Rear-wheel drive and four-wheel drive vehicles are equipped with a differential. The differential contains the ring gear and pinion, which determines the final drive gear ratio of the vehicle. In addition to the ring gear and pinion, the differential also contains a carrier, which houses the “spider-gears”, and the axles. Documentation for devices dating back as far as 1,000 B.C. indicate the presence of some type of differential. The automobile, since its inception in the early 1900s, has utilized the differential and many vehicles in production today continue to do so.
The final drive gear ratio is determined by the number of teeth on the pinion gear, as they are divided into the number of teeth on the ring gear. For instance: If the pinion gear has eight teeth and the ring gear has thirty-three teeth, the gear ratio would be 4.10:1. The higher the first number is, the lower the gear. A vehicle which launches hard from the starting line will have a low-gear ratio (4.10:1 or 4:56:1 are popular among drag racers) despite the higher number. A vehicle which launches normally and is capable of reaching greater highway speeds will have a higher gear ratio (3.23:1 and 3:73:1 are commonly used) despite the lower number.
There are three basic types of differentials which have been used in light-duty passenger vehicles. Open carrier differentials propel the vehicle using only one axle, limited-slip differentials primarily use one axle but, if significant slippage is detected, both axles begin to pull. The locking differential often referred to as a positive-traction differential, pulls using both axles.
Most “business-class” rear-wheel drive passenger cars employ an open carrier differential. We are talking about the lower powered straight and V6-cylinder equipped cars. The economical business man’s coupes used open carriers; nothing too powerful, very few trucks, and certainly not sports cars.
By far the most commonly used differential is the limited-slip variety. Sports cars, light trucks, and full-sized sedans use the limited slip differential. Ford F-150s, Chevy 1500s, and Dodge Rams all use limited slip differentials. Most Camaros, Mustangs, and Challengers use them, too.
This type of differential uses a special clutch system in the carrier assembly to achieve a positive lock-up. High-powered sports cars and heavier trucks, such as the F-350 and the Chevy 3500 often use a positive traction differential. Many performance cars can also be special ordered with a posi-trac differential.
By far, the most common complaint regarding differentials deals with bearing failure. Replacing the bearings is best accomplished as a set. Chances are that if one bearing has failed, the all of the bearings are contaminated. Careful cleaning and removal of all bearing material, metal, and debris is necessary to any successful differential bearing kit replacement. Always replace bearings and races, together. There are three types of differential bearings. They are all tapered roller bearings.
- Carrier bearings are on either end of the carrier assembly. The bearings and races are held in place by two carrier straps, which are bolted on with large bolts.
- Pinion bearings are the tapered bearings on either end of the pinion gear shaft.
- Axle bearings are located towards the extreme end of the axle tubes. The axles ride in the axle bearings which are press-fit into the differential.
Bearing kits are sold for nearly every domestic and import vehicle. These kits normally include all of the bearings, seals, a new pinion nut, crush sleeve, and cover gasket.