How much rear lift before worrying about driveshaft length?

[AMCJeepMJ]

I've searched for this but haven't found a solid answer (maybe just missed it, for that matter), but as the subject line of the thread asks:

 

How much lift can I do to the rear before having to worry about driveshaft length?

 

The MJ in question is my DD 87 2wd with D35 and 4 speed that currently has 3.25" of rear lift due to heavy springs to carry increased loads [sod, wood, etc.].

 

The reason I ask is since I have put the springs on this weekend, I have noticed a vibration that starts at about 55 mph and increases with intensity as speed increases [has recently balanced tires that were smooth as glass prior to spring switch] There is a humming sound being transmitted through the body of the truck that also increases with intensity as speed increases above where it is noticible at 55. The vibration is enough to blur the rear view mirror. All bolts were torqued down to spec during the leaf pack switch. As new poly bushings were installed during the leaf pack switch, a lot more noise is coming from the rear of the vehicle [or perhaps new bushings makes them noticible instead of the sound being absorbed in old squishy bushings]. Steering and vehicle control is not affected by the vibration... its like sitting in a Sharper Image vibrating chair to give you an idea of what the vibration is similar to.

 

Thoughts or comments?

[tbrand]

Did you triple-check your U-joints? I had one go bad, replaced it, and then two months later the other went bad. Shook the whole truck at certain speeds, but the wheel was steady, so I figured it wasn't a suspension issue.

[Pete M]

ujoints are notorious for not liking changes in their operating angles. assuming there isn't any obvious looseness in the driveshaft, I would get new ones. :thumbsup:

[AMCJeepMJ]

Is one u-joint more susceptable to failure after a suspension geometry change than the other?

 

I visually inspected the rear u-joint as I was installing the springs but did not notice anything unusual about it.

[tbrand]

A visual may not do it... I gave my front u-joint a pretty good shake and visual once-over when I had the shaft out the first time, and it looked fine. Just my experience.

[87Warrior]

Did your rear springs alter the pinion angle? The transfer case output angle and pinion angle should be about the same or a couple degrees lower on the pinion to account for torque twist.

[ParadiseMJ]

One of the diffeerences with XJ's and MJ's (even short beds) is that 3" on an XJ MIGHT require an SYE, especially on newer(post 97) models. On an MJ 4-5" OR SOA might call for an SYE.

 

Also, I don't know squat about 2WD rigs but the rear should be the same. Swapping trannys and axles would also be a factor. An inch can make a big difference (according to my wife!!)

 

I have a pinion angle chart that I'll look for and post if I can.

[HOrnbrod]

Did you torque everything down while the rear end was lifted? the proper procedure when changing rear leaf packs, or anything else in the suspension, is to get the bolts just snug, lower it to the floor, jump up and down on the bumper or even drive it around the block, then torque to specs whatever you loosened.

[AMCJeepMJ]

Did you torque everything down while the rear end was lifted? the proper procedure when changing rear leaf packs, or anything else in the suspension, is to get the bolts just snug, lower it to the floor, jump up and down on the bumper or even drive it around the block, then torque to specs whatever you loosened.

I torqued the u-bolts down to 80# and the spring/shackle bolts to 100#, put about 40 easy [careful] miles on it and retorqued to 83# and 105# the next day. I did try jumping on the bumper but the back hardly moves [really tight springs, and I'm over 230]. An inch and a half of the lift is from Motion Offroad lift shackles which should provide [at least in my mind] a degree or two of 'up' angle for where the driveshaft meets the axle, so the u-joints don't have to bend so tightly.

 

I didn't have the opportunity to crawl under the truck yesterday to check the u-joints; schedule was too full.