Steering Wobble Under Braking

[DavidEm]

I have a 2001 XJR with Milan alloys and Brembo brakes. I have recently experienced a quite noticeable wobble through the steering wheel when braking from speeds in excess of 50mph. All four wheels have been balanced to perfection but the problem persists. Interestingly, the previous owner had the same problem just before I purchased the car from him, and he replaced the front discs (Brembo oem) and pads (green stuff). That was about 2K mile ago. All has been OK, up until about a month or so ago. Any thoughts? All help and guidance will be much appreciated, thank you.

[oldtimer]

Hi Jagslag,
Is it wheel wobble or brake judder?

If it is brake judder, then we are into my favourite subject. Wheel wobble sounds more like a suspension joint.

Incorrectly fitted discs reveal themselves after the sort of mileage you mentioned.
You may choose to mark the disc with a marker pen at 12. 3, 6, 9, o'clock and between.
Measure the disc with a micrometer at the eight points and record. The measurements should be taken at about 1/2 to 1 inch in from the periphery of the disc. Translate these measurements on to a sheet of paper. Draw a strait line and mark the findings along this line. Ideally all measurements should be identical. Variations of 40 microns (1.7 thou) is acceptable. Anything much above this is a clear indication of disc excessive run out. New discs that have not had the protective coating removed or the hub mounting face is corroded and not satisfactorily cleaned will definitely cause the problem.

This is not a Jaguar or Jaguar model problem it is applicable across the industry.

I hope this helps - come back if you want a bit more.

If this is the case then Warranty rules should apply

Oldtimer

[DavidEm]

Thanks Old Timer! Then perhaps I should take her along to a brake specialist?

[ptjs1]

You can do a very rough assessment of disc warping just by holding a fixed point against the top edge of the disc and turning the hub. See if the disc is moving in and out as it turns by more than a few mm.

I have heard that Brembos are much more inclined to warp quickly. Do a search on "Brembo" on the XK8 section of the Bulletin Board and I think there are a few threads.

Good luck

Paul

[DavidEm]

Thanks Paul, but wouldn't a warped disc be noticeable when braking at low speeds? Also, if Brembo discs warp after 2K miles from new, I shall be 'downgrading' to Jaguar standard equipment!! I'll pop over to XK8 section as suggested. Thanks once again.

David

[Brakebuster]

brake discs are susceptible to warping for a few reasons,

first is the way its installed, the mating surfaces MUST be clean , as a small bit of debris can cause an imbalance that is then exaggerated when braking

the other is the hot spot caused after long or hard braking by keeping your foot on the brake pedal when stopped, the rest of the disc cools while the part still in contact with the pads stays hot in relation to its self , causing a warp ,

this warp is nearly always indistinguishable by eye, but using a reference point, or a run out dial can be easily spotted ,

replacement is the only cure

BB