So I just replaced my stock rear driveshaft with a Tom Woods and now I have a vibration when accelerating between 20 and 30 mph. Its not a really bad vibration but it's bad enough I know its there. Any ideas on whats going on and how to fix it? Everything has been double checked and is tight.

 

Do you have any adjustable control arms? If your caster is out it will cause some vibrations. Only adjustable control arms (Preferably the uppers) can correct this.

 

Your pinion angle needs to be exactly in line with your driveshaft. No angle should be there.

 

Agree, what's your pinion angle? Do you have arms, upper, lowers, both ?

 

Alright thanks. Im pretty sure thats what the problem is.

 

This is what I was shooting at.... Caster isn't really an issue for a rear axle, but adjustable control arms are the fix for both... (I need to finish me first hit of caffeine )

 

Did you adjust your rear pinion angle? The pinion needs to be in line with the driveshaft as said above and then dropped 2 degrees. This is so when there is tension n the driveline it will be in line. Hope these 2 pics help.

 

Combining a CV (or dual Cardan CV approximation in this case) with a U-joint (single Cardan) on each end of the same driveshaft forces you to have zero angle at the pinion to avoid vibration. In theory, you don't want zero angle on a U-joint because the needle bearings will just rock back and forth in the same spot causing premature failure. In practice it is near impossible to get the angle to exactly zero so you end up with vibration.

The ideal arrangement is to have CV joints on both ends of the shaft (like stock) or have single Cardan U-joint on both ends and the transmission flange and pinion flange be parallel. This way the vibration induced by each U-joint cancels the other. I've seen this arrangement on trucks all the time but never on Jeeps.

If Jeep ever enters the 21st century with a fully independent suspension and protected drive train all these problems will go away.

Driveline 101

Driveline Basics with Steve Johnson - ORN

 

Not sure if I follow your reply. Can't tell if you are questioning or agreeing with what was written. What are you questioning? What I suggested and the other posters suggested is what your listed documentation supports. If you question the 2 degree drop in the pinion that was told to me by the techs at several aftermarket suspension companies and installers because when you get on the throttle the pinion does lift slightly, do to slack in control arm bushings and such so by dropping it slightly it comes as close as possible to a zero angle when under load. As for your concern of premature wear, all the needle bearings do in a u-joint is rock back and forth and so long as they are greased it shouldn't cause any premature wear. The greater the angle of operation generally the quicker the wear. For example, for anyone who has aftermarket driveshafts front and rear, you are more likely to see the u-joint at the axle end of the front driveshaft wear out quicker than the rear, because of the steeper operating angle. That causes a greater rocking back and forth of needle bearings under more pressure than a joint at near zero angle.

While your suggestion of a protected driveline and independent would eliminate the angle issues may be correct, I don't think that many here would back you on that as an independent front end or four wheel independent suspension may increase a jeeps ability to go offroad at high speed it wouldn't help it for rock crawling or overland driving as most here do.

 

I was not questioning anything. Simply supplying additional information for integration into the broader knowledge base.

I'm not saying a 2 degree down angle won't help reduce U-joint wear, it probably will. The down-angle setting is based on the twist caused by torque as you said but it is much more pronounced on leaf spring suspensions (more common on pickup trucks) than with control arms. Besides over-torqueing and extreme angles, U-joint brinelling can also be caused by insufficient angle. (less than 1 degree)

As far as fully independent suspension, that has been the military standard for jeeps (not Jeeps) since 1959.

M151 (MUTT) was produced by Ford, Kaiser and AM General from 1959 to 1982 and saw military service at least until 1999 has fully independent suspension and unibody:
http://www.youtube.com/watch?v=giqGHuWv-eM And was replaced by the AM General HMMWV which also has full independent suspension:
http://www.youtube.com/watch?v=_S7zhTkVepw