Alright here's the story. I recently installed a Moser 12-bolt with 4.11 gears on my 95 M6 Z28, and I'm having a stubborn vibration problem at high speeds (above 70 mph). The car has 2" lowering springs, poly trans mount, BMR rear suspension, LCA relocation brackets. I also had a custom 3" steel driveshaft with new heavy duty U-joints fabricated by a driveline shop, as the stock shaft would have required shortening. The driveline shop that I've been dealing with (the ones that fabricated the steel DS), seems very compotent and they insist over and over that the driveshaft, pinion yoke, ect. are very well balanced and should not be causing any problems.

After the initial install, the vibration was very heavy and kicked in at 70 mph. I had the driveline shop that fabricated the shop access the problem, and they attibuted it to improper drivline angles due to lowering. They reset the driveline angles to -2.5 degrees for the both the transmission and rearend angles, instead of the normal positive trans angle and negative pinion angle. That fixed most of the vibration for a while, until I had the gear oil changed and gears checked after the 500 mile break-in period on the Moser. Since the car was going back in anyways, I told them to check the driveline again to see if they could eliminate the small amount of vibration that was left. The vibration at that time was as follows:
-only above 70 mph in gear
-light (as opposed to "heavy after initial install")
-stopped when clutch pushed in, or letting off gas
-felt constant as speed increased ( ex: accelerating from 70 to 90)

Here's the confusing part. After the break-in service, the driveline shop said the gears looked good, and they checked the balance on the driveshaft again. They said the shaft was very well balanced, and re-installed it. They claimed that they did not change anything else, and left the angles the same (-2.5 degrees). BUT, after leaving the shop the vibration had increased to almost it's previous level! In addition, the vibration would remain even if the cluth was pushed in. The shop could not explain why the vibration had changed as they claimed to not have changed anything. The current vibration is as follows:

-only above 70 mph
-medium/heavy
-remains when clutch pushed in, or trans in nuetral, engine idling
-seems to increase with speed (not completely sure, only driven shorty)

The shop said they tried to mess with the driveline angles again, could not change the vibration at all. They now say the car has -2.7 degrees on the trans side, -2.8 on the pinion. I know that these angles are different the the -2.5 earlier when I had less vibe, BUT they said that they tried a bunch of different angle this last time with no results.

THE SHOPS ANSWER: They claim that the car has an engine vibration between 1900-2200 RPM that is probably causing the vibration. I CAN feel this vibration just revving the engine up to that speed. Or, they think it could be a clutch issue. They insist that the driveline and/or DS is not the problem.


Here are my questions for you guys:

1. Do you think an engine vibration can transmit through the entire driveline? If so, can a mild engine vibration cause what feels like a moderate driveline vibration? The vibration definitely feels like it coming from the rear of the car (pinion yoke).

2. Why would the vibration change so drastically after servicing the rear end (removing cover, new oil, setting girdle pre-load), and R&R the driveshaft?

3. Would a switch to an aluminum DS help? They claim that this steel shaft is balanced perfectly. (But, they did make it.... ) If so, could a shaft like Lingenfelter's be shortened 1/2-1" to fit my application?

4. Could this be a problem with the Trans yoke/dampner assembly? Read some other threads that alluded to this...

5. Will switching to a softer trans bushing help instead of my poly bushing? Had the poly bushing before this install, with no vibes.

 

Have your shop check the following:

1) Check the fit-up of the trans yoke into the transmission. Some yokes have loose splines or the trans has a worn out tailbushing which can cause yoke wobble at certain frequencies. It doesn't take much to cause problems. I found that aftermarket billet yokes were way too loose. Plain ol' spicer yokes seemed to be controlled the best. My trans bushing was oblonged after I broke a drive shaft and the 1 foot section whipped around a while before I got things shut down.

2) Check how accurately the rear U joint runs on-center as installed in the car. Use a dial indicator on the face of the U-joint bearing cap, zero the indicator, then rotate the shaft 180 degrees and check the other cap. Anything off more that .005" will drive you crazy. Mine was way off after a new rear install. The cause of the problem was the solid sleeve setup to preload the pinion bearings threw the pinion yoke way off. Ended up disassembling the rear and going back to a crush sleeve.

3) An aluminum DS does tend to dampen vibrations a little better. I'm not exactly sure why.


I screwed with your exact problem for over a year before I figured it out.

Hope I helped

Steve

 

Thanks for the suggestions. I think I'll have my shop check those things, and also see if they can diagnose the motor vibration.

I'll probably also try switching to a stock-type trans mount to help with the problem, although that's most likely not causing it it's probably compounding the situation.

Question: guys that had vibrations from a poly trans mount, what type was it? All-around, RPM specific, high speed...? Thanks

 

A poly trans mount doesn't cause vibrations. It make it transmits vibrations that are already there instead of dampening them. So it can be any type of vibe.

 

I had the same problem. It turned out that my torque arm was loose, and it was touching the DS tunnel ever so slightly, so when I got up to speed and the wind started pushing down on the car the torque arm would make contact with the tunnel. It's worth a shot.

