Gadgetized

iTrader: (9)

If your vehicle is in low at 60 MPH, or OD at 60 MPH, your driveshaft is moving the same RPM's and the same speed. You could be at redline or at idle and it won't make a difference.

SCOTT_A

iTrader: (1)

I'm bringing this thread back and updating it for what I have found to be solutions to my problem... A lot of threads go on and on and never have closure with the real solution to the problem.
My vibration is still plaguing me. Its WAY better than it was, but I have been searching for a fix and I thought I would share what I have learned.
I always knew my U joint operating angles were equal and opposite. I learned the importance of this from my Jeeping days. The thing with Jeeps and lifted 4x4's is you don't usually ever run over 75 mph, so a steep u-jount angle is ok.
Here is a illustration of my current driveline geometry.


I thought I was following the rules with 6 degrees negative on one end and 6 degrees positive on the other. equal and opposite. And it felt fine going down the road until you broke 80 mph.
I found the Spicer Driveshaft Manual and found everything already discussed in this thread (equal and opposite angles, critical speed charts etc). I also found a chart for "Angle Size vs max RPM". "Spicer engineers recommend true universal joint operating angles of 3 degrees or less. Any universal joint operating angle greater than 3 degrees will lower universal joint life and may cause a vibration. Here is the chart:


So according to this my 6 degrees limits me to 3000 rpm driveshaft speed, thats only ~75 mph in overdrive. So I'm going to fix this by doing what Swerve suggested much earlier, raise the trans tailshaft one inch. Then adjust the pinion angle with angle shims to make the angles equal and opposite. This should reduce my angle, hopefully around 3 degrees.
I have read through a lot of drivetrain vibration threads and a lot of people with dropped trucks suffer from vibration. I hope this helps some others out.
It will be a couple months before the streets are swept clean and I get the truck back out. So an update will be a ways out. I will however run the truck on jackstands.

BlackGMC

iTrader: (31)

I just order my driveshaft for driveshaft specialist in san antonio. They said a 4in AL 0.120 wall would be good for me. He said a 3.5 or 4.0 steel shaft would not be good at my length. Roughly 59inches joint to joint. He said the steel shaft would reach critical speed way to soon.

SCOTT_A

iTrader: (1)

What is up with all the Cat Avatars? You all have cats smoking butts, playing golf, holding shotguns, doing their taxes.... Whats the story?

BlackGMC

iTrader: (31)

I have 4 cats. So i al that crazy cat guy. I only had one prior to getting married. My wife had 2 when we met and talked me into buying a friend for my cat. Then we got married so 2+2=4.

SCOTT_A

iTrader: (1)

Update:
I ended up raising the transmission tail shaft one inch. Then remeasured my angles (2 degrees at the tail shaft) and made an appropriate shim for the rear axle to make the angles on each end, equal and opposite. My u-joint working angles are 2 degrees.
I recently got the truck out for the spring time and took it for a ride. The truck is now super smooth! Its so nice to have some closure with this issue. So to sum up what I learned:
1: U-joint operating angle at the tail shaft and pinion yoke must be equal and opposite AND be less than 3 degrees. See post 52 for explanation.
2: A driveshaft shop should recommend your driveshaft diameter and material per your requested length, power and rear end gears. They will make you aware of potential critical speed problems.

These are not the only 2 rules of driveshafts... but these were my problems.

Scott

SILVERZZ28

iTrader: (3)

Scott. So you have the trans up 2 degrees from the driveshaft and the reared down 2 degrees from the driveshaft?

SCOTT_A

iTrader: (1)

No, The rear end is up 2 degrees and the trans is down 2 degrees. As illustrated in post 52, except instead of 6 degrees, they are both 2 degrees.
Hope that helps

chris99gmc

I didn't read your last couple posts in the thread, but if you're going to have a new DS built, I'd replace the slip yoke as well. Chances are it's worn out, even with a new bushing in the tail shaft, the slight amount of tolerance will be enough to wear out u joints prematurely and also create driveline vibrations... And Use American made solid Spicer u-Joints only.