JonFund, Inc. :: Education, exploration and recreation for the 4 wheel drive and off road community

This is easy to detect and inexpensive to fix. Get under your truck, grab the driveshaft right close to the u-joint and shake the bejeezus out of it. There should be no relative movement between the two parts on opposite sides of the joint. Shake it side to side, up and down, and "rotationally" as well. If a bearing has gone bad, $5 - $15 for a new joint and simple hand tools are all it takes to press the old joint out and a new joint in. A large C-clamp and a couple of carefully chosen sockets can perform as a press, even in the backcountry.

B) Worn or damaged splined slip-joint

The diagnostic procedure in this case is the same as in case "A", above -- except grab your driveshaft right at the slip-joint where the shaft gets longer and shorter with suspension movement. No lateral or radial movement should be detectable. If this joint is worn or damaged, however, it's a trip to the driveshaft shop to get it fixed. The worn stub must be cut off, a new one welded on, the shaft re-balanced, and a new yoke to match must be purchased. Lotsa bucks, and good motivation for greasing this joint on a regular basis.

C) Incorrect U-joint alignment

If you've performed any lifting on your truck's suspension, there's a good possibility that the operating angles of the two joints on opposite ends of each driveshaft are no longer "equal and opposite". With the exception of driveshafts equipped with CV joints, this is the cardinal rule of driveline alignment.

This is hard to imagine, and even harder to explain, but here goes: when a common cross-cardan type of U-joint (a "Hooke's Joint") is operated at anything other than a perfectly straight alignment, the motion of the two halves is NOT identical.

Suppose, for example, that the "driving" shaft (from the transfer case) is rotating at a uniform 2,000 RPMs. The "driven" shaft (driveshaft) on the opposite side of a bent u-joint experiences a motion which AVERAGES 2,000 RPMs, but which ACTUALLY is speeding up and slowing down slightly -- twice per revolution. It's a NON-UNIFORM motion. (The math for this is available on request.)

This non-uniformity or variation in the velocity of the driven shaft increases with the severity of the operating angle. At 25-degrees, the variation in rotational velocity of the driven shaft is nearly ten percent. That is, for two brief moments during every revolution, the driven shaft is rotating at nearly 2200 RPM; and for two brief moments it is turning at only 1800 RPM. The driven shaft still AVERAGES the same 2000 RPMs of the driving shaft, however.

Ideal alignment is to reverse this process at the bottom of the driveshaft -- to input a non-uniform motion, and get a resulting uniform motion of the axle pinion. If the operating angle of the second joint is not equal and opposite to that of the first, however, a non-uniform motion of the axle pinion will be the result. Hence, driveline vibration.

If you're rolling down the road at a constant speed, and the engine/transmission is naturally trying to maintain a uniform motion, the two ends of your driveshaft are each trying to accomodate a DIFFERENT, non-uniform motion.

Corrective measures include tipping the engine/transmission or tipping the axle with tapered shims between the spring and axle -- with equal u-joint angles being the only cure short of a CV joint.

One final aspect of U-joint alignment that deserves atention is "u-joint phasing". Not only must the U-joint angles be equal and opposite (so that the speed variations are equal in amount), but also, the joints on both ends of the driveshaft must be oriented so that these speed variations occur at the proper time. Both yokes must be in the same plane. That is, if an imaginary dowel were inserted in place of the U-joint cardan on each end, the dowels should be parallel.

Special Case for CV Joints

A "constant velocity" joint is precisely what its name implies -- one that is NOT subject to the variations in rotational velocity described above. Both sides of a CV joint -- the "driving" side and the "driven" side -- turn with a uniform rotational velocity at all times.

If the upper end of your driveshaft has such a joint, then the lower end needs to operate with a uniform motion as well. And the only way for a STANDARD double-cardan joint to function without variations in its angular velocity is at a ZERO DEGREE operating angle. For this reason, the axle pinion should be pointed directly at the transfer case such that the non-CV joint is as close to ZERO degrees as possible.