XK 4.2 vibration/juddering front brakes

[ParaHandy]

Not long ago, June, I replaced the front discs and brakes (with aftermarket products) because they were worn and I was experiencing, not severe but worrying, front brake judder when braking from 70-80mph. The discs are the standard 326mm and the calipers the single 60mm diameter piston; MY06 coupe with 35k miles.

The suspension bushes all appear satisfactory; in all other respects, the car appears and drives as good as it gets.

After 1,000 miles (1 month) the vibration is back. I have re-aligned the wheels which were nearly perfect anyway, tried various tyre pressures with no success.

Owners of similar cars that I have talked to have no experienced of this.

I'm going to shortly check if the new discs have distorted when I replace the rears but I am wondering if anybody can offer advice, any of which which would be much appreciated.

Douglas

[Brakebuster]

The ' runout ' on these cars is critical

if the disc is out of square by even a small amount this will cause issues , withthe disc removed, and the hub scrupilously clean ( mating surafaces ) install the disc and tighten back, use a guide to see if there is any runout , a guage should be used , but a fixed guide will give an indication

also

heat soak on stationary hot discs WILL distort aftermarket

BB

[robv12]

yup, BB has it in a oner!!, these cars are very susceptible to brake judder, as are 350's which I have had my own similar issues with, at the recent seminar this was a good topic of conversation, both XJK and David Marks our tech guys, were adamant fit only genuine discs, ensure they are fitted to a perfectly clean hub, check the run out with a dial gauge and then ensure the mating surface on the wheel is perfectly clean as well

hope this helps

Rob

[ParaHandy]

Thank you both. A bit of an eye opener. I had to be convinced by the independent to fit the aftermarket parts rather than Jag or OEM. I'll have them checked.

[ParaHandy]

A bad ending. I removed the whole lot and replaced with Jaguar parts. The judder has gone. An expensive lesson.

There ought to be an induction to the JEC with your technical people advising to a broader community not to do what I did.

[dhdove]

A bad ending. I removed the whole lot and replaced with Jaguar parts. The judder has gone. An expensive lesson.

There ought to be an induction to the JEC with your technical people advising to a broader community not to do what I did.

I feel for you, an expensive lesson indeed.

You could put all the old parts into a stout bag, return to the independent who persuaded you to fit them and turn them upside down from a height onto his (hopefully) expensive desk and ask for redress for his incompetence.

To be fair the Club does its best with Model seminars for potential owners ( always join the club and attend the seminar before buying the car). Failing that there is this Bulletin Board which will always give you the benefit of other members hard earned previous experience, especially with regard to technical matters.

In my experience with technically advanced performance cars like these there are two ways to go, the cheap way and the right way not that this helps you now, but you will be able to impart your hard earned knowledge to another member of the Club in the future here to help them avoid making the same mistake.

In any event, glad you're sorted now

TonyS

[oldtimer]

Take great care when dropping "stuff" on peoples head.

About twenty five years ago asbestos free brake pads were introduced which caused monument problems with vibrations. Of course all the experts said it was poor quality discs.

At the time I worked for a major parts supplier and this was a very high profile problem especially as the Warranty costs were going through the roof. At the time it seemed prudent to accept the Warranty rather than put the larger business at risk.

Engineering had to address this situation. One of the tasks was to make a video with disc fitting instructions which also included filming of the brake pedal osellations in sympathy with the disc vibration. This video together with a DTI (Dial Test Indicator)and universal DTI mounting bracket was distributed free of charge through out the network.

The Disc run out should not exceed 40 microns. measured with a DTI mounted within the caliper. However secure, the DTI should not be mounted remotely as this will guarantee inaccurate readings.

When making site visits to address customers complaints we were often met with a hostile reception. Comments like --- well perhaps not on here. Oh and the DTI has been loaned to the garage up the road. Absolutely no intention of using it - you can't tell me etc.

By marking the disc at eight points of the clock with a marker pen and then measuring the thickness of the disc at those points and translating them into a straight line the vibration frequency curve can be seen.

It should be pointed out that this is /was an industry wide problem. On occasions whilst trying to fit new discs the disc mounting face although absolutely perfectly clean was found to be distorted leading to the conclusion that this was the result of over exuberance with a impact wrench.

So, going back to the beginning it may be worth measuring the displaced discs and also consider the possibility of a repeat performance of break judder in another thousand miles.

I hope that helps just a little
Oldtimer

[ParaHandy]

I hope that helps just a little

The difficulty in substituting asbestos is surely well behind us?

Whilst struggling to understand why these brakes so comprehensively failed, I came across Eurac one of the leading grey iron foundries of its type in Europe whose technical papers I found quite helpful.

[ParaHandy]

(always join the club and attend the seminar before buying the car). Failing that there is this Bulletin Board which will always give you the benefit of other members hard earned previous experience, especially with regard to technical matters.

Look, I've owned Jaguars for years and done 100,000s miles without knowing of the existence of JEC. The point I'm making is that it isn't much use if the two gentlemen who were so adamant in their opinion, did not offer that opinion to a wider audience.

[dhdove]

(always join the club and attend the seminar before buying the car). Failing that there is this Bulletin Board which will always give you the benefit of other members hard earned previous experience, especially with regard to technical matters.

Look, I've owned Jaguars for years and done 100,000s miles without knowing of the existence of JEC. The point I'm making is that it isn't much use if the two gentlemen who were so adamant in their opinion, did not offer that opinion to a wider audience.

Douglas
Sorry? I don't understand your post!
Who are the adamant gentlemen?

TonyS

[Brakebuster]

he is having a go at someone he doesnt know in an organisation he didnt know about saying something he couldnt be bothered to research at the time......

i think

BB

[mike020150]

The good news in this story is that the car has been fixed by using OE (original equipment) parts. For me, the bad news and the main issue is Jaguar's and other manufacturers' approach to parts pricing. The mark-ups are so great that it guarantees any less than scrupulous parts' manufacturers lots of pricing scope to tempt owners away from OE dealerships. Sadly, with items like brake discs, getting specs from aftermarket sellers doesn't amount to more than the approximate diameter and thickness of discs, not the limit of run-out or the material composition, only some of the details I'm certain Jaguar specify to their suppliers. Mike.

[kingzetts]

Let me add my experiences to this.

About 18 months back when I bought my XK8, the PO had recently changed the front discs. I was plagued by a front wheel wobble which I only solved when I had the front wheels balanced on the car. The tech who did this suggested the front discs may be out of balance.

I swapped the XK8 for an XKR with Brembo brakes this year, and needed to fit new discs. I was unwilling to pay OE prices and opted for aftermarket discs. Bearing the XK8 experience in mind, I decided to have the new discs balanced before fitting. Luckily there is an excellent precision balancing firm in Worcester where I live, who I have previously used to balance my e-type engine.

They turned up a mounting mandrel and balanced all 4 discs which they said were "miles out" and would have wobbled like crazy. I could clearly see where they had ground away metal to achieve a balance. The car with the new discs runs smooth as silk. The cost was around £120 for all 4 discs but would be less in future as they now have the mandrel on the shelf.

Talking to them they said that unbalanced discs are surprisingly common, even when purchased as OE Jaguar or from Brembo. Certainly in future I will be having any replacement discs I fit balanced, whatever the source.

I had never previously thought of discs as a source of imbalance, but as discs have got larger and more of them are internally vented I can see how this is more of a problem as while solid discs are likely to carry any imbalance in the inner bell, internally vented discs are less likely to be naturally balanced by the machining process as any casting imperfections in the inner vented areas cannot be machined out.

Worth thinking about if you have persistent balance problems.