need help with misinformation

[hotrod22]

I bought a (stock)front dual-cardan shaft out of a Cherokee at the junkyard for $50, had my local machine shop measure,(I drove it down there w/no rear shaft in 4wd) then lengthen/rebuild w/Spicer u-joints for $130. So, I've got $180 in a dual-cardan rear shaft that works just fine.

 

[ChiXJeff]

if i assume correctly the "double cardan" are the ends, and the slip spine is the center like the front shaft, now what good would a double cardan be without the center slip section? wouldn't this just add 2 more u-joints to break and still not allow any type of shinking and extending action?

The driveshaft you'll end up with after installing an SYE on your NP242 will have a double Cardan u-joint at the tcase end, and a single standard u-joint at the axle end. There will be a slip shaft in the middle. The driveshaft you have pictured has a couble Cardan at both ends, and does have a ship shaft in the middle (covered by the black bellows.)

Any driveshaft that has to deal with suspension cycling will have a slip joint somewhere. It may be a slip YOKE like the stock XJ rear driveshaft, it may be a slip shaft in the middle if you've got an SYE. But no matter how you look at it, it has to be able to expand/contract on demand.

If you've got a situation that requires power transmission IN A FIXED GEOMETRY, then a fixed length shaft can be used. Not on a Jeep, though.

 

[funvtec]

thanks alot everyone, you cleared alot up for me

 

[Eagle]

The advantage to a double cardan joint is that it allows a greater operating angle without the u-joints self-destructing in a very short time.

Something that has not been mentioned in this thread is that u-joints need to be "in phase." Basically, if you think of the stock rear shaft, this means that the u-joint operating angle at the transfer case wants to be as close as possible to the same angle at the pinion end. If not, there is a vibration set up and the u-joints wear out.

When you replace a drive shaft with one that has a double cardan at one end, you now have three u-joints in the shaft ... two in the double cardan unit, and one on the other end of the shaft. The two in the double cardan unit are always "in phase," or self canceling. Which means that if the third one has any angularity at all, it is always out of phase. So when installing a double cardan rear drive shaft, you should also shim the rear axle so that the pinion is parallel to the drive shaft, making the u-joint operating angle zero.

 

[funvtec]

this is a very good thread with good info thanks again, this ll makes perfect sense and will help me greatly when i go bigger