The purpose of the final drive gear assembly is to supply the Final wheel drive 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 tires 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 transmitting mounted in the front, the ultimate drive and differential assembly sit in the trunk of the vehicle and receive rotational torque from the transmitting through a drive shaft. In RWD applications the final drive assembly receives insight 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 rotate at different speeds as a car encircles a corner.
A RWD final drive sits in the rear 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 contain a pinion gear and a ring gear. The pinion equipment gets the rotational torque from the drive shaft and uses it to rotate the ring gear. The pinion gear is much smaller and includes a much lower tooth count compared to the large ring equipment. Thus giving the driveline it’s final drive ratio.The driveshaft provides rotational torque at a 90º angle to the direction that the wheels must rotate. The final drive makes up because of this with the way the pinion gear 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 exact centre of the band gears teeth, at moderate to full load. (The gears push away from eachother as load is certainly 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 teeth are curved which in turn causes a sliding action 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 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 portion of this article) Many final drives home the axle shafts, others use CV shafts just like a FWD driveline. Since a RWD final drive is external from the transmitting, it requires its own oil for lubrication. That is typically plain equipment oil but many hypoid or LSD final drives need a special type of fluid. Refer to the assistance manual for viscosity and various other special requirements.
Note: If you are likely to change your rear diff liquid yourself, (or you plan on starting the diff up for support) before you allow 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 installed, which implies that rotational torque is created parallel to the direction that the wheels must rotate. There is no need to modify/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 equipment(s) will mesh with the final drive ring gear. In almost all situations the pinion and band gear will have helical cut teeth just like the rest of the transmitting/transaxle. The pinion gear will be smaller sized and have a lower tooth count compared to the ring gear. This produces the ultimate drive ratio. The ring gear will drive the differential. (Differential procedure will be explained in the differential section 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 usually a simple (cheap) design that uses 4 gears (sometimes 6), that are referred to as spider gears, to drive the axle shafts but also permit 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 part 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 so are driven by it. Rotational torpue can be 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 straight 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 begin to rotate as they drive the axle part gears, allowing the outer wheel to speed up and the within wheel to slow down. This design is effective so long as both of the driven wheels have traction. If one wheel doesn’t have enough traction, rotational torque will follow the path of least level of resistance and the wheel with small traction will spin while the wheel with traction will not rotate at all. Since the wheel with traction isn’t rotating, the vehicle 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. This 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 get rotational torque and invite the vehicle to move. There are many different designs currently in use today. Some work better than others based on the application.
Clutch style LSDs derive from a open up differential design. They have a separate clutch pack on each of the axle aspect gears or axle shafts inside the final drive housing. 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 part gears which put strain on the clutch. If an axle shaft wants to spin faster or slower than 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 good turn at low acceleration (parking), the clutches offer little level of resistance. When one drive wheel looses traction and all the torque goes to that wheel, the clutches level of resistance becomes much 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 fluid isn’t changed at the proper intervals, the clutches may become less effective. Leading to small to no LSD actions. Fluid change intervals vary between applications. There can be nothing wrong with this design, but remember 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 always rotate at the same speed, even in a convert. This is not an issue on a drag race vehicle as drag vehicles are driving in a directly line 99% of that time period. This can also be an edge 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 great modification for vehicles made 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 driving through a gradual turn (parking). The effect 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 no spider gears, which are the weak point of open differentials.