Back end wiggles when coming off the power
 

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'
https://cimg1.ibsrv.net/gimg/www.jag...b92e860d45.png

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

https://cimg2.ibsrv.net/gimg/www.jag...18523ce85f.jpg

The only way to adjust the rear ride height is by altering the spring perch that rests on each shock absorber body, or by buying shocks with adjustable perches.

As far as I know you can use spring packers (CBC5435) up to a maximum of three each side, so raise the rear height a little. In hindsight, though, I wished I'd have gone for the height-adjustable Gaz shocks.

Do you mean putting them under the springs on top of the perch? That would be doable, I think. Also, is it possible your springs are a bit tired?
Greg

Yep, that's the plan. I think it's the cheapest way to go, initially, before shelling-out for the height adjustable Gaz shocks. Maybe raising the rear suspension might bring the nose down a touch, too? Once car is back from the garage I'll get a photo of the ride-height and post it up.

Spring are new Eibach for an Aston DB7, so it could be that they are specified differently to XJS springs, but I can't find any info around that.

Does Eibach say they'll work correctly on the XJS?

IMO, spring manufacturers play it fast-n-loose with specs and applications....which is why we see so many ride height issues after spring replacements

Cheers
DD

I bought the springs on Ebay, based on what I'd read on the internet around DB7 V12 springs fitting a V12 XJS, but as you allude to, I guess there's a difference between a part fitting and a part working correctly for the application!

The ride height used to look fine before, but I had to change the suspension as a couple of the shocks were leaking and the springs were rusty. I think the car should have a slight nose-down appearance, but at the moment it sits completely level (maybe like an Aston DB7!)

I think Jaguar intended the car to sit level...so you might be OK.

If you want a slight nose down appearance...I like it, personally.... lower the front ride height a bit. IMO

Cheers
DD

Quick update - the new cage mounts are in and the shimmy seems better. However, I tried some heavy braking, from as low as 20mph, and the car still shot sideways, so guess this is a brake related issue!

What year is your car? Does it have ABS?

Sounds like an ABS problem I had on my 1990.

Sorry, it's a 1991 pre-facelift V12.

Yup, decades ago on a far older critter, I learned that x's rear brake bias will get one sideways or even 180.


AKA, the bootlegger turn. Yank on the hand brake hard. the shine car does the 180. befuddles the revenoooers coming the other way, now!!


Carl

The problem I had was the car veering to one side under emergency braking. The reason is that the front ABS sensors get covered in crud, partially old wheel bearing grease, then the wheel ABS signal is weak. Below a certain speed, the signal is too weak to be detected, at which point the system assumes that wheel is locked, and releases the brake.

Braking hard from, say 60, mine would brake nice and straight, until below about 30mph, when it would suddenly veer to the left.

Fixed it by cleaning and adjusting the sensors.

Hmm, will give the sensors a clean, if I can, as that's a pretty easy first thing to try. My original issue with losing control of the car under braking was at high-speed, but it also seems to do it under heavy braking from low speed.

Interestingly, after a few trial emergency stops yesterday, it seems to be braking straight and true now, from low speed at least. Not sure I'd trust it to stop me from high speed as yet, though. I've seen some threads around faults with the ABS valve block, particularly after brake-work and bleeding, which is when my problem started. Looks pretty daunting, though.

Nope.....still not right. It handles ok if I chuck it into a low speed corner, but seems to feel unstable at speed, even in a straight line, and especially under braking. I'm fed-up with chucking new parts at it so have booked it in with KWE, as they seem to be the experts with these cars.

I will be very interested to know what they find. My original list or possibilities centred on a rear axle problem, and nothing you have mentioned since makes me think otherwise. You have all the symptoms of rear wheel steer. Good luck with it, and start saving!
Greg

Have you checked the front brake hoses?
I had a collapsed one on mine that caused it to dive to the left.
After replacing them both with braided hoses I had a full system bleed done and it is fine now.

Thanks! I've not checked all the things suggested by contributors to the thread, because some, I don't know how to, but have tried all the 'easy' stuff and still no joy. I can't imagine it's going to be cheap, but I'm not getting any enjoyment from the car as it is. It really feels like it's much less than the sum of its parts, if that makes sense:

Manual gearbox conversion
All the AJ6 Engineering tuning bits
Electric fan conversion
Gaz shocks
Eibach springs
Rob Beere cage mounts
Poly bushes in most bits
3.54 diff
New discs, pads & calipers......

List goes on, and it's still a bit of a dog to drive!

Worn fulcrum arm bearing! I never managed to get to KWE but fortuitously, a classic jag specialist has just opened near my home, so I took it in there. It's now stable at speed and handles great. There's still some diving under braking, so I guess it was two or more faults causing my problems. Brakes next.....

Hi, I'm not far from you in East Surrey and keen to hear of nearby specialists. Can you provide a contact for your Jag specialist. Thanks and good luck with the car.
LeeP

Here you go:

https://www.turnerclassics.co.uk/

the chap who worked on my car was 'John' and really knows his stuff.