The car, not me, that is. If I’m driving above about 50mph, the back-end of the car seems to wiggle a bit when I come off the accelerator. Can be quite un-nerving if I’m going quickly. I thought it must be worn bushes or suspension, but I’ve just replaced all the dampers with Gaz adjustables and the springs with Aston DB7 Eibach items. The entire front of the car has been re-bushed, so I’m kind of excluding that. It’s a V12 with a manual conversion and I notice that there’s quite a lot of up and down movement on the gearstick, so might it be a worn gearbox mount? I’ve read about the gearbox mount on the auto giving problems, but not the manual.
Not sure if this is related or not, but also when I get to about 50mph and above, the car vibrates – not a harsh vibration, but it makes it hard to see out of the wing mirrors as they’re shaking a fair bit.

Any ideas, gratefully received!

 

Have you carefully checked the V-mounts for the rear subframe?


Cheers
DD

 

The car should not vibrate like that, there is a definite fault. Could be any of the following, besides Doug's sensible and equally likely suggestion:
Propshaft out of balance/UJs shot
Gearbox mounting insecure/shot, gearbox output shaft worn
Rear axle lower suspension arm outer or inner fulcrum bearings loose/shot; lower suspension pivot bracket attached to the diff loose
Diff loose in the cage
Rear axle radius arm bushes shot/loose; radius arm cracked/rusted out
Axle cage itself cracked
Driveshaft UJs shot, flange bolts loose
Diff input shaft bearing shot, flange bolts to prop loose, diff shot.


If the gearstick is jumping about I would start with examining the mounting and the propshaft. I spent nearly a year last year chasing an annoying shimmy in the steering and a funny vibration through my seat that might have been rear wheel imbalance and was not. Turned out to be a faulty front balljoint (the steering shimmy) and a propshaft going home (the vibration). new props are still available from jaguar and are not dear, considering what a big piece of kit they are.
Greg

 

I would definitely check the rear radius arm bushes and mounts as they will cause the symptoms you describe if worn.

 

Wow, thanks for all the advice on things to look at. Thinking about it, I did have to get the propshaft changed a few years ago, though the vibration was a bit stonger that time. I'll do a visual inspection of the v-mounts, but not sure how to check the manual gearbox mount.

Hoping it's nothing to do with the rear cage or any of the bits attached as the mechanic has just removed it to swap the diff to a 3.54 and replace the rear calipers and discs. I'm assuming he'd have spotted if something was amiss. Rear radius arm bushes were changed a few years ago, but I think that was just the two big ones that attach to the underside, not the ones that attach to the cage.

Probably not likely, but could it be caused by the settings on the rear dampers? They're set pretty soft at the moment, so maybe too soft?

Thanks again!

 

Personally I'd approach the "wiggle" on closed throttle and the vibration as two different issues. IMHO the wiggle would likely be a suspension location issue....that is, something is not being held in its proper position. Vee-mounts, radius arms bushing, etc as mentioned.

Stretching a bit, perhaps excess wear (and hence free pay) in one of the hubs (or even differential) which causes a wheel camber/geometry issue. This might account for a vibration as well. Get the rear wheels safely in the air and give some hard pulls with hands at 6-12 and 9-3 o'clock. Do the rear wheels move a lot?

Cheers
DD

 

Hi, not had this with my XJ-S but have with MGBs. Apologies if you have already checked this but make sure rear tyres are not low on pressure and equal either side.
Good luck,
LeeP

 

To my shame, I've not got any further forward in trying to sort this, as I'd not really used the car. However, I did drive to work a few weeks ago and was travelling pretty quickly on the motorway when someone pulled-out in front of me. When I braked sharply, I lost control of the back end of the car, corrected, then again it broke away, and then finally managed to gather it up, through luck not skill...... I'm assuming this instability is related to the shimmy when coming off the power that I've been experiencing before, only a more pronounced effect, due to the greater/more sudden deceleration. Anyway, I don't really feel confident driving the car now, so will start investigations soon as per everyone's suggestions.

The vibration has gone now, though, with a new set of tyres and wheel balancing

 

My car did the same thing. In my case, the back two
of the V mounts had unbonded, the large radius and small radius bushes had unbonded, and the inner fulcrum bearing rusted up and the fulcrum shaft had worn an oval shape into the rusted bearings. I guess what I’m saying is it took a lot for me to notice and it wasn’t all one thing! Very hard to check those inner bearings to see if they’re rusted solid. In my case, the bottom arm was clearly sitting cockeyed since the hole was oval.

