Important Driveshaft Criteria

There are several factors that go into driveshaft selection. In order of importance they are strength, critical speed, and weight. Selecting the proper driveshaft means taking a deeper look at the requirements of every application, because every application is unique. Careful consideration of these three factors will ensure reliable use with the highest performance.

Strength:

The first thing to take into account is the strength of the driveshaft a racer will need. The more power you have and the more a car weighs, the stronger the driveshaft needs to be. The weight of the car is one factor that is often forgotten when selecting a driveshaft. A 4000 lb. car with 1000 hp will be much harder on a driveshaft than a 1800 lb. car with the same power. There are three factors that make a driveshaft strong; material, diameter, and u-joint size.

In terms of material, steel and thicker walls will be the strongest choice, though there are obvious drawbacks with the added weight. Using high strength alloys allows thinner tubing walls to take out extra weight without sacrificing strength. Aluminum, especially when using high quality 7075 tubing and ends, offers a well balanced mix of strength and weight. Carbon fiber is by far the lightest driveshaft material with good strength. Mark Williams Enterprises offers a large array of material choices including: Mild steel, 4130 chrom-moly, 7075, 6061, a proprietary aluminum, and carbon fiber.
Increasing diameter will significantly increase the strength of a driveshaft, no matter the material. For example, increasing diameter from 3" to 4" will increase strength by about 80%. Increased diameter has some other benefits that we will discuss later.

The two most common u-joints used in drag racing are the 1350 and 1480 series u-joints, both offered by M-W. For the most part, 1350 series u-joints will work in almost every situation. They offer the best mix of strength, precision and weight. They are used in every class all the way to pro-stock. 1480 joints are reserved for extra heavy duty applications, mainly used in Pro-Mod and other high power classes especially when tire shake is an issue. U-joints are usually the weakest piece in the system.

Critical Speed:

Critical speed is a situation where the natural frequency is the same as the rotating speed of the driveshaft. When those two coincide vibrations are multiplied and will eventually shake the driveshaft so hard it will fail. When considering a driveshaft, it is extremely important to make sure that the car will operate at speeds lower than the critical speed.
The three factors that change critical speed are, length, diameter, and the ratio of weight to material stiffness. Just like a wind chime, the longer a tube is, the lower its natural frequency is, and the lower the critical speed. likewise diameter and how stiff a driveshaft tube is affects the critical speed. There are a lot of factors that go into critical speed but in general, a larger diameter and lighter tube will allow a driveshaft to spin faster.

Weight:

After strength and critical speed are addressed the weight of the driveshaft can be considered. As opposed to strength and critical speed, the weight of the driveshaft directly affects the performance of the car. Not only will a heavier driveshaft add to the weight that rolls down the track, but it will take more power to spin a heavier driveshaft.
For the sake of comparison, we will use 45" center-to-center, 3.5” o.d. driveshaft with a M-W 39004 Turbo 400 trans yoke. Steel, of course, is heavier than aluminum or carbon fiber. A 3.5" o.d. shaft made of chrom-moly tips the scales at 20.6 pounds. Aluminum is by far more popular, and M-W’s exclusive 7075 alloy driveshafts are 13.7 pounds. A 3.5" o.d. 7075 shaft weighs about 33% less than an equal size chrom-moly shaft. This translates to 33% less power needed to accelerate the driveshaft to operating RPM, and the faster the driveshaft spins the more power that is saved.