get the vibration through to the chassis and you will notice.

[MOTORHEAD]

I would replace the caliper at this point. Sometimes when all testing and checking fail to expose the problem it's time to start the process of elimination by replacement. if the diff is out now is the perfect time to do it.
Good luck, these things can be very frustrating !!!!!!!!!!

[joyridin']

I agree with Motorhead. At this point in time, start by replacing a rotor at a time. It almost sounds like a rotor with a cracked air vein.

[Eljaro]

you do not have to get new calipers, you can rebuild them, replacing pistons and seals. Check here The very best reconditioned brake calipers, jaguar axle componenets, hubs, differentials superflex polyurethane bushes also wheel bearing kits, drive shaft ujs, cobra axles for dax, ak, for parts.

[ollykiwi3]

Thanks again for your input.
pistons and seals Were all new as were the rotors.
I'm tending to think it could be a sticky piston or still a air bubble.
Will re check once diff is back in.
New pads of different type will be fist before changing calipers.
The.016" is between pad and calipers at the top and bottom which allows the pads to float back and forth with the direction of rotation of the disc. If you put your foot on brake with wheels off the ground and rotate the wheel the pads move with the disc until they hit the caper. This is.016".
The rotors are solid so no air vent in mine.

Any thoughts on why the pads are wedge shaped from top to bottom?
Leading edge wearing faster? This would mean the poison does not sit flat on the pad and the pad could chatter until more load applied right?

Thanks again

[CHANMADD]

Have you checked that the four bolts on top of the diff are tight and also do you have longitudinal rods .?

[Jac Mac]

Since you have a balance bar find a real quiet or private stretch of road, dis-able or remove the pushrod to the front brake master cyl and try it with rear brakes only, then reconnect front brakes and disconnect rears & try again. That should help decide whether its caused by the brake rotors/calipers & at which end of the car.

Also remember that jag rears with the fixed length halfshaft & u-joints can suffer from the old unequal u-joint angle problem, is there enough deflection in the rear end geometry to induce this condition under braking...eg toe in or out in combination with ride height. This could be the case since you get no pulsation in the pedal. What happens is the half shaft accelerates/decelerates twice in each revolution at one end in relation to the other, since you have your foot on the brake the tire end does the acel/decel bit and you feel it through the car or to be more correct-your butt.

[MOTORHEAD]

Are both the pads wearing unevenly ? How about the pads on the other caliper?
With uneven pad wear either the piston is cocking or the caliper is moving out of alignment with the rotor. Have you watched it while someone operates the brakes?
You could even set up a back-up camera to watch the brake operation under on-the-road braking operation, or as I said, replace the caliper, it's been driving you crazy for what, a YEAR ????

ps; Is the caliper a floating type or solid mount?

[ollykiwi3]

Jac Mac - I don't believe its this kind of sensation with the u-joints. it goes away with more pedal effort and only with light pedal presure. again, this only happens at low speed around20-30mph. I will try new pads fisrt and then single out the front and rear end once back on the road.

chanmadd - yes all bolts are tight, and the rods are also tight. although I did find the fulcrum pivot mount loose, but cant see how that affects the brakes as they are on the diff head. that's all fixed now, but again, cant road test for another couple months.

motorhead- both sets of pads are exactly the same, both wedge shaped with the same amount of wear and on the same plane. the calipers cant be misaligned as the pads would have wear on opposite sides. one side thick, and opposite thin. these are both thin at top.
its as if the top is wearing faster than bottom. so like a I say, I think the piston is now not sitting square on the pad. don't think all four pistons could be cocked? seeing as all are the same.
have had a look underneath while operating, and there is a little soak up required when applying brake but not sure how much is too much. replacing calipers is expensive here and will be a last resort.
wish I had of recorded it before I took car off the road.

[RICK LAKE]

ollykiwi3 Olly I got a real problem with this problem. I have never had calipers cause a braking problem unless the sliders where no good. Brake pads will cover a number of problems.
I believe you said this has been happening for a long time. I am thinking you have a hard spots in the rotors or rotors. Take them to a machine shop and have them resurfaced. Tell them you want new bits on the machine and the main shaft checked for true. Less than .001" on the brake machine. Resurface both rotors. Clean both stub axle surfaces with 3m wheel. After cleaning install your dial indicator on each stub shaft and check for out of round. Both should be under .002", if not have machine true. Install the rotor and recheck the out of round with the rotors bolted to the stub axles. Again shoudl be less then .002" I checked my jag set from XJ6 and I have about .0017" play all the way around. My does 95% track and auto cross. New pads and same rotors I installed back in 98 and then change pads and resurfaced rotors in 2006. I would change the rotors if nothing shows up. China steel is crap. If this doesn't fix the problem, Go to Jag and find a good tech and see what he said. Can you go 20-30 in reverse?? See if the schutter is there. What happened with the parking brake test?? This should give you the same feedback as rear brakes?? Good Luck Rick L. Ps have an old service manual for Jag, going to look up info and see if any thing will help.

