When you hit the throttle, the engine spins the flywheel. The flywheel transfers energy to the transmission that is then transferred to the differential, spinning the tires and putting the power to the ground, right? The driveshaft is merely a link between the transmission and rearend, right? This is the common train of thought about the driveshaft, anyway — it’s just a link. The fact of the matter is, however, that while you can’t gain horsepower through the driveshaft, you can certainly lose it.
Driveshaft balance needs to be considered anytime you increase the power output (of a stock engine) and, subsequently, increase the speed of the engine. Most factory driveshafts are balanced between 3,000 and 3,500 rpm. Pressing the driveshaft to spin beyond that range can have a parasitic effect.
Professionals suggest that when balancing a performance driveshaft, a minimum of 5,000 rpm to as high as 7,500 rpm should be used as guidelines. This ensures a properly tuned driveshaft that reduces vibration and efficiently transmits power to the wheels, for greater performance.
Length And Diameter
After balance, the length and diameter of the driveshaft are the most common and directly impacting factors affecting the performance of the unit. Critical speed is the rpm at which the driveshaft becomes unstable and begins to bend into an S shape. The longer and smaller round a driveshaft is, the lower its critical speed. Critical speed is felt as an excessive vibration, which could cause the unit to fail. To calculate your unit’s critical speed, the length, diameter, wall thickness, and material module of elasticity must be known. Below is a critical speed chart. You can also click here for a calculator to help you determine what yours is.
The material make of the driveshaft is just as important as its length and diameter. An OEM steel shaft is rated for no more than 350 lb-ft or 350 to 400 hp. For high-performance use, there are two types of steel used: DOM or drawn-over mandrel seamless tubing and chrome-moly steel. DOM steel is better than OEM steel, handling up to 1,300 lb-ft and 1,000 to 1,300 hp and also has a higher critical speed — a good choice for a vehicle not requiring a lightweight unit.
Chrome-moly is a step up from DOM and is the strongest possible material (usually seen in Pro Stock cars). Chrome-moly steel tubing can be heat-treated as well, raising the torsional strength 22 percent and increasing the critical speed by 19 percent. Steel is heavy, though, which increases the load on the drivetrain.
Aluminum is probably the most common performance driveshaft material. The light weight of aluminum reduces rotational mass, freeing up horsepower from the engine and reducing parasitic loss. A great choice for most muscle cars, an aluminum driveshaft will support up to 900 lb-ft or 900 to 1,000 hp. It’s understandably not as strong as steel, however, so some driveshaft shops don’t have twist guarantees on aluminum driveshafts.
The most efficient are Carbon-fiber tubes — but they’re also the most expensive. When you’re 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. They also have the highest critical speed module of elasticity, meaning the shaft won’t flex at slower speeds, unlike other material components. Couple that with the highest critical speed and the light weight, and a carbon-fiber driveshaft can free up as much as 5 hp over a stock steel driveshaft.
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. It makes sense why professionals are necessary in this process, due to the specialized knowledge and equipment needed.
The choice in U-joints makes a difference, too — and not just the brand. Load capacity isn’t the only determining factor in choice. For most cars, 1310-series U-joints are common. For performance applications, 1350-series joints are probably a better choice. The larger the series number, the larger the trunnion diameter, which equates to more torsional strength. (Torsional forces are those exerted in a twisting motion.) Changing to a larger-series U-joint isn’t a simple task either, because you can’t just buy bigger joints. All the yoke-slips, bolt-ons, and weld-ins 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 transmission 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 will eventually fail.
The type of joint (solid-body v. greaseable) is important, too. The Spicer-style solid-body U-joints come lubed for life and don’t have grease zerk fittings. This makes them a little stronger since they don’t have the stress risers created by the opening for the zerk fitting as in a greaseable U-joint.
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. In most applications a cast pinion-yoke is usually strong enough to handle up to 800 hp. That number has some fudge room, though, because, for instance, 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.
Reaching critical speed is what causes driveshaft failure. This complex formula is used to calculate the critical speed for every driveshaft. All driveshafts have a critical speed, custom to it’s particular length and diameter. The module of elasticity of the shaft material is an important part of the equation. Getting these numbers can be a little tricky, as most shops keep the specific numbers close to the vest. For steel, the basic MOE is 30, aluminum is 10, and carbon-fiber depends on the manufacturing processes used, so no numbers are available.