Driveline 101

 

Contents:


Introduction:

Have you noticed a vibration or rumbling noise when you are driving down the highway? If you have a lot of miles on your vehicle, have you  modified the suspension or drive train in any way, you may be experiencing driveline vibration.

Ideally you want the two ends of a double-U-Joint 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 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 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, I was still 1 above the transfer case and I needed to add another 2-3 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.


Terminology:

U-Joints:Universal Joint

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.

CV Joints:

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 angular velocity variations of one joint are canceled 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.
Single Cardan is a term for a drive shaft 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 drive shaft 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. Normally a Double-Cardan (a.k.a. Constant Velocity or CV) style drive shaft is used in applications where it is not possible or practical to properly align the ends of a drive shaft for a single-cardan setup. Examples include where the operating angle would be too great over a single cardan joint (see below) a double-cardan allows the operating angle to be split across the two halves of the joint. It is also possible to use two CV joints on a drive shaft which is commonly used where it is not possible to align either end of the drive shaft, such as when both vertical and horizontal mis-alignment occur, or when mis-matched operating angles are present, such as in front wheel drive vehicles, where both up and down motion is present from the suspension travel as well as rotation about a vertical axis due to steering action. Drawbacks of multiple CV joints are their higher cost and complexity as compared to u-joints, their extra length and weight, and their decreased maximum operating angle limitations.

U-joint Operating 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 drive shaft. 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 will decrease the u-joints life. As a general rule of thumb, for each doubling of the operating angle, RPM, or load, the lifetime of the joint is decreased by half. Rated lifetimes are on the order of 3000 hours.
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, drive shaft problems are common in vehicles modified for off-road use.

Measurements:

A frequently asked question is about drive shafts and angles and so forth, is "How much shim do I need for XXX" or is Y shim too much?". Well, there really is no general answer to these general questions, rather the right answer is what works for that particular situation. For example, assuming that the drive shaft is aligned properly in a vehicle with stock suspension, if it is lifted with a block or spring lift, then everything should still be lined up, at least with a single-cardan drive shaft. It's like a parallogram, the angles change, but the side remain parallel. So why do some lift kit makers include shims with their kits? So the correct answer for how much to shim an axle to correct the drive shaft angles depends on how far off the angle is to begin with.

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.


Phasing:

Phasing is a term that describes the alignment of the single-cardan joints on opposite ends of the drive shaft. As discussed above, a single-cardan (or u-) joint does not rotate at a constant velocity if the operating angle is non-zero. The drive shaft speeds up and slows down slightly as it rotates due to the nature of the joint. One way to reduce this is to make sure the joints at each end of the drive shaft are aligned properly. If the yokes on each end of the shaft line up with each other, as seen in the figure below:

Then the affect will be that the two joints will tend to cancel out the speed variations from each other. In most 4x4 applications, the drive shaft will have a slip yoke in the middle to allow for changes in length. If the shaft is ever taken apart, it is important to get it re-aligned properly when it is re-assembled. One way to do this is to mark both sides of the slip yoke. However, you should check that the joints really do align properly, don't assume they are. The reason for the phasing is that the speed variation of the joint is related to its operating angle and its angle of rotation. In order to get the most effective cancellation, the joint yokes *must* be aligned exactly with each other and the operating angles must be identical. Any variation in either angle will show up as un-cancelled vibration, which will get worse at higher speeds.


Fixing Driveline Problems:

Most likely, if you've read this far (or even searched for this page) you may have a problem with driveline vibration. If so, you probably want to fix it. How to fix it depends somewhat on what led to the problem in the first place.

So, assuming there is no physical damage or worn out parts, and you simply have an angularity problem, there are a number of ways to fix it. Basically, you want to correct the angles. How you do that depends on a number of factors:

  1. How the angles got off in the first place
  2. How bad the angles are, especially if the operating angle is greater than 10
  3. The type of drive shaft you currently have
  4. What kind of suspension you have
  5. How much work you want to do to correct the problem :-)

If you have a multi-link suspension, perhaps with coil springs, there are a few options. If the links are adjustable, you should be able to correct the angles with the adjustments. If no adjustments are provided, then you'll either have to get an adjustable link or relocate the suspension brackets on the axle.

If you have a leaf-spring suspension, then there are more options available. Among the options are shims, rotated spring perches, longer or shorter spring shackles, or driveline changes. Below is a table of common lifts and driveline affects:

Lift/DS Single
Cardan
Rear
Single
Cardan
Frnt/Fwd(1)
Single
Cardan
Frnt/Rev(2)
Double
Cardan
Rear
Double
Cardan
Frnt/Fwd(1)
Double
Cardan
Frnt/Rev(2)
Spring None None None Tilt UP Tilt UP Tilt UP
Block None None None Tilt UP Tilt UP Tilt UP
Shackle Tilt DOWN Tilt DOWN Tilt UP None(3) None(3) Tilt UP
Lift Affects on Driveline; Direction to tilt the pinion to correct angles

Notes:

  1. Front axle with shackles forward
  2. Front axle with shackles reversed
  3. Affect varies with length of spring, shackle and drive shaft length