There are two distinctly different types of universal joints in today's vehicles. Rear wheel drive and front wheel drive both use universals. Although different in design, they have the same purpose. Universal joints in a rear wheel drive vehicle serve to be a flexible connection for the driveshaft allowing movement vertically to conform to the movement of the rear axle.
The front wheel drive vehicle has the transmission and engine situated transversely in the vehicle. The final drive axle is an integral part of this type of transmission. A shaft from the transmission to the front wheels, coupled on both ends with a constant velocity joint serves the same primary purpose as a universal joint with one difference. This difference is that it also moves from side to side as well as vertically as the steering is turned to the left and right. Its job is to transfer power from the transmission to the wheels.
The constant velocity joints utilize two large cups approximately 5 inches long and 4 inches in diameter. These dimensions are only approximate for illustrative purposes. The cup has a shaft about 6 inches long by about 1.5 inches thick and has splines for use in locking into the transmission spider gears. The other end is for locking into the spindle. There is a shaft that covers the distance from the transmission to the spindle and each end fits into the large cups. The are four posts--as in the standard universal--on each end of the shaft. These posts have a large singular roller bearing mounted on the posts. These roller bearings are housed inside the large cup and sealed with a rubber boot that contains the grease necessary for lubrication.
The universal used in a rear wheel drive is approximately 4 inches in diameter and has a center with four poles at all four points of the compass. Each are capped with a cup with roller bearings. This arrangement allows free movement on all axis. They are the direct connection for the driveshaft from the transmission to the rear axle. The driveshaft transfers the power to the rear wheels.
