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Discussion Starter #21
Hey Guys,

First off, Steve the shock mounts have been inspected and are fine. Thanks for the suggestion.

Well, we might be getting somewhere. The new driveshaft is on and today we did a few tests. We removed the rear wheels and then ran the car in 4th and 5th at 1-3000 rpm's. Still vibrating. So we can totally eliminate the wheels (they're brand new anyway). Next, we took off the heat shields and reattached the muffler. There is a definite vibration coming from the center bearing area of the driveshaft. We think the low mph/low rpm vibration might be coming from one of three places. They are:

1. The new drive shaft has a mount at the center bearing that has about a 10-12 mm variance in it. The recommended mount is 2 to 4 mm from the front end side of the slot, but you can mount it wherever. They are trying different mount places to see if it helps.

2. The shop seems to think that the angle of the driveshaft connecting to the transmission is putting it in a bind, so they are going to try and lower the tranny housing slightly to achieve a neutral or parallel geometry from the tranny housing through the driveshaft and into the LSD (I think that's right).

3. The new driveshaft could possibly be bad as well.

We'll see what happens. This has been very enlightening to learn about the things this vibration is NOT caused by...hmmm

Anyway, one thing for you guys to comment on...I noticed when the car was up in the air and in fifth gear spinning, the half shafts looked to be spinning out of balance. Since they were suspended in mid-air, they were of course hanging down at their lowest possible point regarding the suspension geometry. My point is this: The half shafts seem to spinning well at the wheel and at the differential, but if you watch both half shafts in the center of the shaft they do not look like they spin in perfect balance. Almost as if they "wobble" a little in the center of the shaft. Is this customary for the half shaft, or could they bend out of balance and bend to a point where they would not be true anymore?

If so, they could be causing these higher speed vibrations. My mechanic said that they may just look like that because the car is off the ground and the half shafts are NOT spinning parallel to the ground like they normally would when you are driving. He said the only really good way to test the half shafts are to put the car on a dyno and watch the shafts for vibrations under normal load.

Anyway, what insight do any of you have regarding the halfshafts. Could they cause a constant vibration between 75-100 mph? Can they bend over time due to force from the engine or maybe just age (sagging)? Any help is appreciated. :byebye:

I'll report again soon.
Robert Collins
 

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Hey Guys,

First off, Steve the shock mounts have been inspected and are fine. Thanks for the suggestion.

Well, we might be getting somewhere. The new driveshaft is on and today we did a few tests. We removed the rear wheels and then ran the car in 4th and 5th at 1-3000 rpm's. Still vibrating. So we can totally eliminate the wheels (they're brand new anyway). Next, we took off the heat shields and reattached the muffler. There is a definite vibration coming from the center bearing area of the driveshaft. We think the low mph/low rpm vibration might be coming from one of three places. They are:

1. The new drive shaft has a mount at the center bearing that has about a 10-12 mm variance in it. The recommended mount is 2 to 4 mm from the front end side of the slot, but you can mount it wherever. They are trying different mount places to see if it helps.

2. The shop seems to think that the angle of the driveshaft connecting to the transmission is putting it in a bind, so they are going to try and lower the tranny housing slightly to achieve a neutral or parallel geometry from the tranny housing through the driveshaft and into the LSD (I think that's right).

3. The new driveshaft could possibly be bad as well.

We'll see what happens. This has been very enlightening to learn about the things this vibration is NOT caused by...hmmm

Anyway, one thing for you guys to comment on...I noticed when the car was up in the air and in fifth gear spinning, the half shafts looked to be spinning out of balance. Since they were suspended in mid-air, they were of course hanging down at their lowest possible point regarding the suspension geometry. My point is this: The half shafts seem to spinning well at the wheel and at the differential, but if you watch both half shafts in the center of the shaft they do not look like they spin in perfect balance. Almost as if they "wobble" a little in the center of the shaft. Is this customary for the half shaft, or could they bend out of balance and bend to a point where they would not be true anymore?

If so, they could be causing these higher speed vibrations. My mechanic said that they may just look like that because the car is off the ground and the half shafts are NOT spinning parallel to the ground like they normally would when you are driving. He said the only really good way to test the half shafts are to put the car on a dyno and watch the shafts for vibrations under normal load.

Anyway, what insight do any of you have regarding the halfshafts. Could they cause a constant vibration between 75-100 mph? Can they bend over time due to force from the engine or maybe just age (sagging)? Any help is appreciated. :byebye:

I'll report again soon.
Robert Collins
 

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Robert,
Did you find out the cause of the vibrations? I know you did a ton of work on yours. Mine 2004 645Ci is driving me crazy with vibration. Feels like the center is shaking sideways. New tires and rims, brakes and rotors. New front suspension parts. Shakes over 50mph. I'm thinking maybe guibo.? Any thoughts? Thanks in advance.
 

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Insight:
"what insight do any of you have regarding the halfshafts. Could they cause a constant vibration between 75-100 mph? "
There are two kinds of vibrations. those you feel and those you hear.
If you do the math you will understand you typically cannot hear the vibration problems that occur at wheel rotations speeds, however the problems in the driveline ahead of the diff output will be audible at the higher speeds. Of course wheel and diff bearing with many rollers and ball bearing can produce audible vibrations.

I anticipate most vibrations in the higher speeds like 80-100 mph are caused by drive shaft and/or diff misalignment.
The drive shaft center bearing needs to be in alignment with the front and rear sections of the drive shaft.
Also, the pinion shaft of the diff should be pointing at the vertical alignment of the output shaft of the transmission. This alignment is done with a laser pointer mounted to the center of the pinion yoke and when the diff pinion is rotated the laser pointer will point directly to a vertical alignment with the trans output shaft.

I did a diff alignment and that solved the high speed vibration that started at 85 MPH.
The diff alignment problem had been in my car since new; with numerous failed dealer repair attempts by the previous owner. Repaired at 45K miles.
 
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