New Tires, New Front Shocks, had rims checked for warping or bent, had tires re-balanced, checked suspension (ball joints, sway bar bushings, etc.), checked U-Joints and Carrier Bearing...All good...

Vibration starts at 47 mph and continues, does not get worse with speed, does not change from 47 to 90, no worse, no better, when slowing down it stops at 47 mph
Feel it under the car, not in the steering...

Any suggestions?
This is a 1997 XJ6, factory wheels, stock size tires, 125k, handling is good, tight and smooth, brakes are fine, rotors and calipers are good.

Thanks!

 

How were the U-joints and carrier bearing checked?

The only way to properly check them is to disconnect and/or remove the shafts and feel the action of the u-joints and carrier bearing. The action should be as smooth as a baby's bottom, of course.

I'm getting nearly the same symptom from my '95. I pretty sure it's one of the above but haven't gathered up the enthusiasm to check them.

Cheers
DD

 

Lack of hysteresis is curious. Control theory sure would be simplified if more stuff acted like that. Also puzzling that neither frequency nor amplitude increase with speed. Is everything solid and properly fastened up front? Bumper fascia? Undertray? To eliminate wind-buffet as cause, catch a very windy day and see if onset speed ever differs from 47 and/or turn around and run downwind and see if it comes in at same speed?

Not very helpful, but if only above 47 mph and only felt under the car and not in the steering wheel...the answer is clear: Stay out from under the car and firmly seated behind the steering wheel when traveling in excess of 45 mph!

 

swap wheels front to rear, just to be sure you can rule out the wheels...if its any worse, or better, then you can re-evaluate them.

Any chance you can get your car up on a lift and safely get the speed up to 47? Maybe driveshaft out of balance?

 

It feels like you flick a switch at 47, doesnt build up doesnt slow down, just instant on and off for the vibration. Very odd, have never had a vehicle do this. It may lighten up a little over 70 mph sometimes...but not everytime.
Tried moving wheels around to see if anything would show up in steering, nothing, took to tire shop to have balance checked, they rebalanced all 4 and checked the rims for any issues. They said all was good, sounded like a balance issue to them. So re did all 4 tires no charge. I decided to ask the forum after driving home from the shop last night and nothing was any different. They said I have a little wear on the right inner tie rod, but that would not cause this issue.
And I assume rear shocks would not do this unless there was a tire bounce to go with it.
It vibrates the same on curves or long straight aways. Same in heavy rain, light rain or hot day or cool night. (Has been doing it for weeks)
Bought new rims in case there was an issue. (ran across a good deal)
Shop said they would move my tires over at no charge, but assured me my current set of rims were fine.

About 2 weeks before I installed the new shocks it would do this, but not everytime. It just started doing it more and more often. Now everytime you leave the house.
This is suppose to be wifes daily driver...not a project...my XJS is suppose to be the Jag I need to work on...

 

I was thinking driveshaft...but would assume vibration would change with speed...putting car in neutral does not change it...up and down shifting does not change it...

U-Joints, to vibrate this bad they would easily show wear.
To check while on the car, you parke the car, put in neutral so there is no pressure between the driveshaft and the tranny. Apply your emergency brake, jack the car up and feel for loose movement, up, down, side to side. If no pressure on it, shaft will have some wiggle at the yoke, but u-joints will be tight.
Now taking them out is best when checking for any wear at all. But this vibration, if they were at fault, it would be easy to see.

I have read in some post that because these cars are so precisely engineered that they will shake or vibrate due to slight wear that most cars would not pick up on until the bad part is much further gone. Which makes it harder to find the culprit since some things need heavy wear to show bad.

 

As your problem is only above 47mph I would agree with doug re the half shafts and u joints and also the main drive shaft and its center bearing ( and if you do the drive shaft have it balanced when it out of the car.

 

Crack in the rubber on the carrier bearing...looked good until getting up in there with a strong light and messing with it...chunk of rubber came out...all u-joints good and tight though, and as far as the driveshaft, no scratches, no dents and factory balance weights still there. So ordering the bearing now.
Anyway, thanks alot for all the suggestions!!

 

Ronniels,

Can you update and let us know if the new carrier bearing fixed the issue?

Sounds a lot like what I'm chasing.

Thanks.

 

Ronniels hasn't posted in 14 months. Not sure if he's still out there.

Replacing u-joints and center bearing went long way towards eliminating an odd vibration in my XJR. I still feel "something" but I'm so over-sensitized at this point that it might be a mental issue :-)

I might add that u-joints being "good and tight" as mentioned above in no way ensures they are not causing a vibration. U-joints must be checked with the shaft disconnected.

Cheers
DD