 

What about a carbon fiber driveshaft?

 

Funny you should mention the Torque Arm/ tunnel clearance problem. The driveline shop already thought this was the problem once, and convinced me to send the car to a fabrication shop to cut the tunnel clearanced. The Hot Rod Fab shop cut out a section of the floor and welded in a steel pocket for clearance. Cost $300 . Didn't fix the problem, and needless to say this added to the frustration. But, at least now I have clearance as the Torque arm was damaging the floor from hitting it. Still have vibration.

BTW, I definitely don't have $ for a carbon fiber shaft. Besides I already paid $350 for the steel one, and I doubt they'll give me a refund on a custom shaft.

As an update, I took the car into my general repair shop to look at it today. They think the problem isn't the engine, as the vibration remains if the clutch is pushed in. They're thinking either an imbalanced/bent tire/wheel, or they think the front U-joint looks a little loose but aren't positive. I don't think they bothered to check the suspected engine vibe yet. I'm taking the car into my tire shop for a free balance check tommorrow to see if that helps. Don't know if it will, as previously the vibe would disappear when clutch pushed in, which means it couldn't be a wheel. After the tire shop, the car is going back the the repair shop on Monday for further work.

This whole thing is just killing me! I'm already broke, and I spent another $100 in labor today for basically nothing, just inspecting. The driveline shop insisted that the problem is in the engine/trans as previously the vibration dissapeared when the cluth was pushed in. Now that the vibe is constant with the trans in neutral, the general repair shop says the opposite because the vibe remains despite disengaging the powertrain. JESUS CHRIST!!

Someone please help!

PS- the broken muffler that I have right now (busted baffels in Flowmaster) couldn't be affecting my engine vibe could it? I'm ordering a new Hooker Aerochamber muffler tonight.

 

Does the vibration happen at the same rpm regardless of the speed? If so, it is probably an engine vibration. If your vibration problem happens at different rpm's, it is not the engine.

Changing out the poly tranny bushing for a stock one will reduce the vibration.

I adjusted my rear end angle according to the Spohn instructions which came with my new torque arm. I measured the angle of the driveshaft (it was around 4 degrees) and then adjusted the rearend to 2 degrees. I haven't had any unusual vibes in the drivetrain.

 

Took car to Tire Pros for balance, front-to-rear rotation. The said that none of the wheels were bent, no existing balance problems, all tires at 32 psi. The vibration seemed the same when I drove home on the highway. Hence, probably not the wheels, as I suspected since the vibe used to disappear when the clutch was pushed in.

Your logic about RPM vs Speed was exactly what I asked my driveline shop, and they claimed that the vibration was still there at that RPM (1900-2200) despite speed. I guess they meant to a lesser degree at lower speed, but I can't feel it at all at lower speeds. The vibe clearly comes in at around 70 mph.

When I take the car back to the general repair shop, I'm going to have them call the the driveline shop and let the two fight it out until one of them caves. They both can't be right, as their explanations are complete opposites.

 

Well, the fact that the problem surfaced after the rear end and driveshaft install would indicate one of those items caused the vibration. You didn't change your clutch or did any other work, right?

 

Well, took the car back to the repair shop today. Their opinion is that "maybe" the clutch is the problem. After they did another $100 of "labor", mostly test-driving, visual inspecting, and hypothesizing, their best guess was a clutch problem. They offered to remove and inspect the clutch for another $500 in labor and replace if necessary. If that isn't the problem, they explained I of course would still be resposible for the labor cost and they would take no responsibility. I promptly took my keys and left before they could charge me another cent. I'm pretty pissed because they did not do any engine testing, as they did not think this could be the problem (complete opposite of driveline's shop's opinion). They spent time checking the pinion angles again, ect., exactly what my previous shop did with no success. So, I paid another $200 in labor for guesses, opinions, and no concrete testing. Christ, if I wanted hypotheses I could sit in my driveway and think them up myself. What am I paying these guys for anyways?

Still stuck with this vibe.... So, in summary if we take believe everything that the shops have told me , all of these things are NOT the problem: wheels/tires (balanced), driveshaft or rearend (inspected several times), pinion/trans angles (many angles tried), engine ("opinion" of repair shop").

The problem is that only leaves the transmission and clutch, which don't seem to be having any other problems themselves, and were exactly the same BEFORE this rearend/driveshaft install . The trans shifts fine, no funny noises or behavior. Cluth doesn't slip at all, no funny engagement, ect. I find it harder to believe the the trans/cluth coincidently broke at the same time as the install. Seems much more likely that one of the installed components would be causing the problem.

So, to play devil's advocate, do you guys think either the cluth or trans could be causing the problems I've described? Someone already mentioned a tailbushing or yoke in on the trans. I told this last shop to check those things, but they were too busy re-checking components at their own perogative. I don't think they checked either of those items.

