Final Drives And Differential Basics
The purpose of the final drive gear assembly is to provide the final stage of gear reduction to decrease RPM and increase rotational torque. Typical final drive ratios can be between 3:1 and 4.5:1. It is because of this that the wheels never spin as fast as the engine (in almost all applications) even when the transmission is in an overdrive gear. The final drive assembly is connected to the differential. In FWD (front-wheel drive) applications, the final drive and differential assembly are located inside the transmission/transaxle case. In a typical RWD (rear-wheel drive) application with the engine and transmission mounted in the front, the final drive and differential assembly sit in the rear of the vehicle and receive rotational torque from the transmission through a drive shaft. In RWD applications the final drive assembly receives input at a 90° angle to the drive wheels. The final drive assembly must account for this to drive the rear wheels. The purpose of the differential is to allow one input to drive 2 wheels as well as allow those driven wheels to rotate at different speeds as a vehicle goes around a corner.
RWD Final Drives
A RWD final drive sits in the rear of the vehicle, between the two rear wheels. It is located inside a housing which also may also enclose two axle shafts. Rotational torque is transferred to the final drive through a drive shaft that runs between the transmission and the final drive. The final drive gears will consist of a pinion gear and a ring gear. The pinion gear receives the rotational torque from the drive shaft and uses it to rotate the ring gear. The pinion gear is much smaller and has a much lower tooth count than the large ring gear. This gives the driveline it’s final drive ratio.The driveshaft delivers rotational torque at a 90º angle to the direction that the wheels must rotate. The final drive makes up for this with the way the pinion gear drives the ring gear inside the housing. When installing or setting up a final drive, how the pinion gear contacts the ring gear must be considered. Ideally the tooth contact should happen in the exact centre of the ring gears teeth, at moderate to full load. (The gears push away from eachother as load is applied.) Many final drives are of a hypoid design, which means that the pinion gear sits below the centreline of the ring gear. This allows manufacturers to lower the body of the car (because the drive shaft sits lower) to increase aerodynamics and lower the vehicles centre of gravity. Hypoid pinion gear teeth are curved which causes a sliding action as the pinion gear drives the ring gear. It also causes multiple pinion gear teeth to be in contact with the ring gears teeth which makes the connection stronger and quieter. The ring gear drives the differential, which drives the axles or axle shafts which are connected to the rear wheels. (Differential operation will be explained in the differential section of this article) Many final drives house the axle shafts, others use CV shafts like a FWD driveline. Since a RWD final drive is external from the transmission, it requires its own oil for lubrication. This is typically plain gear oil but many hypoid or LSD final drives require a special type of fluid. Refer to the service manual for viscosity and other special requirements.
Note: If you are going to change your rear diff fluid yourself, (or you plan on opening the diff up for service) before you let the fluid out, make sure the fill port can be opened. Nothing worse than letting fluid out and then having no way of getting new fluid back in.
FWD Final Drives
FWD final drives are very simple compared to RWD set-ups. Almost all FWD engines are transverse mounted, which means that rotational torque is created parallel to the direction that the wheels must rotate. There is no need to change/pivot the direction of rotation in the final drive. The final drive pinion gear will sit on the end of the output shaft. (multiple output shafts and pinion gears are possible) The pinion gear(s) will mesh with the final drive ring gear. In almost all cases the pinion and ring gear will have helical cut teeth just like the rest of the transmission/transaxle. The pinion gear will be smaller and have a much lower tooth count than the ring gear. This produces the final drive ratio. The ring gear will drive the differential. (Differential operation will be explained in the differential section of this article) Rotational torque is delivered to the front wheels through CV shafts. (CV shafts are commonly referred to as axles)