Universal joints allow drive shafts to move up and down with the suspension while the shaft is moving so power can be transmitted when the drive shaft isn’t in a straight line between the transmission and drive wheels. Rear-wheel-drive vehicles have universal joints (or U-joints) at both ends of the drive shaft. U-joints connect to yokes that also allow drive shafts to move fore and aft as vehicles go over bumps or dips in the road, which effectively shortens or lengthens the shaft. Front-drive vehicles also use two joints, called constant velocity (or CV) joints, but they are a different kind that also compensate for steering changes. On rear-drive vehicles, one sign of a worn U-join is a “clank” sound when a drive gear is engaged. On front-drive vehicles, CV joints often make a clicking noise when they’re worn. CV joints are covered by protective rubber boots, and if the boots crack or are otherwise damaged, the CV joints will lose their lubrication and be damaged by dirt and moisture.