The purpose of the final 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 really is due to this that the tires never spin as fast as the engine (in almost all applications) even though the transmission is within an overdrive gear. The ultimate drive assembly is connected to the differential. In FWD (front-wheel drive) applications, the final drive and differential assembly can be found inside the transmitting/transaxle case. In a typical RWD (rear-wheel drive) program with the engine and transmitting mounted in the front, the ultimate drive and differential assembly sit in the rear of the vehicle and receive rotational torque from the tranny through a drive shaft. In RWD applications the final drive assembly receives input at a 90° angle to the drive wheels. The ultimate drive assembly must account for this to drive the rear wheels. The objective of the differential is usually to allow one input to drive 2 wheels along with allow those driven wheels to rotate at different speeds as a car encircles a corner.
A RWD final drive sits in the rear of the automobile, between the two rear wheels. It 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 ultimate drive. The ultimate drive gears will consist of a pinion equipment and a ring gear. The pinion equipment receives the rotational torque from the drive shaft and uses it to rotate the ring gear. The pinion equipment is much smaller and has a much lower tooth count compared to the large ring equipment. This gives 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 ultimate drive makes up for this with the way the pinion equipment drives the ring equipment in the housing. When installing or establishing a final drive, how the pinion equipment contacts the ring gear must be considered. Preferably the tooth get in touch with should happen in the exact centre of the ring gears teeth, at moderate to complete load. (The gears press from eachother as load can be applied.) Many last drives are of a hypoid design, which means that the pinion gear sits below the centreline of the ring gear. This enables manufacturers to lower the body of the automobile (as the drive shaft sits lower) to improve aerodynamics and lower the automobiles centre of gravity. Hypoid pinion equipment teeth are curved which in turn causes a sliding actions as the pinion equipment drives the ring equipment. In addition, it causes multiple pinion equipment teeth to communicate with the ring gears teeth making the connection more powerful and quieter. The band gear drives the differential, which drives the axles or axle shafts which are linked to the rear wheels. (Differential operation will be described in the differential section of this content) Many final drives house the axle shafts, others use CV shafts such as a FWD driveline. Since a RWD last drive is exterior from the tranny, it requires its oil for lubrication. That is typically plain gear essential oil but many hypoid or LSD final drives require 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 fluid 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. Absolutely nothing worse than letting fluid out and then having no way to getting new fluid back in.
FWD final drives are very simple in comparison to RWD set-ups. Almost all FWD engines are transverse installed, which implies that rotational torque is created parallel to the direction that the tires must rotate. There is no need to change/pivot the path of rotation in the ultimate drive. The final drive pinion gear will sit on the end of the output shaft. (multiple result shafts and pinion gears are feasible) The pinion gear(s) will mesh with the ultimate drive ring gear. In almost all instances the pinion and ring gear will have helical cut the teeth just like the rest of the tranny/transaxle. The pinion gear will be smaller sized and have a lower tooth count than the ring equipment. This produces the ultimate drive ratio. The ring equipment will drive the differential. (Differential operation will be explained in the differential portion of this content) Rotational torque is sent to the front wheels through CV shafts. (CV shafts are generally referred to as axles)
An open up differential is the most common type of differential within passenger vehicles today. It is usually a very simple (cheap) design that uses 4 gears (occasionally 6), that are known as spider gears, to operate a vehicle the axle shafts but also permit them to rotate at different speeds if required. “Spider gears” is definitely a slang term that is commonly used to describe all of the differential gears. There are two various kinds of spider gears, the differential pinion gears and the axle side gears. The differential case (not casing) gets rotational torque through the band equipment and uses it to drive the differential pin. The differential pinion gears trip on this pin and so are driven because of it. Rotational torpue is definitely then used in the axle aspect gears and out through the CV shafts/axle shafts to the tires. If the automobile is travelling in a directly line, there is absolutely no differential actions and the differential pinion gears will simply drive the axle aspect gears. If the vehicle enters a change, the external wheel must rotate faster compared to the inside wheel. The differential pinion gears will start to rotate as they drive the axle aspect gears, allowing the external wheel to increase and the within wheel to decelerate. This design is effective as long as both of the powered wheels have traction. If one wheel does not 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 automobile cannot move.
Limited-slip differentials limit the amount of differential actions allowed. If one wheel starts spinning excessively faster compared to the other (more so than durring normal cornering), an LSD will limit the swiftness difference. That is an advantage over a normal open differential design. If one drive wheel looses traction, the LSD actions allows the wheel with traction to obtain rotational torque and allow the vehicle to go. There are many different designs currently used today. Some are better than others depending on the application.
Clutch style LSDs derive from a open differential design. They possess another clutch pack on each of the axle aspect gears or axle shafts in the final drive housing. Clutch discs sit down between your 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 material is used to split up the clutch discs. Springs put strain on the axle part gears which put pressure on the clutch. If an axle shaft really wants to spin faster or slower than the differential case, it must get over the clutch to do so. If one axle shaft attempts to rotate quicker than the differential case then your other will try to rotate slower. Both clutches will withstand this action. As the velocity difference increases, it turns into harder to overcome the clutches. When the automobile is making a tight turn at low velocity (parking), the clutches offer Final wheel drive little level of resistance. When one drive wheel looses traction and all the torque would go to that wheel, the clutches resistance becomes a lot more obvious and the wheel with traction will rotate at (near) the rate 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 proper intervals, the clutches can become less effective. Resulting in little to no LSD actions. Fluid change intervals differ between applications. There is nothing incorrect with this style, but remember that they are only as strong as a plain open differential.
Solid/spool differentials are mostly used in drag racing. Solid differentials, like the name implies, are totally solid and will not really enable any difference in drive wheel acceleration. The drive wheels usually rotate at the same velocity, even in a switch. This is not an issue on a drag race vehicle as drag automobiles are traveling in a straight line 99% of the time. This may also be an edge for cars that are becoming set-up for drifting. A welded differential is a normal open differential that has got 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 would be advisable over a good differential. Every change a vehicle takes may cause the axles to wind-up and tire slippage. This is most visible when generating through a slower turn (parking). The effect is accelerated tire wear and also premature axle failing. One big benefit of the solid differential over the other styles is its strength. Since torque is applied right to each axle, there is absolutely no spider gears, which are the weak spot of open differentials.