Visitor:
Have you noticed a vibration or rumbling noise when you are driving down the highway? If you have a lot of miles on your truck, have modified the suspension or drive train in any way, you may be experiencing driveline vibration.
From what I've researched, ideally you want the two ends of a double-ujoint drive shaft within a few degrees of each other for maximum u-joint life and minimum vibration. This is actually the operating angle (under load) and not the angle of the drive shaft to the u-joints themselves (that has its own limit).
Since the rear pinion moves up under acceleration (unless you have anti-wrap control on the axle) ideally you set up the static pinion angle to be 1-2 degrees below the transfer case output flange angle. This way, as the pinion twists up, it comes into a good alignment with the transfer case.
In my case, I stuck on a 1.5" longer shackle and from some simple trig, came up with needing about a 3 degree shim to compensate for the extra tilt of the shackle. I never measured the angles at the time. Later, I did measure and found even with 3 degrees, I was still 1 degree above the transfer case and I needed to add another 2-3 degrees to get me passed zero and into the desired range. So I have to conclude that originally my pinion angle was off with the stock shackle as well. Driving experience also confirmed this, I had drive line vibration under load (pinion tips up), but it would go away under coasting conditions (pinion tips down).
So my point is to measure what you have now and see if its OK and how much will it change with a longer shackle.
I am planning to go with a CV-style rear drive shaft in which case you do want to tip the pinion up to point directly at the transfer case, so there a longer shackle works for you.
The universal (or u-) joint is considered to be one of the oldest of all flexible couplings. It is commonly known for its use on automobiles and trucks. A universal joint in its simplest form consists of two shaft yokes at right angles to each other and a four point cross which connects the yokes. The cross rides inside the bearing cap assemblies, which are pressed into the yoke eyes. One of the problems inherent in the design of a u-joint is that the angular velocities of the components vary over a single rotation.
The universal joint was actually invented around 300 B.C. by the ancient Greeks. It was later re-invented in the 16th century by the Italian physicist Geronimo Cardano who used it as a mounting gimble for holding a ship's compass horizontal in rough seas. Finally, it was re-re-invented in the 17th century by the English mathematician Robert Hooke who used it in its common form for transmitting torque. If that name sounds familiar, that is because he also is famous for Hooke's Law, which states that the stress in a spring is proportional to the strain (i.e. the spring rate). Where would 4-wheeling be without springs and u-joints? And I guess Hooke fooled around with a microscope looking at plant cells or something. So for some reason, the term "Cardan Joint" has stuck and is used interchangeably for a universal joint.
CV (or Constant Velocity) joints are a class of joint which are designed to eliminate the variation in angular velocity that plagues u-joints, thus they are given the name Constant Velocity. The simplest CV joint is simply two u-joints connected end to end, usually the center section is called an H-yoke because of its shape. In this manner, the anglular velocity variations of one joint are cancelled by the joint on the other end. Since there are two joints, the operating angle capacity of the double cardan joint is twice that of a single cardan joint.
More complicated CV joints utilize
Single Cardan is a term for a driveshaft with one universal joint at each end of the assembly. So actually there are two single cardan joints in a single cardan drive shaft.
Double Cardan is a term used when describing a one piece driveshaft with three (or more) universal joints. What a double cardan will do, is split a universal joint operating angle into two separate angles that are exactly one half of the original angle.
This is the angle formed between the two yokes connected by a cross and bearings. It may be a simple or compound angle, depending on the geometry of the driveshaft. While u-joints can operate at fairly high angles (usually up to 30°), the speed at which the shaft moves provides a practical limit to the angle as follows:
| SHAFT RPM | OPERATING ANGLE |
| 5000 | 3.25° |
| 4500 | 3.67° |
| 4000 | 4.25° |
| 3500 | 5.00° |
| 3000 | 5.83° |
| 2500 | 7.00° |
| 2000 | 8.67° |
| 1500 | 11.5° |
This table is based upon the joint at rated load and life. Going above the rated load or angle
In the typical off-road vehicle, a suspension lift is done to increase clearance and allow larger tires to be installed. To compensate for the larger diameter, lower gears are installed in the axles. Lets see what this does for the drive shaft, the lift increases the angle of the shaft and the lower gears means the shaft has to spin faster for a given axle speed, both things are working in the wrong direction on this chart. No wonder, driveshaft problems are common in vehicles modified for off-road use.
So, how do you go about measuring drivelines and angles, etc.? At first glance it seems kind of difficult, but I have some easy techniques that make the job very easy.
For setting the drive shaft length, measure it from flange to flange at rest. You should allow at least 1.25" of compression on the rear shaft and maybe a bit more in front (assuming spring shackles in back) to allow for the suspension compression. Then, be sure you have enough spline travel at full droop. If the existing spline length is not long enough (sometime a problem in the front drive shaft) a long travel spline shaft may be needed.
