The Science Of Driveshafts

The chances of breaking the pinion-yoke are small, and the compact design means more meat in the important places, yielding a strong component. That is not to say a cast yoke is bulletproof. When the slip-yoke broke in our '71 Buick GS, it egged out the pinion-yoke saddles. We replaced it with a billet steel yoke from Mark Williams. New yokes usually come with the better joint caps instead of the lighter-weight stock-style U-bolts, which are prone to distorting the U-joint bearing caps.

The chances of breaking the pinion-yoke are small, and the compact design means more meat

Carbon-fiber tubes are the most expensive, but they are also the most efficient. When you are looking at power figures up to 1,200 lb-ft or 900 to 1,500 hp, carbon fiber is a great choice. Carbon-fiber driveshafts are not only strong, but they also have a surprisingly high torsional strength, resisting twisting and reducing the shock factor on the rearend. Carbon fiber also has the highest critical speed module of elasticity, meaning the shaft won't flex at slower speeds, unlike other material components. Coupled with the highest critical speed and the light weight, a carbon-fiber driveshaft can free up as much as 5 hp over a stock steel driveshaft. When winning is everything, that might make the difference.

U-Joints
Once the driveshaft is measured and ready to build, there are a few other issues to contend with. Phasing the U-joints with the weld-in yokes is an important part of the equation. With every rotation of a U-joint at any degree other than zero, a vibration is generated. This shows up as a torsional pulse, which is felt as a significant vibration. By phasing the weld-in yokes to minimize the combined degrees of rotation, the vibration will be drastically reduced.

Which U-joints you use makes a difference-and not just the brand. A U-joint should be looked at in terms beyond load capacity. For most cars, 1310-series U-joints are common. For performance applications, 1350-series joints are the better choice. The larger the series number, the larger the trunnion (the protruding shafts the caps ride over) diameter, which equates to more torsional strength. Torsional forces are those exerted in a twisting motion. Changing to a larger-series U-joint is not a simple task; you can't just buy bigger joints. All the yokes-slip, bolt-on, and weld-in-must match the desired joint size. Crossover U-joints allow you to mate a larger (or smaller) U-joint to the other. For example, you buy a new driveshaft that comes with 1350 weld-in yokes, but your car has 1310-sized yokes for the tranny and rear differential. A 1350-to-1310 joint would have a 1350 on one plane and a 1310 on the other, allowing you to install the driveshaft until you replaced the slip- and bolt-on yokes. While it can be done, using crossover U-joints is not suggested as a long-term solution. The smaller size basically becomes a fuse, and it will eventually fail.

The type of joint, solid-body versus greaseable, is important as well. The Spicer-style solid-body U-joints come lubed for life and do not have grease zerk fittings. This makes them a little stronger, as they do not have the stress risers created by the opening for the zerk fitting in a greaseable U-joint.

Yokes
The slip-yoke and the pinion-yoke also take a lot of abuse in a high-performance application. These are the physical connectors to the transmission, driveshaft, and differential. When the slip-yoke failed on our Buick GS, it wiped out just about everything it could, including damaging the pinion-yoke saddles. In most applications a cast pinion-yoke is usually strong enough to handle up to 800 hp. That number has some fudge room, though, as a lightweight car with street tires and 800 hp will put less strain on the yokes than a 4,000-pound Chevelle with slick and 500 horses. Another option when using a cast pinion-yoke is using U-joint caps instead of the weaker stock-style U-bolt retainers. This will increase the holding power and eliminate the possibility of distorting the caps. New billet yokes typically come with the proper retaining caps.

How To Measure A Drive Shaft
If you have changed transmissions and need to measure for a new one, it is important to measure from the face of the seal on the tailshaft to the flat surface on the pinion-yoke with the pinion-yoke installed and the car sitting at ride height. Changing to a billet pinion-yoke can alter the length by as much as 3/4 inch. maintenance

By Jefferson Bryant

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