Drivetrain Classroom - Driveshaft 102

6. Rear Gear Runout
Often overlooked in the driveshaft installation process is the necessity that the rearend yoke rotate smoothly with minimal runout. The top NASCAR teams and gear shops use a sophisticated checking ball to determine true yoke runout with a dial indicator. They then correct it by clocking the end yoke to the pinion gear or selecting different yokes until the runout has been reduced to a minimum. It is quite common for pinion gearshafts to be bent after final heat-treating, and this procedure is needed to eliminate any built-in orbiting.

7. Driveshaft Verification
After installing your new shaft, take the time to cycle the rear axle assembly through its entire range of motion, just as you would when checking bumpsteer in the front, and monitor the position of the shaft for clearances and obstructions. Remember that you will also be jacking the car from side to side in pit stops and sometimes running with a flat tire or without a wheel on one corner.

Verify that the shaft moves freely through all of these motions and never contacts any solid object. Set your axle travel limiting chains with this in mind so that the shaft can't contact the driveshaft loop at full droop.

8. Driveshaft Inspection
After installing a new driveshaft, it is important to inspect it after its first on-track sessions to ensure that it is in a safe operating range and has not come in contact with the driveshaft loop or other obstructions. Most teams carry only one spare driveshaft with the goal of never needing it. Finding another driveshaft at the racetrack that will fit your car correctly is pretty unlikely, so a little caution goes a long way.

9. Slip-Yoke Inspection
After running several on-track sessions, the next opportunity you have to remove the driveshaft is a good time to inspect the slip-yoke and determine its actual clearance. Noticeable "witness marks" will be evident on the barrel of the slip-yoke, indicating the actual typical running position that the yoke has settled into on the tailshaft housing's bearing surface.

To validate your length, these marks should be in the "happy zone," with no risk of bottoming out and a minimum distance of twice the output shaft diameter engaged in the splines at all times. A shaft that is too short will cause a random vibration or a sudden noisy disconnection. A shaft that is too long will bottom out at the transmission, breaking the slip-yoke, tailshaft housing, and internal transmission parts.

10. Post-Crash Inspection - Race Cars Crash!
From a little bump on the wall to a little rubbin' to a major incident, the driveshaft is always at risk for damage. Breaking a Panhard bar or customizing the rear axle alignment on the Turn 4 wall will put your driveshaft in a very compromised position. Think of the driveshaft as a visual fuse in the drivetrain. Any crash damage serious enough to dent or buckle the driveshaft indicates that the rear end or chassis has been pushed out of alignment.

The driveshaft's operation zone will be moved and all your careful clearance checking will be irrelevant. Hopefully, the damage to the driveshaft will have spared more expensive components, such as the transmission or rear gear, from further damage. Replacing the driveshaft with your spare will not correct the rear axle alignment problem.

Before you frantically pound out the fenders and send the car back on-track, consider the consequences of damage to the driveshaft. Always inspect the driveshaft closely after a crash. If there is any doubt as to its condition, remove it from service and return it to the manufacturer for inspection and repair.

11. Driveshaft Damage and Failure
In normal use, your racing driveshaft will be trouble-free for its usable life. But no one on a professional race team ever gets a bonus for using a driveshaft until it fails.

The consequences of a failure during an event go far beyond the cost of the shaft in terms of lost finishes, prize money, and points. A driveshaft failure at speed will likely cause a crash and serious car damage and injury. So recognizing when a shaft is damaged beyond further use will go a long way toward preventing catastrophic failures.

12. Driveshaft Handling
With machined surface clearances measured in thousandths and balancing accurate to the ounce or gram, your driveshaft is a precision piece of equipment that needs to be handled with care. Modern racing shafts can be easily damaged by careless handling in the shop or under the car. Always hold a shaft with both hands, like you were cradling a baby. Never lean a shaft against a wall or lay it on a bench top where it can roll off and hit the floor. Contrary to Isaac Newton, racing driveshafts are affected by twice the force of normal gravity and will always seek the shortest path to the floor as quickly as they can.

13. Bending
Tube runout is carefully controlled during the production of a racing driveshaft. The manufacturer will measure runout over the length of the shaft after welding and carefully straighten it to a minimum specification before balancing. While rolling the shaft in a set of V-blocks, a dial indicator will show the actual tube runout. Doing this process, before installing your new shaft, will give you reference data to compare when checking a suspect shaft after a crash.