The purpose of the ultimate drive gear assembly is to supply the final stage of gear reduction to diminish RPM and increase rotational torque. Typical final drive ratios could be between 3:1 and 4.5:1. It is due to this that the wheels by no means spin as fast as the engine (in almost all applications) even though the transmission is within 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 final drive and differential assembly sit down in the trunk of the automobile 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 tires. The final drive assembly must take into account this to drive the trunk wheels. The purpose of the differential is usually to permit one input to drive 2 wheels and also allow those driven tires to rotate at different speeds as a car goes around a corner.
A RWD final drive sits in the trunk of the automobile, between the two rear wheels. It really is located in the housing which also may also enclose two axle shafts. Rotational torque is used in the final drive through a drive shaft that operates between the transmission and the ultimate drive. The final drive gears will consist of a pinion gear and a ring equipment. The pinion gear gets the rotational torque from the drive shaft and uses it to rotate the ring gear. The pinion equipment is a lot smaller and includes a lower tooth count than the large ring equipment. Thus giving the driveline it’s last 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 what sort of pinion gear drives the ring equipment in the housing. When installing or establishing a final drive, the way the pinion gear contacts the ring gear must be considered. Ideally the tooth contact should happen in the exact Final wheel drive centre of the ring gears tooth, at moderate to complete load. (The gears drive away from eachother as load can be applied.) Many final drives are of a hypoid design, which means that the pinion equipment sits below the centreline of the ring gear. This enables manufacturers to lower your body of the automobile (because the drive shaft sits lower) to improve aerodynamics and lower the automobiles centre of gravity. Hypoid pinion equipment the teeth are curved which causes a sliding action as the pinion gear 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 more powerful 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 content) Many final drives house the axle shafts, others make use of CV shafts such as a FWD driveline. Since a RWD final drive is exterior from the transmission, it requires its oil for lubrication. That is typically plain equipment oil but many hypoid or LSD final drives need a special type of fluid. Make reference to the service manual for viscosity and various other special requirements.
Note: If you are going to change your back diff liquid yourself, (or you intend on opening the diff up for services) before you let the fluid out, make sure the fill port could be opened. Nothing worse than letting liquid out and having no way of getting new fluid back in.
FWD last drives are very simple compared to RWD set-ups. Virtually all FWD engines are transverse installed, which means that rotational torque is created parallel to the path that the tires must rotate. There is no need to modify/pivot the path of rotation in the final drive. The ultimate drive pinion gear will sit on the finish of the output shaft. (multiple result shafts and pinion gears are possible) The pinion gear(s) will mesh with the final drive ring equipment. In almost all cases the pinion and band gear will have helical cut tooth just like the remaining transmitting/transaxle. The pinion equipment will be smaller and have a lower tooth count compared to the ring equipment. This produces the ultimate drive ratio. The ring gear will drive the differential. (Differential procedure will be described in the differential section of this content) Rotational torque is sent to the front wheels through CV shafts. (CV shafts are generally known as axles)
An open differential is the most common type of differential found in passenger cars and trucks today. It is usually a very simple (cheap) design that uses 4 gears (occasionally 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 definitely a slang term that is commonly used to spell it out all of 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 band equipment and uses it to drive the differential pin. The differential pinion gears trip upon this pin and are driven by it. Rotational torpue is usually then transferred to the axle aspect gears and out through the CV shafts/axle shafts to the tires. If the vehicle is venturing in a straight line, there is absolutely no differential action and the differential pinion gears will simply drive the axle aspect gears. If the vehicle enters a change, the outer wheel must rotate quicker compared to the inside wheel. The differential pinion gears will start to rotate because they drive the axle aspect gears, allowing the outer wheel to increase and the inside wheel to decelerate. This design is effective provided that both of the driven wheels have traction. If one wheel doesn’t have enough traction, rotational torque will follow the road of least resistance and the wheel with small traction will spin as the wheel with traction will not rotate at all. Since the wheel with traction is not rotating, the automobile cannot move.
Limited-slide differentials limit the quantity of differential actions allowed. If one wheel begins spinning excessively faster compared to the other (more so than durring regular cornering), an LSD will limit the speed difference. That is an advantage over a normal open differential style. If one drive wheel looses traction, the LSD actions allows the wheel with traction to get rotational torque and invite the vehicle to move. There are many different designs currently used today. Some are better than others depending 
on the application.
Clutch style LSDs derive from a open up differential design. They possess a separate clutch pack on each one of the axle aspect gears or axle shafts within 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 separate the clutch discs. Springs place pressure on the axle part gears which put pressure on the clutch. If an axle shaft really wants to spin quicker or slower compared to the differential case, it must overcome the clutch to take action. If one axle shaft tries to rotate quicker than the differential case then the other will try to rotate slower. Both clutches will withstand this step. As the quickness difference increases, it becomes harder to overcome the clutches. When the automobile is making a good turn at low acceleration (parking), the clutches provide little resistance. When one drive wheel looses traction and all of 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 speed of the differential case. This type of differential will most likely require a special type of liquid or some kind of additive. If the liquid is not changed at the correct intervals, the clutches may become less effective. Leading to little to no LSD action. Fluid change intervals differ between applications. There is definitely nothing incorrect with this style, but keep in mind that they are just as strong as an ordinary open differential.
Solid/spool differentials are mostly used in drag racing. Solid differentials, just like the name implies, are totally solid and will not allow any difference in drive wheel quickness. The drive wheels always rotate at the same quickness, even in a switch. This is not an issue on a drag competition vehicle as drag automobiles are driving in a directly line 99% of the time. This may also be an edge for cars that are getting set-up for drifting. A welded differential is a regular open differential which has experienced the spider gears welded to create a solid differential. Solid differentials certainly are a fine modification for vehicles created for track use. As for street use, a LSD option will be advisable over a good differential. Every turn a vehicle takes may cause the axles to wind-up and tire slippage. This is most visible when driving through a gradual turn (parking). The result is accelerated tire put on as well as premature axle failing. One big benefit 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.