Ok, in my hunt to find my nasty vibration I decided I better check my driveshaft length. I suspected it may be too long as it was always a little tight getting it in so after doing many other things about this vibration I decided I better check this as all else has failed. I jacked the car up and with the rear at drive height I unbolted the 2 ubolts holding the driveshaft to the rear and slid the driveshaft into the tranny. I measured the movement to be 11/32" (about 0.9cm). I have a Moser rear with a Spohn driveshaft and Spohn TQ arm. Now I am thinking I should have atleast 1/2" (~1.2cm) but I don't. Now the question is is the shaft too long and more importantly could this cause my vibration problem? The vibration I have I can slightly feel at lower speeds and by around 60mph it gets REAL REAL bad. Anyone have any ideas on if this is my problem?

 

Im not real sure about the length issure, I would think that as long as you have some play at all times you should be ok. When the suspension squats does it push it into the tranny and maybe cause it to bottom out in there?

Is your TQ arm adjustable. Could your pinion angle be out of whack? That is the first thing I would look into if you havent already?

 

I have an adjustable Spohn rod end torque arm. I've tried various pinion angles, I've rebalanced the driveshaft, I've got 4 new tires and had them balanced and rotated them to double make sure, I've measured to make sure the wheels are equal front to back on both sides in the wheel wells, I've checked for play in the T56, I've properly clearenced the floor for the TQ arm, about the only things I can think of now are: driveshaft too long, freak chance axle out of balance or bad new wheel bearing, something wrong in the rear diff like the backlash or something which I never had checked. I don't know much about rear end internals so if you guys think the driveshaft is an ok length I guess I'll take the car to a shop and get the rear checked out I've put about 2000 miles on it now. Maybe I trusted Moser too much

 

Everything I have seen indicates you want at least 1/2" clear on the front slider. And that's for a "stockish" HP level. As you up the HP, the requirement increases toward 1". It's conceivable that under load, the DS is bottoming out in the tailshaft housing and transmitting vibration. Not sure how you could verify that other than borrowing a shorter DS.

 

Well I have just over 1/4" that it can slide in.. should I just go and have it shortened about 1/2" that'd give me just over 3/4" and the car is a street car that will likely top out around the 500hp mark when I am done with it. I don't want to waste money if I don't need to but if you think I need atleast 1/2" then I guess something needs to be done sooner or later here. Spohn figured out the length based on what I told them so I figured it should be ok, I'll have to talk to them and see what they say. If anyone has an idea of what the shaft length should be for an T56 car that has a Moser 12 bolt and is lowered with Eibachs let me know so I can measure my shaft or something to get a better idea.

 

The Moser seems to be problematic as far as DS length is concerned. Seems like about 1/2 the people have problems with the stock DS being too long, and the other 1/2 don't. Is it possibly a matter of which pinion yoke they put on it? Does the 1350 yoke cause length problems? Don't know.

I had a problem with an ACPT carbon fiber DS.... front metal part tore loose from the inside of the CF tube. ACPT tried to tell me it was my fault because the DS was too long. I talked to Steve Spohn, and his info was that the stock DS had 1/2" clearance, and he recommended 1" for high HP setups. My ACPT DS was in fact ordered, and delivered 1/2" shorter than stock, so in the end, ACPR lost and had to refund my money.

The shop that does my performance work ordered a new Mark Williams chrome moly DS to replace the ACPT DS. It was also ordered and delivered 1/2" shorter than stock. It worked fine, no problems. My setup at the time was a T56 and a Strange 12-bolt. But the Strange will work with the stock DS 100% of the time. The 1350 pinion yoke is standard on all Strange 12-bolts (I think it is extra cost on the Moser), and I used the Strange for at least a year with the stock DS with no problems.

I think you need to shorten the DS. Will it clear up the vibration problem? I have no way to know. But it will prevent future problems. Its possible you already have damaged the tranny output shaft..... that would explain the vibration as well, and shortening the DS isn't going to correct any damage to the output shaft.

Tough call, but I'd vote for the shorter DS, and keep my fingers crossed.

 

How do you shorten it? You cut the front or cut the tube and reweld/balance it?

 

You can machine the yoke shorter on an aftermarket driveshaft as the yokes are about 1" longer then stock or you can shorten the shaft and reweld and rebalance it. I talked to Steve Spohn today and he said I can use adjustable lower control arms to push the rear back 1/4" to test and see if the driveshaft is the problem. After talking to him about it he thinks it could be the backlash in my rearend causing my vibration problems so I am going to take the car to a local shop and have that adjusted before I decide to mess with the shaft more.