March 1998 Nuts & Bolts

Flippin' It
Instead of using blocks to raise the rear of my K20 Chevy pickup, I've thought about reversing the spring shackles to gain height. However, it appears that the pinion angle will be rotated upward a great deal and might cause a problem. Also, will this shackle flip affect wheelhop or axle wrap?
James Bauer
Juneau, AK

Flipping your shackles from tension style to compression type will add about 6 inches of lift. Since the rear of the springs will be lower in relation to the front pivot end, the springs will sit at an angle that will indeed point the pinion upward. But since the angle of the U-joint at the transfer case will increase, the extra tilt may help reduce driveline vibration. Even so, the axlehousing can be rotated back to its original position by using degree shims under the spring pads, although cutting the pads off and repositioning them correctly is a better alternative. Theoretically, the axle or spring wrap problem would be the same, since the stock springs would be retained. We haven't heard of anyone experiencing worse wrap due to this conversion. If you experience excessive wrap, using stiffer springs, add-a-leaves, or any of the numerous aftermarket traction devices available may alleviate the problem.

W1 = Path of first u-joint. W2 = Path of second u-joint.

Driveline Vibration
I recently purchased a four-door '95 Jeep Cherokee Sport. So far I've lifted the Jeep with a ProComp 3-inch front suspension kit and Rancho rear leaves with 1-inch blocks. Since then I have experienced a lot of rear driveline vibration. To fix it, I tried shimming the rearend, lengthening the driveshaft, and dropping the transmission 1 inch. This helped a lot but didn't completely solve the problem. The driveline angles have been checked and are within three quarters of a degree of each other at the U-joints.

The vibrations occur between 20 and 25 mph and between 35 and 45 mph under load. I have heard that a CV-style driveshaft might solve this problem. Will it?
Jeff Malfatti
San Ramon, CA

Fig. 3 Proper Geometry for CV Joint Driveline

Any time you troubleshoot something on your rig, first try each solution individually. For instance, you say you shimmed the rearend, lengthened the driveshaft, and dropped the tranny. Did you do this all at the same time, or did you add the shims and try it, yank the shims and try the new driveshaft, and then yank it and lower the tranny? Did you try these fixes in different combinations (tranny and shims, tranny and 'shaft, and so on)? By trying all the remedies at once, you may have "overfixed" your problem.

Fig. 4 Proper Geometry for a Conventional Two-Joint Driveline (Centerline of differential pinion and transmission are parallel)

Fig. 4 Proper Geometry for a Conventional Two-Joint Driveline (Centerline of differential

We didn't have a vibration problem on our lifted Cherokee, and the driveshafts were totally stock. We have heard that lifted Cherokees with stock driveshafts can develop some wobble in the rear slip yoke after some miles are put on it, but we suspect that wouldn't cause vibration under load as you described, and it shouldn't be a problem if you lengthened the driveshaft.

Furthermore, we certainly wouldn't lower the transfer case at 3 inches of lift. All the off-the-shelf T-case lowering kits we've seen drop the 'case by angling it down at the back. That alone can cause vibration, since it loads the motor mounts differently. In extreme cases, it may even upset the angle of the front driveshaft. If your Cherokee has an NV231 (NP231) 'case instead of the NV242, then you can use one of the several aftermarket short-extension housing kits that allow you to install a longer rear driveshaft. This shouldn't be needed on your Cherokee, but see "Instant Superhero," Dec. '96, for more info.

For more ideas, we called Tom Wood at Six States Distributors (Dept. 4WOR, 1112 W. 3300 S., Ogden, UT 84401, 800/453-2022). He felt that your problem could be torsional vibration. As a U-joint rotates, it moves through an elliptical path, which causes the surface speed of the driveline to increase or decrease twice per revolution. If the second U-joint angle is equal to the first, it will speed up as the first slows, allowing smooth operation of the driveshaft. If the angles aren't equal, or if the U-joints are out of phase, vibration can occur (see fig. 1 and 2). Wood says a CV-style driveshaft will remedy your problem if our other suggestions don't. If you go with a CV, we'd put the T-case back to its stock position before you begin.

The proper setup for a CV-style driveshaft is such that a straight line can be drawn from the pinion centerline up through the centerline of the driveshaft (see fig. 3) while the vehicle is moving. Remember that the pinion climbs upward under power, so we usually set the pinion a degree or two down from perfectly inline with the 'shaft while the 4x4 is at rest. Still, the pinion angle may need to be angled upward from stock, and if so, Six States recommends adding enough extra gear lube to fill up the rearend to the center of the pinion bearing.

If you're going to stick with a regular U-jointed driveshaft, Figure 4 will serve as a reminder that the T-case and pinion must be parallel to each other (or have equal and opposite angles that cancel each other out) to eliminate vibration. Any custom driveshafts you need can be obtained via mail-order from Six States.

Fig. 2 Cardan or Universal Joint at 30-degree Driveshaft Angle (One revolution)