Making a drive shaft

[northplainsdrifter]

After my axle swap in my CJ, the rear drive shaft is just too short and wore out. I bought a Cherokee front drive shaft as a donor since it has the same size tube and U-joints as mine. My plans are to cut one yoke off my old one and the CV end off the donor and make one that is the right length. I thought about using the whole Cherokee one with the CV but it's too late as I already set my axle up for the 2 U-joint ends. So my question is how do I measure for the correct length? The new drive shaft has 3" of slip joint, do I center that and go off that for my length? Or would I end up bottoming out or over extending? I'm not sure which way the axle will move the most, in or out (up or down). I've searched and searched and can't seem to find a good answer.

[tufcj]

It's all a matter of the geometry of your specific vehicle. Droop and stretch of the suspension, arch of the springs, and drive shaft angle all play a part. The axle moves away from the t-case as the springs compress, so max length at droop is the important measurement.

When I did my front shackle reversal, 3" of travel wasn't enough. If I made it short enough that it wouldn't bottom on compression, it would pop apart on droop. If I made it long enough to stay together, it would bottom, it actually knocked my t-case loose. I ended up having a long slip shaft made with about 8" of travel, overkill, but it is nice.

Best bet is to hang the rear wheels in the air for max droop and measure (flat of yoke to flat of yoke) to be sure the shaft will be long enough on full extension. Then take a measurement at static (sitting still, fully loaded). Position the slip so you have enough to fully extend, and hope there's enough compression left that it doesn't bottom.

The CV end would go toward the t-case, and require a new yoke. Laying things in the V of a big piece of angle iron helps keep things straight, but what are you going to do about balance? 1 ounce of weight off at 3000 RPM is like swinging a 5 LB off center weight.

I don't like homespun drive shafts as I've seen them fail and rip thru floors and actually flip a car over when one broke near the front and dug into the ground and "pogo'd" it over. One place where I think paying a pro is money well spent.

Bob
tufcj

[northplainsdrifter]

I do feel confident in my buliding abilities to make a safe driveshaft. The Jeep see's very little pavement so I will address the balancing at a later time if need be. You gave me some good info so I will do a little more measuring tonight. I'm pretty sure I have enough slip in the drive shaft, it just wasn't long enough before. Thanks, Bob!

[northplainsdrifter]

After researching a little more, I found a good article, http://www.4x4wire.com/jeep/tech/shaft/diydriveshaft/ I measured at full droop and sitting level. I came up with about 1 3/4" difference in lentgh. I decided to center the slip yoke and make my cut. The new drive shaft was a bit thicker so I ground the yoke to fit. I''m picking up a dial indicator tonight and will mock everything up tonight. I don't feel like I have a safety issue as the yoke slides into the new tube like from the factory and is welded together.

Here's my progress so far.





I'll take a few more pics when I true it up.

[northplainsdrifter]

I finished up the drive shaft yesterday and I'm really happy with it so far. The best I could get it trued up was about 10 thousandths out of round. I tried for a long time to settle with that. I'm assuming it is the irregularities in the tube. I ran it around town and down the freeway with no vibrations other than the tires! All in all I'm happy with it.