How is a CV joint different than a U-joint?
Why use a CV joint instead of a U-joint?
A CV joint is commonly used on front wheel drive (FWD) and all-wheel drive (AWD) cars and trucks. Constant velocity (CV) joints are used on each end of a drive shaft to allow the drive shaft to supply rotating power to the wheels yet allow the drive shaft to move up and down as the vehicle goes over bumps. CV joints also allow the drive shaft to supply power to front wheels, allowing them to receive power during turns.
A universal joint (U-joint) is most commonly used on the drive shaft of rear wheel drive (RWD) vehicles. U-joints allow the drive shaft to provide power to the rear differential yet still allow the differential to move up and down when going over bumps. U-joints work just fine on a drive shaft of a RWD vehicle because the U-joint angles are the same at each end. If the differential rises by 20° both U-joints rotate at the same angle.
Why can’t car makers use U-joints on FWD vehicles?
Front wheels must move up and down and left and right, creating different angles between the two joints on a single drive shaft. FWD vehicles have two drive shafts, one to drive each front wheel. Each drive shaft has two CV joints. One CV joint on the drive shaft connects to the transmission and the other to the wheel hub. The CV joints allow the front wheels to move up and down and turn left and right.
If those drive shafts had U-joints instead of CV joints, the U-joints would have to operate at different angles as the wheels were turned by the driver. In fact, front wheels can turn up to 45° while still being allowed to move up and down at the same time. U-joints can’t operate at those angles. As less steep angles, U-joints on each end of a drive shaft generate a cyclic vibration. The greater the angle, the greater the vibration. So obviously, U-joints are unsuitable for use as front axles.
CV joints, on the other hand can transmit power through variable angles maintaining a constant rotational speed without vibration or stress.
How do CV joints work?
There are many styles of CV joints but the tripod and Rzeppa style CV joints are the most common on FWD vehicles. The Rzeppa CV joint is used on the wheel hub side of the drive shaft, also called the outer joint. The drive shaft is splined to the inner race. As the shaft turns it applies torque to the inner race which transfers the torque to the balls and then to the housing which is splined to the wheel hub to drive the wheels. The entire joint is filled with grease and covered by a pleated rubber boot. The boot is clamped to the housing and drive shaft with special clamps. The Rzeppa CV joint allows a much greater range of motion than a typical U-joint or a tripod joint.
A tripod or “plunge style” CV joint consists of a housing, also called a tulip. The drive shaft connects to a three-legged “spider” end with bearings. Torque transfers from the transmission to the tulip and then to the bearings and spider. The spider is splined to the drive shaft which transfers the torque to the outer CV joint. The tripod joint is mainly used on the transmission side of the drive shaft. It’s designed to allow the drive shaft to move up and down, as well as in and out to accommodate the elliptical arc of the drive shaft as the wheel travels over bumps.
A tripod CV joint is also filled with grease and protected by a pleated rubber boot.
What goes wrong with CV joints?
A CV joint can last the life of the vehicle because it’s packed with grease. The “wear” portion is the protective rubber boot. As the CV boot ages, it develops cracks between the pleats. If those cracks open, the CV joint will fling the grease out of the joint. At that point the joint is exposed to water, road salt and grit. If the joint isn’t quickly cleaned, regreased and rebooted, the grit and salt will corrode the inner workings of the CV joint, causing it to vibrate, make clicking and popping sounds, expecially on turns, and eventually fail.
How far can you drive with a torn CV boot?
How much of a gambler are you? It’s really that simple. As the inner workings of the CV joint wear, the joint becomes less stable and the driveshaft eventually breaks. It’s not as simple as just leaving you stranded. The driveshaft usually breaks while it’s spinning, swinging wildly around and damaging all components it contacts. That can include broken fuel and fluid lines, damaged or broken wiring harnesses, and even damage to the transmission case, power steering pump or air conditioning compressor. In short, when a CV joint fails, the spinning drive shaft can easily cause up to several thousand dollars in damage. If you’re a risk taker, feel free to continue driving with a torn CV boot. Otherwise, get it to a shop. Once the boot is torn and the grease is gone, it’s best to replace the entire axle shaft with a rebuilt unit. Replacing just the boot is risky.
©, 2016 Rick Muscoplat
Posted on by Rick Muscoplat