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Something we see several times a year with all of the races and drag strips we go to is a car on the starting line spitting out a driveshaft. Something we see less often but that can be more catastrophic is a car at the top end spitting out a driveshaft. It is one of the most common failures to occur on modified cars.
Before we go any further, let’s take a moment to examine what a driveshaft really is. In essence, a driveshaft is a lengthy coupler, connecting the output shaft of the transmission to the input shaft of the pinion gear in the rear end (we’re talking front engine and rear wheel drive). Driveshafts are sometimes rated in horsepower, but what they’re really transferring and the forces they are subjected to is torque.
The torque subjects the driveshaft to a rotational shearing, in a sense torque is trying to twist the driveshaft at every point along its entire length, like twisting a lid off a jar. If this torsion exceeds the strength of any point in the driveshaft—the tube itself, a U-joint or CV-joint, or where the joints are bonded or welded to the tube—then the driveshaft will fail. The shear force is not constant, it is basically a function of the difference between the input torque to the shaft from the transmission and the inertia of the load on the output end at the axle. And there is a large spike in the shearing force for a car launching at high RPM on sticky tires on a stuck drag strip.