Williams is pretty close. New yokes usually come with better joint caps, instead of lighter-weight stock-style U-bolts, which are prone to distorting the U-joint bearing caps.
Another option when using a cast pinion yoke is to use U-joint caps instead of the weaker stock-style U-bolt retainers. This increases the clamping force and eliminates the possibility of distorting the caps. New billet yokes typically come with the proper retaining caps.
Along with balance, the length and diameter of the driveshaft directly affect the performance of the unit. Determining the required length for the driveshaft necessitates looking at several factors. The distance from the rear yoke to the transmission seal is the most important measurement because it determines the overall length of the driveshaft. Measure this length with the pinion yoke installed and the car at ride height. The pinion yoke influences the measurement, and changing from a cast-steel yoke to a billet pinion yoke can alter the length by as much as 3/4 inch.
Provide these measurements to the driveshaft shop and they can create the complete shaft with the required slip yoke and predetermined run-out for the slip yoke. For most applications, a run-out of 1 inch is more than enough to provide the play needed for suspension travel, so do not let a shop convince you to accept more run-out than that. Some transmission shops insist on running out 1-1/2 inches, but this could be disastrous and lead to driveshaft failure. With that much of the slip yoke hanging out of the transmission, there could be less than 3 inches of splined yoke in the transmission, thus creating a wobble in the yoke that would cause a heavy vibration at various RPM. Stick with the 1-inch rule.
Always measure the driveshaft length at drive height. If the vehicle is too low to crawl under it on the ground, jack up both ends and use jack stands under the rear end and front suspension; be careful to make sure all the stands are at the same height. The slightest variation in the suspension can throw off the measurement, resulting in a driveshaft that does not fit.
The function of “critical speed” (CS) factors into the length versus diameter rule. Critical speed is the RPM at which the driveshaft becomes unstable and begins to bend in the middle. This is also known as “jump roping.” The longer and smaller (diameter) a driveshaft is, the slower its critical speed. Critical speed is felt as excessive vibration, and if run at CS too long, the unit will fail. To calculate the critical speed, you must know the length, diameter, wall thickness, and material module of elasticity. Then, using the critical speed calculation formula, you can plug in the numbers to calculate the driveshaft’s critical speed.
Driveshaft material is just as important as its length and diameter. Original equipment manufacturing (OEM) steel driveshafts are for just that, OEM power. An OEM shaft is rated for no more than 350 ft-lbs, or 350 to 400 hp. For high-performance use, drawn over mandrel (DOM) seamless tubing and chrome-moly steel are the two materials used. DOM steel is better than OEM steel, handling much more torque, up to 1,300 ft-lbs and 1,000 to 1,300 hp. DOM steel can be spun faster, as well, with its higher RPM rating, making it suitable for any stock LS application. This is a good choice for any car that does not need a lightweight unit.
The step up from a steel shaft is chrome-moly, which is the strongest material available. It’s used in 3,000-hp Pro Stock cars. Chrome-moly steel tubing can be heat treated as well, raising the torsional strength 22 percent and increasing the critical speed 19 percent. Steel is heavy, which increases the load on the engine bandit so that it takes the engine longer to get to speed.
Reducing driveline weight is important, so lighter materials are sometimes a better choice. Aluminum is the most common performance driveshaft material. A lightweight aluminum shaft reduces rotational mass by freeing up horsepower from the engine and reducing parasitic loss. Aluminum driveshafts are strong but cannot hold as much torque as steel. Therefore, some custom driveshaft shops do not have “twist” guarantees on aluminum driveshafts. An aluminum driveshaft supports up to 900 ft-lbs, or 900 to 1,000 hp, making it a great lightweight choice for most muscle cars.
Carbon fiber, also an option, is the most efficient in terms of parasitic loss, but it is also the most expensive; it is not needed for high-performance street use, but often is used for high power figures, up to 1,200 ft-lbs, or 900 to 1,500 hp. Carbon-fiber driveshafts are strong and have a surprisingly high torsional strength, resisting twisting and reducing the shock factor on the rear end. Carbon fiber also has the highest critical speed module of elasticity, meaning the shaft doesn’t flex at slower speeds, unlike other material components. Coupled with the highest