[Nick 427]

I have the same problem same symptoms and have tried pretty much everything that you have.
I think I have solved my issues by doing the following
It seems that my brake bias is not applying as much force to the rears as other cobras and hence it is glazing the disc/pads
I jacked the rear of my car put it in gear and applied the brakes till they started to smell
I then went for a drive and the brakes felt better immediately.
I was cruising most of the day and the brake shudder reappeared towards the end of the day although very slight
I am going to do the same thing again only give it more revs and heat them up more than the first time and see how I go
I believe this is the problem and if the glazing keeps reoccurring I am going to fit a softer brake pad
Hope this helps

[ollykiwi3]

Hi all
I have done a major rebuild over the last six months and am finally back on the road.
I installed new pads to see what would happen,and it seems nothing has changed.
The first drive was ok and I thought all was good until the brakes got warm.
The chatter isn't there when it's cold but appears after the brakes warm up a bit. I don't even use them much.
Again,it's like a dry windscreen wiper bouncing across the windscreen.
It's a long thread but please read previous solutions and answers for the history before commenting.
A brake place told me it could be material transfer that is a mismatch causing it to become"sticky"so to speak which causes the pads to chatter under very light load.
I can't believe I'm the only person in the jag or cobra community with this problem.
I still want to try different pad material but ebc pads are expensive and if that's not the problem then I just wasted $200.
Again,it's not a shudder,its a high freq low speed,low load "chatter".
I will try and record it next time out and upload if that works.

Nick427-I might give that a go.i have trw pads & rotors on mine,but I was going to go harder or better with ebc Reds. Have you had any more luck?
I tried the bias thing ang got the rears to lock first but that was at higher speed and I guess more weight transfer to front making rear lighter anyway,but it still didn't help.
Besides,it was crap at stopping like that.


Any further thoughts?

Thanks and merry Christmas

[Tom Wells]

Olly,

A definite vote for the RedStuff EBC pads here. I have found them to be noise free and very low dust for many, many track day and street driven miles on two different cars. Probably more than 20K miles.

I did follow the EBC break in recommendations pretty closely.

My Jag rear has a CWI Porsche vented rotor setup but I think the stock rotors would behave the same. No vibration of any kind either on the street or the track.

I'd strongly recommend using the same RedStuff on the front disks. Don't know what the effect of mixing pad brands would be in regard to relative caliper line pressures and friction coefficients vs temp. Might be OK - might be problematic - wouldn't want to find out on the track!

As to front/rear caliper bias, the front calipers should always lock up first. If the rears lock and the fronts continue to turn, you won't be able to steer.

Think about trying to stop an arrow while it is in the air by slowing its front tip with a finger; if the rear part isn't perfectly in line with the front, if your finger moved slightly off the trajectory, the whole arrow will rotate using the front as a pivot, and very rapidly!

In a Cobra this is called a snap spin. When this happens you will usually figure it out after the fact.

Not as much of a factor in conservative street driving but a big one in an emergency on the street or during spirited driving on a track.

Just saying...

As an added thought about ten years ago I had a shudder in low speed stopping that went away when I switched to the EBC pads. No recollection whether it was the front or rear and not saying it will work for you necessarily but here is one anecdotal precedent.

I am certain you will wear out this problem!

Tom

[Tom Wells]

Olly,

OK. I read more and have a couple more what-ifs:

You said

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??

As air in the brake caliper warms up it will expand. In any case, the max pressure on the pad will be less because of the compressibility of the air bubble.

Your question reminded me that bleeding the rear calipers on my car was a real chore. I must have done it 5 times before I was mostly sure there wasn't any air left inside. When I bleed the fluid to do a change I try not to introduce any air.

Message: be certain there is no slight foam or air left in the calipers.

Second point, I noticed you mentioned the calipers but didn't respond to Rick's suggestion about looking into and testing the emergency brake, I second the testing of the e-brake that he suggested. The actuating mechanism for that piece can be irksome.

Next, are the rotors centered in the calipers? There's usually a 0.020" shim inboard of the rotor, also 0.005" shims are available if needed.

That's all I got today....

Tom

[Cobra #3170]

I had a similar problem on my Ford GT it turned out that the break in did not transfer a consistent amount of pad material to the brake rotor. I ended up replacing both the rotors and pads and then carefully following the break in procedure. This made sure that pad material was deposited on the rotor evenly and I did not have a stick slip condition on the rotor. There is another possible condition known as "thick Thin wear" of the rotor where the brakes pulsate because the rotor has thin and thick spots around the circumference. You have to mic the rotor every inch or so around it's circumference to find this condition. It was common on Ford Trucks and some passenger cars in the 1990's and caused by aggressive pads and slight run out over time.

[ollykiwi3]

Tom- thanks for your thoughts.in regard to rear brake bias and lock up,I only did that briefly for isolating the issue.brakes are back to where they should be,front locks first.

As for the handbrake test,I'm pretty sure I did that a long time ago. I don't really see the relevance as the two are independent and it only does it with foot pedal pressure.its also intermittent.very hard to repeat consistently. But the handbrake pads do keep wearing? I have return springs on the arms now,but maybe not enough.i keep having to adjust them?


Rotors are centred.
I'm going to bleed them again this weekend and see if that helps.i will be looking into the ebc Reds soon.

Cobra3170- interesting,that's kindof what the brake guy was saying.im not sure if the sensation you had was pulsating due to material build up or more like mine and being a high pitch chatter.i don't get any pedal feedback.
But another thing to look into all the same.

I did record it yesterday but too much background noise was picked up.will try again today.

Ok.thanks guys.

[Tom Wells]

Olly,

But the handbrake pads do keep wearing?

i keep having to adjust them?

This may be a clue...

Mine haven't worn in 32K miles.

Tom