The purpose of the final drive gear assembly is to supply the ultimate stage of gear reduction to decrease RPM and increase rotational torque. Typical last drive ratios could be between 3:1 and 4.5:1. It is due to this that the wheels never spin as fast as the engine (in almost all applications) even though the transmission is in an overdrive gear. The ultimate drive assembly is linked to the differential. In FWD (front-wheel drive) applications, the final drive and differential assembly are located inside the tranny/transaxle case. In a typical RWD (rear-wheel drive) software with the engine and transmission mounted in leading, the ultimate drive and differential assembly sit in the trunk 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 objective of the differential is certainly to allow one input to drive 2 wheels and also allow those driven tires to Final wheel drive rotate at different speeds as a vehicle encircles a corner.
A RWD last drive sits in the trunk of the vehicle, between the two rear wheels. It really is located in the housing which also could 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 ultimate drive gears will consist of a pinion gear and a ring equipment. 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 lower tooth count than the large ring equipment. Thus giving 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 equipment drives the ring gear inside the housing. When setting up or establishing a final drive, how the pinion equipment contacts the ring equipment must be considered. Ideally the tooth get in touch with should happen in the precise centre of the band 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 implies that the pinion gear sits below the centreline of the band gear. This enables manufacturers to lower your body of the automobile (because the drive shaft sits lower) to increase aerodynamics and lower the automobiles center of gravity. Hypoid pinion equipment tooth are curved which causes a sliding actions as the pinion equipment drives the ring gear. In addition, it causes multiple pinion equipment teeth to be in contact with the band gears teeth making the connection stronger and quieter. The ring gear drives the differential, which drives the axles or axle shafts which are linked to the rear wheels. (Differential operation will be explained in the differential portion of this article) Many final drives house the axle shafts, others use CV shafts just like a FWD driveline. Since a RWD last drive is external from the transmitting, it requires its own oil for lubrication. That is typically plain equipment essential oil but many hypoid or LSD final drives need a special kind of fluid. Refer to the assistance manual for viscosity and various other special requirements.

Note: If you’re likely to change your rear diff liquid yourself, (or you plan on opening the diff up for services) 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 to getting new fluid back.
FWD final drives are very simple compared to RWD set-ups. Virtually all FWD engines are transverse mounted, which implies that rotational torque is established parallel to the path that the wheels must rotate. There is no need to alter/pivot the path of rotation in the ultimate drive. The final drive pinion gear will sit on the end of the output shaft. (multiple output shafts and pinion gears are feasible) The pinion gear(s) will mesh with the final drive ring equipment. In almost all cases the pinion and ring gear will have helical cut teeth just like the rest of the transmission/transaxle. The pinion equipment will be smaller and have a lower tooth count compared to the ring gear. This produces the final drive ratio. The ring gear will drive the differential. (Differential operation will be explained in the differential portion of this article) Rotational torque is delivered to the front tires through CV shafts. (CV shafts are generally referred to as axles)
An open up differential is the most typical type of differential found in passenger vehicles today. It is definitely a simple (cheap) design that uses 4 gears (sometimes 6), that are referred to as spider gears, to drive the axle shafts but also allow them to rotate at different speeds if necessary. “Spider gears” is usually a slang term that’s commonly used to describe all the differential gears. There are two various kinds of spider gears, the differential pinion gears and the axle aspect gears. The differential case (not casing) receives rotational torque through the ring equipment and uses it to operate a vehicle the differential pin. The differential pinion gears trip on this pin and are driven because of it. Rotational torpue is certainly then transferred to the axle part gears and out through the CV shafts/axle shafts to the tires. If the vehicle is venturing in a directly line, there is no differential action and the differential pinion gears will simply drive the axle part gears. If the vehicle enters a turn, the outer wheel must rotate quicker than the inside wheel. The differential pinion gears will start to rotate as they drive the axle part gears, allowing the external wheel to speed up and the within wheel to slow down. This design is effective so long as both of the powered wheels possess traction. If one wheel doesn’t have enough traction, rotational torque will observe the path of least level of resistance and the wheel with small traction will spin as the wheel with traction won’t rotate at all. Since the wheel with traction is not rotating, the automobile cannot move.
Limited-slide differentials limit the amount of differential action allowed. If one wheel starts spinning excessively faster than the other (more so than durring normal cornering), an LSD will limit the swiftness difference. That is an advantage over a regular open differential style. If one drive wheel looses traction, the LSD actions will allow the wheel with traction to obtain rotational torque and invite the vehicle to move. There are several different designs currently in use today. Some work better than others based on the application.
Clutch style LSDs are based on a open differential design. They have a separate clutch pack on each of the axle aspect gears or axle shafts inside the final drive casing. Clutch discs sit down between the axle shafts’ splines and the differential case. Half of the discs are splined to the axle shaft and the others are splined to the differential case. Friction materials is used to split up the clutch discs. Springs place strain on the axle aspect gears which put strain on the clutch. If an axle shaft really wants to spin faster or slower compared to the differential case, it must overcome the clutch to do so. If one axle shaft tries to rotate quicker compared to the differential case then your other will attempt to rotate slower. Both clutches will resist this action. As the velocity difference increases, it turns into harder to conquer the clutches. When the automobile is making a tight turn at low acceleration (parking), the clutches offer little level of resistance. When one drive wheel looses traction and all the torque would go to that wheel, the clutches level of resistance becomes a lot more apparent and the wheel with traction will rotate at (near) the velocity of the differential case. This type of differential will likely require a special type of liquid or some type of additive. If the liquid is not changed at the correct intervals, the clutches may become less effective. Leading to little to no LSD actions. Fluid change intervals vary between applications. There is certainly nothing wrong with this design, but keep in mind that they are only as strong as an ordinary open differential.
Solid/spool differentials are mostly found in drag racing. Solid differentials, like the name implies, are completely solid and will not enable any difference in drive wheel acceleration. The drive wheels usually rotate at the same velocity, even in a convert. This is not an issue on a drag race vehicle as drag automobiles are driving in a directly line 99% of the time. This can also be an advantage for cars that are becoming set-up for drifting. A welded differential is a normal open differential which has experienced the spider gears welded to make a solid differential. Solid differentials certainly are a fine modification for vehicles designed for track use. As for street make use of, a LSD option would be advisable over a good differential. Every switch a vehicle takes will cause the axles to wind-up and tire slippage. That is most obvious when traveling through a gradual turn (parking). The result is accelerated tire wear as well as premature axle failing. One big advantage of the solid differential over the other styles is its power. Since torque is applied directly to each axle, there is absolutely no spider gears, which are the weak point of open differentials.