Understanding Driveline Alignment and Eliminating Driveshaft Vibration and Failures

When putting your car together after an update, or when building up a car from the frame off, you need to consider the drive line alignment when mounting the engine and rear end. Because this is one of the basic design features of the car, it can and must be done initially to save a lot of work later on. The quality of the final product depends on this.

This topic was discussed as far back as the mid ‘60s in car magazine as one of the most important considerations for hot rodders in general. It was firmly impressed upon the readers that if you didn’t get this right, there could be plenty of problems down the road. That fact has not changed over the years.

When working with high horsepower applications and when making modifications, we need to consider that our driveline angles may be changing in the process, and that each part will be taking much more force and must be upgraded if we don’t want to run into serious problems with broken driveshafts, yokes and bearings.

We now have available a good amount of information about drive shaft technology and geometry. This technology had initially been developed for production cars, but high horsepower applications require a closer look and slightly different approach to this information. The advantages of this knowledge are in the area of reduced power loss and increased component life. To begin with, we need to know what to call the various components related to our driveshaft.

Drive Line Terminology – The component names for the driveline parts are as follows:

Slip Yoke – this is the yoke at the front of the driveshaft that goes into the transmission and can slip to take up the slack of chassis movement.

Weld Yoke – is the yoke at the each end of the drive shaft that attaches to the pinion yoke at the rear end and the Slip yoke at the front. These are welded to the shaft tubing.

Universal Joint Kit or U-Joint (UJ) – is the actual part that forms the rotational connection between the driveshaft and the transmission and rear end.

Tubing – is the metal tubing between the weld yokes.

Pinion Yoke – is the yoke that is attached to the pinion shaft at the rear end.

Controlling Driveline Vibration – The sources of vibration in our high performance drivelines, in order of importance with the most critical at the top, are:

1) Run-out

2) Improper Drive line Angles

3) Looseness in the fit of any of the parts

4) Unbalanced parts

5) Component Deflection

6) Reaching Critical Speed

We will examine some of these causes and see how we can cure many of them just by using better parts that are designed for the high horsepower environment.

Drive Line Angles – Basically the transmission output shaft and the pinion shaft need to be parallel to one another, and if they are, the angles they make with the drive shaft will be the same and opposite. And, when a U-Joint operates with any significant amount of drive line angle, it creates a problem. The bearings speed up and slow down twice per revolution of the driveshaft. This causes an oscillation in the power train. The more angle that we have, the higher the peaks of oscillation we see and therefore the greater chance of vibration.

Drive shaft angles are not only measured from a side view, but also from a top view. Some designs can have a lateral displacement of the rear of the drive shaft from the front. That creates a drive shaft angle at both the transmission and the pinion. So, we can align the drive shaft from a side view to zero angle and still have some degree of drive shaft angle present.

There is the thought that the driveshaft must have some angle to the pinion and transmission yoke so that the U-Joint bearings will rotate to help lubricate them. This is true, but we should keep the driveline angles as low as possible and most importantly keep the angles equal and opposite at each end of the drive shaft.

Changing Driveline Angles – We can change either the front (transmission output shaft angle) or the back (pinion angle) height of the driveshaft. The front may take more effort, so we usually make changes to the back, or pinion angle to match the transmission angle.

To change the driveline angles, you need to check to see if the overall driveshaft to pinion or transmission angle is excessive. In almost every case, the transmission shaft is higher than the pinion shaft. So to reduce the angle to the driveshaft, you would need to drop the transmission mount height and angle the engine/transmission down to the rear.

Over four degrees could be considered excessive for transmission/pinion to driveshaft angle. In this case, you will need to change the engine/transmission angle first. This can possibly be done by changing the height of the transmission mount. This may, or may not, be possible.

Then you can change the pinion angle by adjusting the lengths of any links in a typical four link aftermarket system. For older leaf spring systems, companies offer wedge inserts that go between the leaf spring and the spring pad on the axle tube. These are available in different degrees of angle.

If your car must have drive line angles from a design standpoint, the angle of the drive shaft to both the transmission output shaft and the pinion shaft should be equal and also opposite.

Yoke Design – The only attachment design for holding the U-joints that is suitable for high performance applications is the strap design. The U-bolt attachment that is commonly used on passenger cars and trucks is not considered acceptable for several reasons.

One reason is that it has less strength than the strap design, two it may distort the bearing caps if over-torqued, and three, it grips the cap at three points whereas the strap design grips it in four places which translates to less distortion of the caps.

U-Joint Kit Designs – There are basically two designs of U-joints available. The popular standard zerk designs are used for production vehicles and have a zerk fitting for lubrication. Inherent in this design are hollow shafts that provide the means for the grease to reach the bearings. This hollow design also makes the part weaker than if it were solid.

The other design is called the Sealed Design, or Solid U-joint and has no grease fitting and therefore a solid core. This unit has precision seals that keep the lubricant with the bearings while also sealing out dirt. Because it is solid, the sealed design is therefore much stronger.

Proper lubrication of the sealed U-joint is simple but can be overdone. We always want to coat the bearings, but not excessively. We also need to fill the trunnion cavity with grease, but not overfill it. If too much grease is applied, then when we attach the caps, the pressure from the grease trying to escape will literally blow out the seals and ruin them.

Adjusting Run-out at the Pinion Yoke – A major cause of driveline vibrations is when the pinion yoke has a measurable degree of run-out. We can test the run-out after the installation of the U-joint in the pinion yoke by using a dial indicator attached to the rear end housing.

We measure at both sides of the u-joint and then if the offset is different, we must remove the pinion yoke and rotate it on the pinion shaft to find a position that will index more correctly. This is a very important and necessary step in reducing driveline vibration.

Balancing the Drive Assembly – The only true way to balance a drive shaft is through the use of a Two Plane balancer, or one that simultaneously balances both ends and has the capability to Cross Talk between ends. This allows the equipment to derive a dynamic model of the forces of the imbalance to determine the force vectors involved and formulate a solution that takes into account both ends influence in the overall balance of the shaft.

A simple automotive drive shaft balancer is fine for grandma’s car, but for high performance and drag racing, with the very high RPM we experience, we need more precision. It would be a very good idea to send your driveshaft off to a company with a proper balancer rig if your area does not have one.

And remember that the higher RPM your driveshaft will encounter, the larger the diameter of the tubing you will need. In the above list of causes of vibration, the last one, Reaching Critical Speed means that if the tubing is too small, it will flex at high RPM and wobble. The larger the diameter of the tubing, the less chance this will occur.

Snap Ring Failure – A common failure in high performance drivelines is when the retainer snap rings come out of the yoke. There is a simple and easy way to reduce this occurrence by applying a spot of epoxy to the ring. This prevents the ring from collapsing and falling out of the ring groove. Be sure that the snap ring is seated completely inside the groove in the yoke before applying the epoxy.

Conclusion – The important things to remember for high performance applications are to use drive line parts that are made specifically for extreme use when possible. Make sure you install the components correctly to reduce drive line vibrations and parts failures.

Check the alignment of your system and correct any miss-alignment. When buying the drive shaft, make certain it is up to the task for the intended RPM and horsepower range you will be running in your type of car. Periodically check driveline parts for cracks or for signs of a bent or dented shaft.

The professional drag racing teams that run the top series always use new drive line parts for each race. We don’t necessarily need to do that at the street level, but knowing they do that says something about the importance those teams place on the driveline components. That same concern should be shared by all high performance automotive enthusiasts.