u-joints: the clunk in a rear wheel drive vehicle
if your vehicle sends power to the rear wheels, there is a driveshaft running down the middle of it, and universal joints at each end letting it flex as the suspension moves.
they are simple, they are strong, and they wear out.
what a worn U-joint sounds and feels like
- a clunk when you move off, or when changing between drive and reverse
- a vibration that comes in at a particular speed and grows with it
- a squeak at low speed that changes with road speed rather than engine speed, which is a dry joint asking for grease
- a ticking from underneath as you pull away gently
greasable and sealed
older vehicles and many trucks have U-joints with a grease nipple, meant to be greased at every service. grease them and they last a very long time. miss it and the needle bearings run dry, brinell the cups, and fail.
many newer vehicles have sealed joints with no nipple. nothing to maintain, and nothing to do when they wear except replace them.
which yours has is worth knowing, because a greasable joint that has never been greased is a maintenance failure rather than a wear item.
why not to leave it
most failures are gradual and give plenty of warning. the one that does not is a U-joint that lets go completely at speed, dropping the driveshaft. on a vehicle that is doing 100 km/h, that is a serious event, and it can take out the transmission tailshaft, the floor pan or the fuel and brake lines on its way.
it is not a common failure and it is not worth being frightened of. it is worth not ignoring a clunk that has been getting louder for a year.
what we check
play in the joints by hand, the condition of the boots and seals, and whether the greasable ones have actually been greased. it takes a couple of minutes on a hoist and it is the difference between a scheduled repair and a roadside one.