 

Hey, whitch torque arm do you have, is it the stock replacement or the track pack? I have the track pack and it makes noise/ vibration, that's just how it is. It's not as extreme as your problem, but it was at one time. I can't remember what fixed my problem. I know that sounds stupid, but I changed so many things I cant pin point whitch one it was. I checked all the same things you did, I even had my rear taken appart and had the axels and pinion yoke checked for run out, and everything came back with in spec. Here is the turning point, it's probably the last thing you want to hear but, I went drag racing and launched the car @ 6000 rpm with a 150 shot spraying into the motor off the line and I went about 3 inches and blew the cai and maf appart. I ended up breaking the output shaft in 2 and because of that the engine over revved causing the back fire (I have a window switch, but for some reason it didn't work that day maby a bad ground either way after re checking/ re routing the wires, for a cleaner install, everything worked like it was supposed to.) So I had the trans rebuilt with a viper spec output shaft, so I had to get a viper sized yoke (spicer part, nothing fancy) I had the shaft balanced (the exact same shaft from the exact same shop), and the problem went away. The thing is I had the trans rebult only a month before the output breakage. I had the factory output checked and it was straight, so I didn't want to spend the $400 at the time (stupid move, it ended up costing double to take the trans appart again). So after writeing this post and looking back on it it just might of been that stupid factory yoke, the one with the damper on it, maby you should try to find a yoke that does not have one of those on it and try the re balance again. By the way, and I don't mean to rub salt in your wounds, but I fixed my DS tunnel with a 2 lbs hammer with a coupple rags duck taped to the striking side. Good luck with the project and I hope I may have helped you some.
Aleks
P.S. I went back to the track with my new viper output shaft and repeated my 6000 rpm launch and went 11.21 @ 123.

 

Well, went to the trans shop and I have a new idea. BTW, I tried swapping in a stock rubber trans mount, did nothing to help the problem. BUT, I think the the torque arm may be contacting the floorpan at different places than just the back by the pinion! When inspected, there were rub marks along the floorpan along the length of the floor, and clearance was VERY tight. We tried at the trans shop using a crowbar to bend the floor away from the torque arm from the length of the tunnel. Along the top length of the torque arm bar, the clearance at first was only about 1/8 - 1/4 inch. With the crowbar, we got an additional maybe 1/2 inch. I didn't make much difference in the vibration, but I think I'm on the right track anyways looking for something rubbing/contacting the floorpan (torque arm or otherwise).

This is because with the symptoms I'm having with my vibration, no single component seems logically to be causing it. The vibration seems to be variable depending on minor changes in setup. It's only at high speed, and varies on how it behaves. Above 65 mph, you can get the vibration to stop by pushing the clutch in and/or waiting a few seconds or revving the engine with the trans in neutral. This does not seem like behavior consistent with a specific defective component like a driveshaft, clutch, ect. This DOES seem like symptoms of something rubbing like the torque arm against the floor. The intensity and nature of the contacting component could be changing based on any variable that affects the load/position/ect. of the driveline.

If this is so, this is a good thing. Getting clearance is a whole lot cheaper than a new clutch, driveshaft, ect. WHEW!! I truely feel that this is the cause, as the more I think about it the vibration just "feels" like something hitting the floor. It transmits through the whole floor of the car, and you really feel it on the bottom/back of your seat when driving. Hopefully this will fix the issue. Any more input on possible clearance issues would be appreciated. Thanks.

 

you know, I have kinda the same thing going on. I have come up with an idea of what it could be, but since there is six inches of snow outside, and it is really cold, driving the car right now is not that big of a deal, so fixing it is not either. But here is something for you to think about. You got this vibration "after" you installed the 12 bolt, so, maybe the 12 bolt is not perfect! Let me explain, on my car, I have the moser with 3.73 gears, an aftermarket torque arm, and 1 1/2 inch eibach lowering springs. I have a 3.5 inch aluminum d/s, and my angles are set! Now I know that my rear end is centered under the car left to right, but what about front to rear? If one tire is slightly more forward than the other, the tire would start to side-scrape as you drove, the faster you went, the more scuffing would occur. I only have an inch and a half drop, so you with a two inch drop I would imagine it would be worse. I know that my adjustable pan hard rod centered the rear cause I could measure it and change it, but my LCA's are not adjustable, and who is to say that the mounts are perfectly placed? Or that the holes are put in the exact places they need to be? Just something for you to check yourself, get a tape measure, jack your car up (I can imagine it sucks big time), set the rear axle on jack stands so the cars weight is on it, take off the back tires, and measure from the front most part of the housing ends on both sides and see what you got. I have no idea what is within spec, but if you are off 1/2 or anything like that, that could be a factor.

Now the only reason I suggest measuring from those points, is because there can be no bent rim, no tire difference, nothing really to interfer with how far the "housing" is sits back. Try this, it is free, and if it is part of your problem, well, yeeeaaahhhh for you! Hope this helps, Jason

 

can someone tell me how and what tools (and where to get them) i need to check my DS angles