Not sure if this will be of any help, but when ever I see these centre "steady" drive shaft bearing used, they were always rubber mounted, if I remember correctly.
I wonder if it could be as simple as that.
Yours looks to be hard mounted right to the frame.

 

Doug.

Great news that you got the Beast out, PITA about the vibration !

First things to check :-

On any shaft the input angle (drive end (Gearbox)) to the shaft should be the same within 1 degree as the output end (center bearing) and also from the center bearing to propshaft.

Next make sure you have out the slip joint together in phase, otherwise you will create an issue.

Load vibration is usually due to phasing issues, the UJ output varies on every rotation if it is not straight, the UJ on the opposing end should cancel this out if aligned with the drive end.

It would be a pain, but you could have double Cardan joints fitted to the prop if this is the issue, these are CV joints and don't have the alignment issues that UJ's have.

It could be the center bearing, but I would have expected this to get picked up by the company that balanced the prop.

You could try isolating the center bearing, but if there is a vibration it will only help reduce it's effect in the car, not actually stop it.

 

Thank you both. Jeff, I duplicated the bearing mount direct to the floor pan as in the XJR so unfortunately that's not the cause. Jon, I assumed the flex couplings were not meant to see a constant angle, so all of that is on the single u-joint (it's no wonder I have an issue under load). Assuming I treat the flex couplings like a u-joint I'll need to raise the bearing about 3/4" to equalize the angles. I guess I do need to refill that argon bottle after all - damn. Any tips on siting centerlines through a diff and trans you can't access? Wishing those shafts had built in lasers. 2 steps back ...

 

Is it simpler to install a CV joint in place of the UJ, that would get over the angle issue. Either way its a royal pain !!

The output shaft after a UJ has a sinusoidal speed curve in relation to the input speed as it rotates, the magnitude of that is amplified as the angle gets greater, the optimum is a max 3 deg, I think you have nearly 5 deg if my maths is correct, 3" change over 30" .

 

Thanks Jon. I pulled the driveshaft to take more accurate measurements. It turns out the trans in its current position has a 6 degree downward slope, so the angle needed at the forward flex coupling and the u-joint works out to right around 3 degrees. Unfortunately raising the bearing to accomplish this puts the angle of the flex coupling at the diff at 4 degrees (u-joint is not centered on the shaft - actually closer to diff). I can raise or lower the trans mount and locate the bearing to equalize these angles at less than 3 degrees with no problem. Before I go there I'll swing by the driveshaft shop to confirm it all makes sense. I want to make certain these flexible couplings function the same as u-joints in terms of the phasing issue you described, Jon. Also wondering what impact the addition of a third joint has on the total system (I know trying to solve the math hurt my head).

Here's a picture of the shaft setup - with the motor and diff fixed except for mountings I'm wondering why there are both flex couplings and the u-joint. I hope it wasn't a mistake trying to carry over all of this engineering vs going with a more conventional driveshaft setup. The idea was to keep things as isolated on the beast as they were on the XJR in an effort to achieve the quiet and comfort of the modern ride - we'll have to see.

 

The Metalastilk couplings are designed for up to 5deg constant angular deflection and up to 10 deg momentary, if you can split the angle up to give the UJ 3 deg or less and the rest on the Metalastk couplings, then all should be good.

When your at the driveshaft shop, check out the option of replacing the UJ with a CV joint (double Cardan, Thompson Coupling or similar) basically it 2 UJ's back to back, these cancel out the harmonics hence the "Constant Velocity" name. They are used on driveshafts because of the range of angles required when the suspension moves up and down.

The flex couplings protect the gearbox and diff from vibrational loads, but I cannot think why they have adopted 2 flex couplings and one UJ, when the single UJ inherently transmits a variable output to the rear shaft (unless the UJ is dead flat that is), I doesn't make sense to me, I would def look at trying to install a CV coupling instead if at all possible.

Do you know what the alignment was on the donor car ?

 

Doug, are you saying you can change the trans mount to make it even? if so why not just measure it and get a custom drive shaft? That is what I did as that is not very expensive, but maybe I am not understanding your situation?

 

Alan, the flex coupling flanges on the XJR trans and diff are fixed, so a slip joint would need to be incorporated into any shaft. Conventional u-joints would also require adapter plates to mate them to the flex coupling flanges, and I was told the addition of that hardware can cause balancing issues at speed. And finally there's the reason I mentioned above - I assume the flex couplings contribute to the overall isolation of the drive train on the XJR, and I wanted to preserve as much of that as possible on the beast. If I can't work this setup out it's simple enough to go the conventional route as you suggest.

Jon, the folks on the left coast who shortened and balanced the XJR driveshaft tell me the flex couplings should see zero angle, and the single u-joint just behind the center bearing should see the maximum 3 degrees you mentioned. I asked if there shouldn't be an offsetting angle in either or both of the couplings and they said no - they repeated just keep the angle at the u-joint to 3 degrees or less. My next step is to answer your earlier question about the setup on the donor car - I'll get my wife's XJR up in the air this weekend and see how it's configured to (hopefully) confirm things.

By deleting spacers on the trans mount and diff carrier I should be able to reduce the overall shaft angle to 2 or 3 degrees. Fortunately the way the bearing mount is configured I can also shim it higher to test different angles under load before locating anything permanently. Unfortunately all of this requires me to open up the top of tunnel behind the shifter for clearance, which means the interior and welding blankets have to come out. Damn

 

By removing shims and raising the bearing mount we were able to achieve about half a degree at the flex couplings and just under 3 degrees at the u-joint. This required raising the shaft tunnel about 1/2" - welding on the tub at this stage in the project is not fun. If this doesn't work the only way to reduce the angles further is to lower the engine mounts. If it comes to that there's always the option of stuffing a period VW motor in the boot or sending the beast to the crusher. Keep your fingers crossed for me.

 

I'll keep my toes crossed too, a right royal pain, but I'm sure will your skills that the Beast will be spared the crusher !

 

Doug,
I am thinking positive thoughts!
Lin

 

Been down this road as well with a 56 ford wagon street rod and others.
Im not saying anything because your work has been unreal.
You can lower the mounts by size or adjust the mounting pads for them and an adjustable tranny cross bar.
Even on a tank like the 56 we needed to raise the tunnel height.
Its all about the angle.
If you have the clearance on the engine, lower the mounts all way round.
Good luck my friend.
GTJOEY1314

 

Any progress Doug?

 

Appreciate all the positive vibes. We should have the beast on the road again tomorrow. Will let you know if it's cruise or crush.

 

it will take some fabrication, but changing out the U joints for CV Joints might be a solution.

 

Haven't spent much time on the beast lately. We finally were able to resolve the driveshaft vibration - after a lot of trial and error shimming the bearing we determined everything was happy with the angles equalized over the u joint and the two flex couplings - right around 1 degree at each. We still don't have the front door window frames in and the beast isn't registered so the final high speed test will have to wait.

I finished the boot interior by cutting and fitting the XJR panels - the intent was for this to be a temporary fix but it came out better than expected - will probably look this way 10 years from now.

I kinda regret installing the parking assist sensors in the rear - just don't like the look. I thought they'd disappear with everything in black, but there's a lot going on visually between the sensors and the mounting bosses. Unfortunately I drilled through the tub to install the damn things so there's no easy fix - bumper and tail light housings off, weld patches, bodywork and reshoot and blend. Hopefully I'll stop looking at the stupid things before that happens. Until next time.

 

Good news re the driveshaft Doug, boot looks great, and to be honest, I think your being too fussy re the sensors, I see what you mean, but you won't sit staring at the rear end, and let's face it not many will get to see it that close !!! Well only in a traffic hold up !

 

Doug,


Your car looks fabulous! Great job and hopefully you can start putting miles on her!

 

Congrats Doug!
Lin

 

This picture pretty much tells the story of what I've been up to lately - going backwards on the project for the most part. I was getting frustrated waiting on the front window frames to get the car registered and conduct some high speed runs, soooooooo (don't tell anyone). While the car had handled well on my earlier runs up to about 50 mph, this wasn't the case at high speed. My test included a section of road with a series of severe dips and crests to test the suspension at full compression and extension, and hitting it at about 75 mph was interesting to say the least. The car went from feeling stable and planted to unstable and floating - not exactly what I was looking for after all the suspension upgrades.

My first thought was less than perfect geometry from the steering rack, but I've used this setup on another Mark 2 and had no such handling issues. With everything else fresh including new polyurethane bushings everywhere I knew nothing was loose. A proper alignment hadn't been done yet but I knew everything was close to spec, and this went way beyond an alignment issue. That left the mods to the front suspension carrier as the possible cause - either the cut down cross member was flexing or the modified shock towers were flexing.

We dropped the front end and connected the upper shock mounts with a tie down strap and pulled. We were able to pull the towers together 3/8" with moderate force, so I'm pretty sure this movement under suspension load is causing the handling issues. We took a lot a measurements at various points on the carrier and the culprit appears to be the cut down center box section.
I'm working with the race shop that did the original mods to come up with a fix - we have enough clearance to increase the height of the box section another 1/2", and instead of a single vertical gusset down the center we'll probably weld in a few lengths of 2" square tube between the upper and lower plates. I'm also researching different alloys for strength, so if anyone knows the best spec for high strength steel for this type of application I'd appreciate hearing from you.

If this doesn't work we'll have to find a way to extend the towers and connect them with a tie bar across the top of the engine. Space is at a premium up there so I'm hoping we can resolve the problem at the carrier itself. In the meantime we went ahead and pulled the engine to fix an exhaust leak and address some clearance issues. The fun never stops.