1.5" bommerang shackles affect driveline angle??

I put a 1.5" lift shackle on the rear to compensate for a heavy bumper,, my pinion was already pointed TOO high (above the driveshaft) I have 6 degrees of reverse shim to point the pinion further down,, i guess my question is is it smart to put more than 6 degrees of reverse shim or should I just get some "regular" 8" rear springs without the built in pinion angle compensation?? Im getting bad vibes by the way

I have rustys 6" rear springs, and they are made with some degrees to help with a bad pinion angle

This brings up a good point.

Do the angles at the ends of the driveline have to be equal and opposite? Or can they be equal and the same?

Lemmie clarify. If the driveline drops 10* from the tcase, can the axle yoke be pointed UP ~10* to cancel vibes? Well, I guess technically it would be pointed up 20* from parallel to the tcase output shaft. This setup should make both driveline angles equal (10*) but the same direction.

Or does the axle yoke need to be roughly parallel to the output shaft of the tcase to eliminate vibes? This setup is what everybody says you need, and makes the angles equal and opposite directions.

This is the crappiest diagram I could think of to illustrate what I am asking:

. _
... \_ Equal but opposite angles.
.
. _
... \
.... | Equal but same direction angles.

Any knowledgeable input here? Ignore all the periods in my "pic". It seems to ignore spaces and they were the only way to line up the lines. The horiz/vert lines represent output shaft and axle yoke. The diagonal represents driveline.

equal and opposite from your diagram. An imaginary line coming out of your transfercase output shaft and one coming out of your pinion should be parallel. Unless you have an SYE, then your pinion shoud point straight at your T case out put.

HTH, Dan

well the are withing a * or two but it is actually pointed slightly above the t-case output, and because I still have vibes Im guessing thats worse than being pointed below, right??
I do have a SYE

Teh pinion will turn up under power a little. One * Below will put it right in line will on the throttle. That's what you should be shooting for.

Dan

thanks

I took the shackle out,, and the vibes are nearly gone,, I think I still have a DS balance problem,, so we'll see

Read this....

http://www.4xshaft.com/driveline101.html

Answers all your questions

Vug

With an SYE, your u-joint angle on the axle end must be zero degrees, or just under zero degrees. This means that the driveshaft and pinion should be perfectly parallel.

Here is why (a synopsis of the above article). A universal joint running at zero degrees rotates in a perfect circle. As you introduce angle to the joint, the arc of travel becomes an ellipse, which means the assembly slows down and speeds up twice per revolution. In a standard driveshaft with a u-joint on both ends, the angles have to be very near equal (and opposite) so the arc of travel of both joints is equal. Otherwise the u-joints will be "out of phase" and you will get a harmonic vibration. This is a droning sound, that "cycles". An out of balance shaft will just vibrate, it won't make the cyclical droning sound.

A double cardan shaft, which we often refer to incorrectly as a CV (constant velocity) shaft, has two u-joints on the t-case end. The purpose is that these joints cancel out each other's angle. This means that the single joint on the axle end must travel in a circle, or it will be out of phase with the double cardan joint. Hence the zero degrees.

Back to the original question - a longer shacle, boomerang or not, raises the pinion by lowering the axle end spring mount (it turns the leaf mounting surface up). If removing the shacke cured the vibes, then you need to lower your pinion back to zero degrees with a shim so the u-joint angle as measured from the cap is -1 to 0 degrees in relation to the driveshaft. Rusty's leafs assume a SYE, which is why they point the pinion up. Along with a shackle, you should be close to parallel without a shim.

Hope this helps.

Nay

thanks everyone,, its as close to 0* as i can get,, thats without the shackle and WITH 6* of reverse shim,, can I use more than 6*??