Drivetrain setup continues to be a baffling and controversial subject. It's baffling because it's three-dimensional, and it's controversial because differing applications require differing solutions. Quarter- and eighth-mile drag or oval track race cars have different setup requirements than cars used on the highway. The following information applies to street rods that are driven on the street by drivers who travel for fun and appreciate comfortable rides.
Why do we have to think about these angles at all? We wouldn't if the engine, transmission, driveshaft, and pinion all shared the same centerline. As it is, these components are never on the same centerline, so universal joints are used to get power around corners. Unfortunately, as the front U-joint gets the power delivered to the driveshaft, it transforms smooth engine power into pulsating power. This happens because the U-joint travels an elliptical path caused by the angle through which it operates. You can visualize this by looking at a dinner plate straight on. If you tilt it the round plate appears to become an ellipse. In traveling this ellipse, the U-joint speeds up and slows down twice per shaft revolution. A second U-joint having an equal but opposite angle is used to convert this pulsating power back into smooth power feeding the pinion. The bigger the working angles are, the more violent these speed changes become.
The Dana Corporation's Spicer Driveshaft Inc., publishes charts showing joint bearing life versus joint working angle at various rpm. It has been our experience that street rodders will be aware of a general busyness in the car caused by joint angles greater than about 3 degrees. The busyness is felt because engine power in the driveshaft is always pulsating power. Here are three examples of working angles of 3 degrees or less.
As everyone knows, a pinion moves, especially in leaf-spring cars. On a street driven vehicle, the goal is for the pinion to move equally on either side of ideal to produce an acceptable compromise for driver comfort. Deliberately placing the pinion at a lesser angle than ideal is common on the dragstrip where power is always full on. Using such a setup on the street will result in an annoying vibration every time you back off the throttle. During long downhill runs with the engine braking this will be particularly aggravating.
Street rods are nothing if not stylish. Sometimes a car's design will clash with the functions required for driveability. What are the consequences of bending or ignoring the functions described above?
Uncanceled U-joint working angles deliver pulsating power to the pinion, ring gear, axles, and wheels. Resistance from the ground causes these pulsations to feed back up the powertrain, contributing to vibrations in seats, mirrors, gearshift levers, and other miscellaneous parts. The pulsations can also destroy parts like transmission clutch packs, tailshaft housings, rearend bearings, gear sets, and axles. Figure 4 shows a real world example.