Racing Driveshafts - Driveshaft 102

After reading Driveshaft 101, you should be able to see the potential for problems in this photo. Unfortunately, this driveshaft will probably have a short life. Final assembly is a bad time to find out that your chassis builder and your mock-up process didn't allow enough operating clearance for your driveshaft.

Everyone who successfully completed Driveshaft 101 last semester, er, in last month's issue, now has a better appreciation of the challenging environment that the racing driveshaft must survive in and the many decisions that need to be made when specifying the best component choices for your application.

In Driveshaft 102, we will go into the shop and discuss many important concepts of working with the racing driveshaft.

In the top NASCAR teams, dedicated drivetrain specialists have learned many of these concepts the hard way through years of expensive trial and error. Here, for the first time, we will reveal many of the tips and techniques that they have developed to first specify the correct shaft and then ensure its reliable operation to meet its difficult challenge.

1. Driveshaft Measuring The length of a racing driveshaft is typically specified from the center of the front U-joint to the center of the rear U-joint in 1/4-inch increments. If you are replacing an existing shaft that is working well, the length to order is obvious. But for a new car or a changed driveline configuration, it is critical that you measure carefully. Guessing is not an option when ordering an expensive custom driveshaft.

Once you have narrowed down all the component choices and verified the actual parts that you will be using, it is time to measure for your new shaft. You want to duplicate the exact configuration the driveshaft will end up in to determine the correct length to order.

It is also important to measure with the same brand of slip-yoke that you will actually be using. Notice that the cast yoke on the left is longer than the billet yoke on the right. The cast yoke will bottom out externally on the rear seal, while the billet yoke will often bottom out internally on the tailshaft before it reaches the seal.

It is important to mock-up the actual environment in which the shaft will run. The mechanic must check that the wheelbase measurement is set, the pinion angle is set, and the measurement is taken at true ride height. With the car on high stands and the rear springs removed, raise the rear axle with your floor jack until it simulates ride height.

If available, use a shorter shaft with the same slip-yoke and U-joint configuration to determine the correct length. Push the slip-yoke all the way into the transmission and mark the barrel at the rear seal where it stops. Then, set the rear U-joint snugly into its saddle and mark the slip-yoke barrel again at the seal to show the free-play.

The correct length driveshaft should have about 1.25-1.40 inches of possible forward movement before the slip-yoke contacts the tailshaft internally or its housing seal externally. Add or subtract from the known center-to-center length of the measuring shaft to determine the measurement that will best achieve this desired clearance. A typical stock car trailing arm suspension will cause the shaft to move forward 1/4-1/2 inch in normal use, yielding an actual running clearance of about one inch.

2. Length Determination by Measurement If you don't have a similar shaft to measure with, use a long ruler to take the measurement from the end of your tailshaft housing to the centerline of the U-joint on the rear pinion yoke. Your driveshaft supplier can use that measurement to determine the correct length using the components you choose.

This dial indicator setup is being used to check the actual runout on an assembled rear gear yoke. The goal is to have the yoke run absolutely true. NASCAR specialists will spend hours trying different combinations of parts and positioning to achieve smooth operation.

3. Driveshaft Inspection Once you have your new driveshaft in hand, examine it closely to make sure each U-joint clip is evenly and smoothly engaged in its circular grooves and there are no sharp edges that can catch debris. Verify the center-to-center length to be as ordered with a straight-edge ruler, not a floppy tape measure. Inspect the spline of the slip-yoke inside the barrel for packing debris and lubricate the barrel of the slip-yoke with a light wipe of a good-quality, high-temperature grease, such as your chosen wheel bearing product.

4. Balance Weights Take a marker and outline the balance weights on the shaft. Then, if a weight is lost during use, it will be visually apparent that a weight is missing. If the driver mentions a sudden vibration, this may be the cause.

5. Driveshaft Installation Place a shop towel over the driveshaft loop to protect the paint and carefully guide the shaft into the tunnel while aligning the slip-yoke with the output shaft. Avoid any contact with the chassis or loop by the shaft to prevent a dent.

The slip-yoke should slide smoothly onto the transmission output shaft with a slight wiggle for alignment of the splines. If there is any binding, do not force the engagement.

It may take several tries to find a smooth entry angle or sequence of movement to insert the shaft easily while you are lying under the car. Once you figure it out, write it down or practice it with your crew members so you can do it quickly at the track when it counts. Damaging your nice new driveshaft at the track in the process of a frantic gear change will ruin your day.