The driveshaft is about as exciting a high-performance component as lug nuts. But as you might guess, there are some important aspects to driveshaft performance that should not be ignored. While most performance street cars will never exceed the performance parameters of a stock driveshaft, the advent of Fastest Street Car racing and top speed events like the Pony Express 100 and Silver State races have made the driveshaft a very important component in the ultimate performance drivetrain.
Every driveshaft has a certain speed, or rpm limit, called a driveshaft’s critical speed. At critical speed, the driveshaft will tend to vibrate and eventually destroy itself. Tex Racing supplies transmission and driveshaft components for many Winston Cup and circle track racers and has experience in the nuances of building driveshafts that can take the rpm and torque of heavy, high-performance race cars. According to John Ritter of Tex Racing, the important variables contributing to a driveshaft’s critical speed are the driveshaft length, tubing diameter, material and driveshaft speed. The idea is to decrease mass while increasing strength so that the driveshaft rpm will never reach the shaft’s critical speed.
Shortening the driveshaft, increasing its diameter and changing to aluminum are the quickest ways to boost critical speed. For example, Winston Cup cars use a steel driveshaft around 48 inches in length that is 3½ to 4 inches in diameter. This is sufficient to run 8500 driveshaft rpm in competition and avoid the driveshaft’s critical speed.
An example of a low driveshaft critical speed would be a 57-inch-long driveshaft with an 0.76 overdrive transmission, 3.90:1 gear, and a stock steel 3-inch-diameter driveshaft. Spin this driveshaft with an engine rpm of 5,000, with a 24 percent overdrive that generates 6,200 driveshaft rpm (5,000 x 1.24 = 6,200 rpm). The critical speed for this driveshaft is 4,500 rpm. Above this critical speed, the shaft will start to vibrate and eventually bend. When this happens, the driveshaft will quickly fail.
There are hundreds of combinations of critical speeds depending upon driveshaft rpm, length, material and diameter. It’s beyond the scope of this column to detail every combination, but you can consult your favorite driveshaft maker or call Tex Racing for more information. The more you know about drivetrain dynamics, the safer your car will be for the fun stuff you put it through.
