Although it would be desirable to have the drive shaft in line with the engine crankshaft, this design is not practical. In the first place, the wheels move up and down because of road irregularities. Second, the frame moves up and down in relation to the wheels. How much it moves depends on the amount of weight in the automobile body and the limits of the suspension springs and linkage. So a compromise is made, and the “workable” design is intended to provide a line of drive as straight as possible under average conditions.
The universal joints, Fig. 46-2, serve to compensate for changes in the line of drive by transmitting power from a driving shaft. Most cars use two or three universal joints in the drive line between the transmission and differential, Fig. 46-3.
