A driveshaft will typically fail in the universal joints at either end before it does anywhere else. Also known as the Cardan or double-Cardan joint, the universal joint is what allows the driveshaft to angle up and down in response to suspension movement. Universal joint failure typically starts out as a very slight increase in clearance; this increase in clearance allows the yoke to build up momentum before engaging, eliciting a clunk or bang and hammering of the joint's remaining bearings into slurry.
The propeller shaft was one of those things that never made it across the Atlantic; well, the name didn't, anyway. Known in the colonies as a "driveshaft," this vital component is what links your transmission to the wheels and makes the car go. Driveshafts are a bit more complicated than they might seem at first blush, and can fail in a number of different ways.
Driveshafts spin very quickly; roughly 2.5 to 4 times as fast as the tires. A perfectly round driveshaft will rotate smoothly, but any irregularity in driveshaft mass distribution will result in moderate-to-severe vibration. A small dent in the driveshaft or missing balance weight will cause a slight vibration at high speed, while a severe dent or completely bent driveshaft will spill your coffee and rattle your eyeballs.
Twist and Stop
While fairly rare, driveshaft twisting isn't unheard of in high-torque, high-weight applications. A twisted driveshaft is exactly what it sounds like, and often results from "neutral dropping" the transmission by revving it up in neutral and dropping it into drive. Odds are best that a neutral drop will just break the tires loose, but really large, heavy tires may grab well enough to twist the shaft. Worst case scenario is that the twisting shortens the driveshaft enough to pull it out of the transmission.
While this may seem hard to believe -- especially if you've seen certain TV programs that "prove" it's nearly impossible -- the National Hot Rod Association requires protective driveshaft loops for a reason. Driveshaft bending, twisting or snapping will often either break the transmission case or pull the shaft out of the transmission and drop it on the ground. Forward motion can drive the transmission into the ground, either breaking the rear end or causing the car to pole-vault onto its nose.
- "Heavy Duty Drivetrains: System and Component Application"; Charles R. Jones; 1991
- "Sure Your Car's Fast, But Is It Safe and NHRA Legal?"; Jeff Smith; Hot Rod Magazine; February, 2009
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