Rear U-joint to final drive flange orientation?

[tomstin]

I've been chasing a drive line vibration. Had the drive shaft balanced, new centering sleeve, flex collar, center mount and u-joint. And had two out of round rims that were discovered doing road force balancing. Ended up dropping the rear sub frame and replaced all the bushings and wheel bearings. All of the parts were worn so not a total waste. Car feels much tighter, but still have the vibration at 50 mph and becomes real bad as speed increases. Doesn't matter what gear, or neutral, it is road speed related. Moved the tires, no change.

Put the car on jack stands and once in gear you can feel the vibration throughout to car. Removed the tires, still there, a little less as there is less rotational mass?

Removed the half shafts from final drive, still vibrating.

With the car in gear on the jack stands, the front of the drive shaft is smooth as silk, same for the front section in front of the center support bearing. The rear section, just behind the center support bearing, nice and smooth. Just in front of the rear u-joint you can feel the vibration.

So, the question, the final drive flange has a painted white mark on it. Does it make a difference how the rear u-joint mounts to that final drive flange?

I have a run-out gauge coming Tuesday.

[TobyB]

It really isn't supposed to matter.

I have had a bent diff input flange do that-
a dial indicator eventually found it.

I have also had a loose pinion shaft do it-
it wasn't loose enough to run out of true when I spun it on the bench,
but it was deflecting under load- it was worst at neutral loading.
That didn't show up until pretty high speed, though.

If the u-joint's coming apart, that'll do it- but I'm sure you've checked it 3 or 4 times!

hth

t

[tomstin]

I have also had a loose pinion shaft do it-
it wasn't loose enough to run out of true when I spun it on the bench,
but it was deflecting under load- it was worst at neutral loading.
That didn't show up until pretty high speed, though.
t

Thanks Toby, you have me thinking now. I did notice the diff seemed a bit warm considering the wheels where off the ground and had only run for a few minutes. Also vibration was worst with the axle and wheels on (more load). I’ll see if the run out gauge shows anything. Also would support little change in the vibration after the drive shaft was balanced.

[TobyB]

Yeah, I went through a lot of exasperation with the driveshaft before I found the bent flange...
....by the pinion episode, I was smart enough to realize that a new diff was the only variable.

t

[tomstin]

Differential flange collar run-out is .002. For the face of the collar I'll need to get creative to check that with the differential in the car. I would rather not pull it...but, may need to go there eventually.

I cleaned the face of the collar and the drive shaft mating surfaces and they meet up with a firm clunk. It doesn't "appear" to be anything preventing a clean, consistent connection.

Drive shaft run-out near the rear u-joint is in the neighborhood of .018. I rotated the u-joint on the differential flange one hole at a time and the run-out is very similar. But that's tough to be 100% accurate as I needed to move everything with each change but its remarkably similar.

The only thing I didn't change when I had the drive shaft balanced was the nylon spline bushing and locking nut. I have one of those on its way. The next step is to pull the drive shaft, check the run-out of the face of the differential collar, replace the nylon spline bushing and see where that takes me.

More to come...

[tomstin]

After cleaning up the yoke on the differential I noticed some witness marks that gave me a hint how the driveshaft was originally installed. Also check the yoke flange for run-out and that looked good.

Removed the driveshaft and replaced the splined plastic collar and nut on the slip yoke and reassembled. Very little change if any.

Spent some time with Mr Bentley last night and the only other thing I want to check is I may have put too much pre-load on the center support bearing? As a last gasp, I'm going to re-position that and see what happens.

If that doesn't do it, I think I'll order a re-manufactured driveshaft. I hate throwing parts at a problem, but that's where I'm at with this vibration.

When spinning, both the front of the driveshaft is smooth, same just before the center support bearing, very slight vibration behind the center support bearing and much worst just before the u-joint. The differential collar is smooth.

[tomstin]

To close this out, I finally ordered a new Beyer drive shaft and the vibration decreased dramatically. Looks like the local guy didn't center the u-joint properly.

But I still had a vibration that I felt in my seat. I decided to go from front to back and check every joint and bushing. I found a bad front strut bearing with a LOT of play and would bang when the wheels were off the ground while wiggling the front wheels. I replaced the front struts and bearings and now the ride is smooth as a baby's butt!

My thought on the strut bearing is that there was enough play in that bearing to vibrate through the body versus the steering wheel.

Moral to the story, sometimes a vibration you feel in the seat, may be in the front suspension. In the past, I believed that a vibration in the front you would feel in the steering wheel.