 

It is the height of foolishness and extremely dangerous to drive the car in this condition, other peoples' lives matter. The rear axle is clearly not located properly and uncontrollable rear wheel steer is the result. One gypsy's kiss should have been enough. Two is a kindly Fate and it has been extremely kind to you.

 

gotta love these Jags, when i 1st got my 78 XJS ,didnt run, i was backing it off the delivery truck, and using the E-brake handle , its started to roll to quickly, so applied E brake!

the complete rear subframe/cage broke loose from the V mounts and twisted sideways, locked up the wheels against the body.

YES a lot of careful work and money before things were back together , my take is Jaguar never intended the cars to remain in service for 30/40 yrs with rust conditions taking place.

i still love the old bucket tho!

 

HewhodaresRoyv2,

As others haev alluded to, I expect there's something more fundamental with the rear axle movement. However, you did question the softness of the settings on the Gaz shocks. If you've got the ones without the adjustable height platform, it can be quite difficult to get the settings right initially. I wanted mine firm so naively assume that position 20 out of 32 would be about right. Wrong!! The car was almost dangerously undriveable and I then found that most of racers never even used settings past about 17.

Try backing out the stops back to zero and reset them at about 11. It may not be perfect for you but it's roughly right and you'll have eliminated one of the possibilities of the poor handling. If you've got the adjustable height platform units and the Eibach springs and you've set the ride height to factory spec, then set the shocks at about position 9 to start with.

Good luck

Paul

 

Many thanks again for all the suggestions.

I'll check the settings of the dampers, but I think they were set at 11 on the dial thing. What I have noticed, though, is that the front of the car sits too high, after the change to the Gaz and Eibach combination and I think this is because the rear is sitting too low. I got the DB7 V12 springs, so I wonder if those are slightly different to the one's sold by XJ Restorations? I don't think this would cause the wayward back-end, though.

Greg in France, point taken, though in my defence, the car was newly MOT'ed and nothing was picked-up about worn mounts, bearings etc and I wasn't really to know that a small twitch coming off the accelerator would translate to the rear end braking away under emergency braking conditions. As I said, the car's not been driven since and won't be until the handling is rectified.

I've had a look at the uprated cage and radius mounts from Rob Beere and wondering if those are worth the extra outlay, if it transpires that I need them. Will check for play in the bearings, too and change those if needed.

Thanks again.

 

I had a new 66 mustang and after a few months I had to hit the brake hard. Back end started for the shoulder of the road. Took the car in to Ford and they found a leaking rear shock. The traveling rear end was fixed. This may be worth a look.

 

Unless you are going trackdaying, uprated cage mounts arenot needed (even though Rob Beere's stuff is all very good). It would be bestto remove the axle and thoroughly check everything; though if you find theradius arms are wobbling about, that would be a principal reason for thetrouble. If that were to be the case, it would still be best to check everything, as theirpoor condition would put extra strains on the wishbone fulcrums, hub bearings,etc.

 

early years XJS V12 race cars built by Grp44(1976/1980), mounted the rear cage solid, with custom aluminum blocks,plus some other small tweaks rear suspension.

like hollow antiroll bar tubes and solid mounts.

like a vid by Sir Williams said, engineers were NEVER satisfied with the rear cage/suspension, it had great soft ride , but wallowed about under hard acceleration.

mine acts like that, just put 4 new Sachs shocks and all radius bushes, and still on hard accel. (my duaghter says feels SCARY, and it aint Halloween yet).

 

Can't fit in the car with no crash helmet, so alas, no track-days, but it has been converted to 5sp manual and has all the AJ6 Engineering bits and a 3.54 diff, so probably a bit more strain on the cage mounts than average. As suggested, I think the cage may have to come out again, which would be a shame as it's only done about 150 miles since it was dropped, but needs must... First I'll get the cage mounts and radius arm bushes inspected.

Will keep you all updated and thanks again.

 

In that case, uprated cage bushes a very good idea. Thanks for the update and good luck finding the causes.
Greg

 

Quick update:

I've had the car checked by a mechanic and he confirmed that the rear cage mounts were shot. I've got the uprated Rob Beere ones, which will hopefully be fitted in the next couple of days.

Had a quick look at the ride height and I think the rear is too low. I measured from the bottom of the cage to the floor and got 170mm at the rear and 170mm from the cradle to the floor at the front. I'll order some spacers for the rear to bring the ride-height up a bit, but would still like the front a bit lower. I think all the 1991 'Le Mans' cars with the 16" wheels look a bit under-tyred at the front, but the facelifts with 16" wheels look fine.

 

V12 'Le Mans'


.....and here's a facelift: