Drive Shaft Thumping

[TJBIII]

I replaced the carrier bearings and bushings on my '97 TT. Now if I wind up 2ND when I get to the high torque range and the back end drops, it feels and sounds like the DS is trying to come up through the floor. It will act up in 1st also, I have not tried the other gears you get going too fast. If I back off on the gas and the car levels, the noise and vibration stops Any Ideas?

[J. Fast]

Try dropping the driveshaft and clocking 90 degrees and reinstalling. The other thing is be sure your exhaust and hangers are out of the way. When you swapped the carrier bearing did you service the Lobro?

[TJBIII]

I have to admit, I do not know what a "Lobro" is. We checked the exhaust and hangers.

[J. Fast]

I have to admit, I do not know what a "Lobro" is. We checked the exhaust and hangers.

The Lobro's precise location is adjacent to the rear carrier. It's a pineapple looking joint that sits on the engine facing side of the rear carrier bearing.

The OEM propshaft has three dampeners. (2) carrier bearings, and (1) Lobro. The Lobro joint is positioned between the two carrier bearings to dampen the "stretch and rake" harmonics in the driveshaft. The (2) carriers are for the torsenial or "twist harmonics and vibrations" of the driveshaft.

If you're still getting a propshaft vibration it could be due to these things.

1) the lobro is not floating or functioning improperly
2) the carrier bearings were not installed properly or they were faulty
3) the propshaft was not sync'd properly
4) the propshaft kicked the balancing weights.
5) vibration is being transferred from something else and is unrelated to the driveshaft.

Do you have access to a Steelman Wireless chassi ear, or a chassi ear of sorts? You could certainly get to the root of the problem quickly if you could borrow one or check one out from NAPA or Autozone.

[TJBIII]

Thanks for the tutorial. I did not want to write War and Peace in my first post. The problem was very severe when I first drove the car after the work was done. When I took the car back at about 100 miles. I had a very difficult time demonstrating the problem. Besides the thumping there was a rotational clicking sound, along with the thumping. Both issues are minimal now, I had to drive the car myself because the mechanic would not push the car hard enough. I do not believe in miracles, things do not fix themselves. I am afraid that something is wrong and will fail at the worst time. I will have the lobro checked. It makes sense since the only time it fails is when the torque makes the rear end lower. I do not understand the syncing point.

[J. Fast]

Sync:

When you separate the propshaft you must mark the companion flanges and the ball positions at the lobro for reassembly. Once taken apart and serviced they must be reinstalled and attached in the exact same position they were placed by the MFR. Your driveshaft was dynamically balanced from the Plant. Take note of the weights welded to the driveshaft invarious locations, if you change the weight distribution by clocking or re-installing any section out of sync it's quite possible the driveshaft will not balance correctly and it will vibrate.

Additionally, ensure the vent at the lobro is not filled with grease. It's important that the lobro can relieve pressure at the joint as it rotates. If the driveshaftt humm's around 55mph it's likely caused when the lobro vent hole is clogged with grease and the bearings overheat. It causes the joint to act's like a spring instead of a float.

[TJBIII]

Again, thank you. I'll reply after I apply your suggestions.

[TJBIII]

I have the car back. They could not find anything wrong, plus the guy working on it was going on vacation. Now that I have had some time to test it, I have some questions. I want to be able to guide them through the repairs, since obviously they have not done one of these before.

I am assuming the lobro is bad. I do not know at this time if they took the lobro apart. I am pretty sure they did not. One of my assumptions is that if it is a DS balance issue, the thumping would be related to ground speed, i.e. it always starts thumping at 55 mph no matter what gear you are in. It does not do that, it reacts to torque. The only way I can get the thumping to start is floor it. In 1st at about 3500-4000 the thumping gets going pretty good, I get the most noise and vibration in 1st, most torque. In second around the same RPM it acts up but not as violently. I can also hear some clicking sounds. It sounds like something is trying to turn but binding. When you stand on it in 1st, things happen pretty quickly, so you only have a couple of seconds of time to figure out what is happening. I have not tried the higher gears, too much speed! I can do without a ticket.

Now the questions, does that sound like lobro? I plan on having them put it up on a lift and run it. There is a video that shows a bad lobro. The DS in the video seems to flex at the lobro.

Next question, how do you identify a bad lobro? How do you service it, and keep all of the parts in the correct orientation? Can you get a new lobro if yes, how do you balance it? I tried looking on line for a new one but came up empty.

Any help would be greatly appreciated.