The purpose of the final drive gear assembly is to provide the ultimate stage of gear reduction to diminish RPM and increase rotational torque. Final wheel drive typical final drive ratios can be between 3:1 and 4.5:1. It really is due to this that the wheels never spin as fast as the engine (in virtually 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 ultimate drive and differential assembly can be found inside the transmission/transaxle case. In a typical RWD (rear-wheel drive) software with the engine and transmission mounted in leading, the final drive and differential assembly sit down in the trunk of the vehicle and receive rotational torque from the transmission through a drive shaft. In RWD applications the ultimate drive assembly receives insight at a 90° position to the drive tires. The ultimate drive assembly must take into account this to drive the trunk wheels. The purpose of the differential is definitely 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 last 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 ultimate drive through a drive shaft that operates between your transmission and the final drive. The ultimate drive gears will contain 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 includes a much lower tooth count compared to the large ring gear. 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 ultimate drive makes up for this with what sort of pinion gear drives the ring gear inside the housing. When installing or establishing a final drive, the way the pinion gear contacts the ring gear must be considered. Ideally the tooth get in touch with should happen in the precise centre of the ring gears teeth, at moderate to complete load. (The gears push away from eachother as load is definitely applied.) Many final drives are of a hypoid style, which means that the pinion gear sits below the centreline of the ring gear. This allows manufacturers to lower your body of the automobile (as the drive shaft sits lower) to increase aerodynamics and lower the automobiles center of gravity. Hypoid pinion equipment teeth are curved which causes a sliding actions as the pinion gear drives the ring equipment. In addition, it causes multiple pinion gear teeth to communicate with the band gears teeth which makes the connection more powerful and quieter. The band gear drives the differential, which drives the axles or axle shafts which are connected to the trunk wheels. (Differential operation will be described in the differential portion of this article) Many final drives house the axle shafts, others make use of CV shafts like a FWD driveline. Since a RWD final drive is external from the tranny, it requires its oil for lubrication. This is typically plain equipment oil but many hypoid or LSD last drives require a special type of fluid. Refer to the service manual for viscosity and other special requirements.

Note: If you’re going to change your back diff liquid yourself, (or you plan on opening the diff up for provider) before you allow fluid out, make certain the fill port could be opened. Absolutely nothing worse than letting liquid out and having no way of 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 established parallel to the direction that the wheels must rotate. There is no need to alter/pivot the path of rotation in the ultimate drive. The final drive pinion equipment will sit on the finish of the result shaft. (multiple output shafts and pinion gears are feasible) The pinion equipment(s) will mesh with the ultimate drive ring gear. In almost all situations the pinion and ring gear will have helical cut the teeth just like the rest of the transmitting/transaxle. The pinion gear will be smaller sized and have a much lower tooth count compared to the ring equipment. This produces the ultimate drive ratio. The ring equipment will drive the differential. (Differential operation will be described in the differential section of this article) Rotational torque is delivered 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 vehicles today. It can be 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 allow them to rotate at different speeds if necessary. “Spider gears” is certainly a slang term that is commonly used to describe all of the differential gears. There are two different types of spider gears, the differential pinion gears and the axle part gears. The differential case (not casing) gets 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 definitely then used in the axle part gears and out through the CV shafts/axle shafts to the wheels. If the automobile is venturing in a straight line, there is absolutely no differential actions and the differential pinion gears will simply drive the axle side gears. If the automobile enters a turn, the external wheel must rotate quicker than the inside wheel. The differential pinion gears will start to rotate because they drive the axle side gears, allowing the outer wheel to speed up and the within wheel to decelerate. This design works well provided that both of the driven wheels have got traction. If one wheel does not have enough traction, rotational torque will follow the path of least resistance and the wheel with little traction will spin while the wheel with traction will not rotate at all. Because the wheel with traction isn’t rotating, the vehicle 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 regular cornering), an LSD will limit the speed difference. This is an benefit over a normal open differential style. If one drive wheel looses traction, the LSD action will allow the wheel with traction to get rotational torque and allow the vehicle to move. There are many different designs currently in use today. Some work better than others depending 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 within 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 separate the clutch discs. Springs put pressure on the axle side gears which put strain on the clutch. If an axle shaft wants to spin faster or slower compared to 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 rate difference increases, it becomes harder to overcome the clutches. When the vehicle is making a tight turn at low velocity (parking), the clutches offer little resistance. When one drive wheel looses traction and all of 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 (close to) the swiftness of the differential case. This kind 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. Resulting in small to no LSD actions. Fluid change intervals vary between applications. There is definitely nothing wrong with this design, but keep in mind that they are only as strong as a plain open differential.
Solid/spool differentials are mostly found in drag racing. Solid differentials, like the name implies, are completely solid and will not allow any difference in drive wheel rate. The drive wheels usually rotate at the same swiftness, even in a change. This is not a concern on a drag competition vehicle as drag automobiles are driving in a directly line 99% of that time period. This can also be an edge for vehicles that are becoming set-up for drifting. A welded differential is a regular open differential that has acquired the spider gears welded to make 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 solid differential. Every switch a vehicle takes may cause the axles to wind-up and tire slippage. This is most apparent when generating through a slower turn (parking). The result is accelerated tire put on in addition to premature axle failure. One big advantage of the solid differential over the other types is its strength. Since torque is used right to each axle, there is no spider gears, which are the weak point of open differentials.