Roadie

Like many others, I'm also chasing down a vibration...

The car is an M6 with Moser 9", 3.70 gears, and is lowered about 1.5". The pinion angle is set correctly. The vibe starts at just over 80mph (3600 rpm at the driveshaft).

I've already tried 2 driveshafts (my stocker that I know is balanced and the new 3" steel heavy duty one). I've also tried clocking them 180 degrees from where they were without any improvement. Both driveshafts are the right length (7/8" of movement before bottoming out in the transmission)

A few threads mention alignment of the driveshaft, and since I just replaced the clutch and transmission, I thought I'd look there next. I may have the transmission slightly out of alignment since there's 1/2-3/4" adjustment from left to right in the transmission mount.

What is the best way to check for alignment of the transmission? I thought about getting a threaded rod with the same threads as the hole in the ourput shaft of the transmission. Then, thread the rod in, and adjust the transmission until the rod is pointing straight back at the rear end yoke.

Thoughts?

OutKast

You have just described my same issue. I'm also running the Moser 9" with the M6/M29 tranny. If you figure it out please let me know. I spent a good amount of time under the car last night re-checking the angles and measuring the DS to no avail. Well, my DS is about a half inch short. I am going to order a new steel DS for it but the fact that you're having the same issue with a DS of the correct length makes me think that a new DS won't fix the vibration.

How do your angles measure out? When I measure the angle of the DS I get about 2* and when I measure the angle along the bottom of the TA mount I get 4*. The difference of the two (which is 2*) should be the pinion angle. Right???

Tonight I'm going to measure the center of the rear wheel to the center of the front wheel on each side and see if it's sitting square under the car. I eyeballed it for several minutes last night and it looks straight but the tape won't lie.


See my post HERE - lots of good tips and info has been posted along with links to others who have the same thing happening.

Roadie

OutKast: You're thread was one of the ones I read, but I didn't want to stomp all over it with my problem.

I also have -2* pinion angle. I'm not as worried about the rear end because nothing changed there when the problem started. I swapped transmissions... So I'm leaning towards a transmission alignment problem. I think i'll do what I said above and make sure it's pointing directly to the rear yoke. I can't think of any better way to determine the left/right alignment.

I also saw in a thread where a different height due to transmission mount thickness made a difference, but I didn't have a problem with the same transmission mount before the swap, so I'm ruling the mount out as a possibility.

SABLT194

Check the U-joints at both ends with a dial indicator on the face of the caps to verify that the U-joint is running on center. Mine was .020" off on a 12 bolt and it drove me nuts for a year. They should run within .005" TIR. Also make sure your slip yoke is a reasonably tight fit into the transmission tail bushing. Wobble in the slip yoke is a killer. Both of these were sources of serious vibe for me.

Roadie

The slip-yoke is tight. And there's no way the u-joint can be off center the way things are mounted (1350 with moroso girdles)... Click here for a pic

I'm still thinking alignment. I found the thread size and am going to pick up a threaded rod tonight. I'll keep everyone posted.

SABLT194

You are assuming that all of the parts are machined correctly and that the pinion yoke is attached squarely to the pinion. This is not always the case.

Steve

Roadie

I'd just like to re-emphasize this point... This problem started when I swapped transmissions.

My main concern in this thread has been "HOW do I make sure the transmission/driveshaft is aligned correctly?" Does anyone have a better idea than threaded rod in the output shaft?

Roadie

So, I pulled the driveshaft tonight and threaded a 36" rod into the output shaft of the transmission. At the end of the 3', the rod was a little over an inch off center, close to touching the torque arm.

I loosened the trans mount and moved the tailhousing to the right so that the threaded rod was on center with the rear end yoke (used a string from trans to rear to be as accurate as possible). I reinstalled the driveshaft and took it for a spin. BINGO! No vibe at all through 110mph.

Thanks to all the threads I read that got me looking in this direction. A big thumbs up to those talking about driveshaft alignment.

OutKast

Sweet, can't wait to get home and give it a try

97 6SPEED Z

Roadie, congrats on getting your driveshaft vibration problem solved. I'm also a firm believer that alignment is the key to solving many of these type of driveshaft related vibration problems. Now, do a fellow board member a big favor; what is the thread size of that 3 foot threaded rod you bought to thread into the end of the tranny output shaft? TIA for that piece of info, and congrats again.

Roadie

it's a 12x1.75 metric thread... But I couldn't find a threaded rod in that size anywhere. A 7/16-14 bolt will screw right in all the way in by hand until it bottoms out, with a tiny bit of play (7/16 is just a hair smaller in diameter than 12mm, and the threads are as close as can possibly be). So, I picked up a 36" 7/16-14 threaded rod from the hardware section of Home Depot and it worked beautifully.

97 6SPEED Z

Roadie, thanks again for that piece of info, and good luck with your car.