I have just purchased a 1979 F250 SuperCab 4X4 with a 6" Skyjacker suspension lift and 37's. It rides remakably well, the only problem is a vibration which feels like its coming from the rear. It only comes in between 55 and 60 then goes away. Would the pinion angle cause this or would a pinion angle vibration be present all of the time? The previous owner passed on a set of axle shims to me, he said the vibration never bothered him so he didnt bother with putting them on. It seems to me if it was pinion angle I would feel the vibration all the time not just at 55-60....but I am deinately no expert.....any thoughts??

 

Often times drivetrain vibration issues can be speed sensitive e.g. an out of balance tire my only be noticable between 35-40mph etc. etc. so i wouldn't necessarily rule out the pinion angle being the cause of your vibration problem.

 

Driveline vibration well come and go and certain speeds. Unfortunately, you can't fix it by simply shimming the pinion. With a single cardan driveshaft, both ends must be at the same angle. If you shim the pinion, you will decrease the angle at the pinion joint. However, the other end will still have a pretty good angle to it. This could actually make the vibration WORSE. You could try playing with the carrier bearing.......but the correct fix would be to replace the rear driveshaft with a double-cardan style and THEN shim the pinion.

 

After looking at it closer the pinion angle doesnt look that bad...I will post a picture soon. I think I should have the wheels checked for balance first as that may be the case since I didnt have them installed I dont know for sure.

 

do you feel the vibration in the seat or on the truck....this could be a driveshaft or transfercase problem or differential

can you see the bed or a side of the fender bounce during these speeds.. then its a rotating mass like the wheels or tires

Vibrations are some of the hardest things to eliminate especially with a lift and larger tires.....ask me how I know???

 

This is why my original thought was pinion angle, I can feel it in the seat like it starts at the transfer case and goes all the way to the rear. After driving it this morning and looking at it again (the 1000th time) the pinion angle doesnt look that bad, but the angle in the two piece shaft after the center support is worse.

 

Is the driveshaft in phase? that is are the yokes parallel to one another?

 

Yeah....that's what I was afraid of. If everything else is as it should be (shafts in phase, no out of balance tires) ........you could TRY and lower the support for the two piece shaft. If that does not help, then you'll need to to add a double cardan (aka CV joint) between the first and second driveshaft.

 

Hillbilly....I think you are spot on....but the center support has been shimmed 1" already. I believe I am more than likely going to have to change out the rear shaft to the cv style, any idea how much that will set me back?

 

Well, I can't say for sure.....because I don't know the exact driveshaft setup. Maybe you can post some pics??

 

Once on a while the center support requires more shim, this is a coin toss really. What works for one, does not always work for others.

Be very sure that the driveline is straight and that there is not a compound bend.
This will cause the exact symptoms as described because the u-joint speeds are always different from joint to joint, and a vibration will occur.
Joints can work at an angle as long as the angles match. Throw one off, and the shaft is out of control. The rotating joints start to move on a separate plane, and the joint starts to wobble, as it tries to rotate on an eccentric pattern.
The same is true when a shaft is out of phase.
Compound bends are not easy to detect, and often can lead to some real troubles.
Take a little time to tune the angles, and you can work out the problem.
This might be a day project, and could take a few cold adult beverages but it can probably be solved.

Another idea is that the slip joint is dry or worn. While this is possible, it may not be probable, as this could cause some other issues, but as long as you are under the truck, it is worth a look.


Here is an example of how speed can vary:

 

How much can the center support be shimmed?

 

I don't know if this is any help, but my 73 F100 2wd truck did the same thing at the same speed. Maybe not a 4wd issue? This was my high school truck and was over 20 years ago. Seemed like it was the front end, but I never did try to fix the issue.

 

Until the point where the vibration becomes worse. Ive had some pretty wild space between the carrier and frame to remedy the problem.