Anytime you increase the power output of a stock engine and subsequently increase the speed at which the engine operates, you need to look at driveshaft balance. Most factory driveshafts are balanced between 3,000 and 3,500 rpm. Spinning the driveshaft past that range can have a parasitic effect. Steve Raymond from DynoTech Engineering tells us, "We have had several NASCAR teams tell us that our driveshaft saves them 3 to 7 horsepower at the wheels of their race cars. That's why balance and design are important and why we manufacture shafts for about 85 to 90 percent of the NASCAR teams." Steve tells us that DynoTech suggests balancing a performance driveshaft at a minimum of 5,000 rpm and as high as 7,500 rpm. This ensures a properly tuned driveshaft that reduces vibration and efficiently transmits power to the wheels.
Solid vs. Greasable U-joint
Greasable or lubed style U-joints have several disadvantages. Because of their lube system, they are not as strong. The internal lube channel results in a smaller cross sectional area of material (less steel to resist the applied torque). They are naturally out of balance by using the zurk fitting on the body, and, with the added machining for the lubrication cross holes, you inherently induce a small amount of imbalance. They are warm forged, which is a less desirable process for strength. They require regular maintenance, but can last almost forever. Since they are lubed, they can leak grease at high speed. Solid-body U-joints have several advantages. Because of their solid design, they are inherently stronger because they make use of the greatest cross-sectional area possible. They are naturally balanced due to the symmetry of the part. They are cold forged, which is a more desirable manufacturing process for strength. They have a nylon sealing system and are lubed for life by the manufacturer. They might not last "forever," but the expected life is 100,000-plus miles (Drawing B).
The critical speed of the driveshaft is the point where the first-order resonant frequency is excited by the speed of rotation. Avoiding long mathematical equations, it's the point that the driveshaft starts to jump rope, so to speak. There are several ways to handle the issue of critical speed, such as shaft material, size, and design. With something as simple as changing from a steel shaft to an aluminum shaft, you could see as much as a 12-percent increase in critical speed. Items such as balance, balance weight location, and joint clearance all have a direct effect on critical speed (Drawing C).
What Do You Really Need?
So what's the perfect driveshaft if you're looking at performance characteristics only? Here are our thoughts: You would like as lightweight and strong a shaft as possible. If you run your car on a chassis dynamometer, you will find that peak acceleration takes place at the same time as peak engine torque. This is because acceleration is defined as force divided by mass. In the case of your car, the mass is constant, so as force increases so must your acceleration. Therefore, the key is more torque! The torque that is most important is that at your drive wheels. The reason that engine torque and drive wheel torque differ is due to drivetrain loss. Drivetrain loss is the consumption of power needed to rotate the drivetrain, including friction and component mass. By reducing component mass (driveshaft weight), you consume less power. The less power you use to rotate your drivetrain, the more you have to propel your car down the strip or street. Lighter weight driveshafts give you the best options for reducing drivetrain loss. The second key factor in the ideal driveshaft is critical speed. You want to make sure your driveshaft operates well below its critical speed, or you will experience endless vibration problems and related excessive wear on mating components.