The purpose of the ultimate drive gear assembly is to provide the final stage of gear reduction to diminish RPM and increase rotational torque. 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 almost all applications) even though the transmission is in 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 transmission/transaxle case. In a typical RWD (rear-wheel drive) application with the engine and tranny mounted in the front, the final drive and differential assembly sit in the trunk of the automobile and receive rotational torque from the transmitting through a drive shaft. In RWD applications the final drive assembly receives insight at a 90° position to the drive tires. 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 as well as allow those driven tires to rotate at different speeds as a vehicle goes around a corner.
A RWD last drive sits in the rear of the vehicle, between the two back wheels. It is located in the housing which also could also enclose two axle shafts. Rotational torque is used in the ultimate 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 equipment gets the rotational torque from the drive shaft and uses it to rotate the band gear. The pinion equipment 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 provides rotational torque at a 90º angle to the path that the wheels must rotate. The final drive makes up for this with what sort of pinion gear drives the ring gear in the housing. When installing or setting up a final drive, the way the pinion gear contacts the ring equipment must be considered. Ideally the tooth contact should happen in the specific centre of the ring gears tooth, at moderate to complete load. (The gears force from eachother as load is usually applied.) Many last drives are of a hypoid design, which means that the pinion equipment sits below the centreline of the ring gear. This allows manufacturers to lower your body of the car (as the drive shaft sits lower) to increase aerodynamics and lower the automobiles centre of gravity. Hypoid pinion gear teeth are curved which in turn causes a sliding action as the pinion gear drives the ring gear. It also causes multiple pinion equipment teeth to communicate with the band gears teeth making the connection stronger and quieter. The ring equipment drives the differential, which drives the axles or axle shafts which are linked to the rear wheels. (Differential procedure will be described in the differential portion of this content) Many final drives home the axle shafts, others make use of CV shafts like a FWD driveline. Since a RWD last drive is external from the transmission, it requires its oil for lubrication. That is typically plain gear essential oil but many hypoid or LSD last drives require a special type of fluid. Make reference to the provider manual for viscosity and additional special requirements.

Note: If you are going to change your rear diff liquid yourself, (or you intend on starting the diff up for services) before you allow fluid out, make certain the fill port could 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 means that rotational torque is established parallel to the direction that the wheels must rotate. You don’t have to modify/pivot the path of rotation in the ultimate drive. The ultimate drive pinion equipment 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 instances the pinion and band gear could have helical cut the 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 ultimate drive ratio. The band gear will drive the differential. (Differential operation will be described in the differential section of this article) Rotational torque is sent to the front wheels through CV shafts. (CV shafts are commonly known as axles)
An open differential is the most typical type of differential found in passenger cars and trucks today. It is certainly a very simple (cheap) style that uses 4 gears (occasionally 6), that are known as spider gears, to operate a vehicle the axle shafts but also allow them to rotate at different speeds if required. “Spider gears” is a slang term that is commonly used to spell it out all of the differential gears. There are two different types of spider gears, the differential pinion gears and the axle aspect gears. The differential case (not housing) gets rotational torque through the ring equipment and uses it to drive the differential pin. The differential pinion gears ride on this pin and so are driven by it. Rotational torpue is then transferred to the axle side gears and out through the CV shafts/axle shafts to the tires. If the automobile is venturing in a directly line, there is no differential action and the differential pinion gears only will drive the axle aspect gears. If the vehicle enters a switch, the external wheel must rotate quicker compared to the inside wheel. The differential pinion gears will begin to rotate because they drive the axle side gears, allowing the external wheel to speed up and the inside wheel to decelerate. This design is effective as long as both of the powered wheels have got traction. If one wheel does not have enough traction, rotational torque will follow the road of least level of resistance and the wheel with little traction will spin while the wheel with traction will not rotate at all. Since the wheel with traction is not rotating, the automobile cannot move.
Limited-slip differentials limit the quantity of differential action allowed. If one wheel begins spinning excessively faster than the other (way more than durring regular cornering), an LSD will limit the quickness difference. This is an benefit over a regular open differential design. If one drive wheel looses traction, the LSD action will allow the wheel with traction to get rotational torque and allow the vehicle to go. There are several 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 side gears or axle shafts within the final drive casing. Clutch discs sit between the axle shafts’ splines and the differential case. Half of the discs are splined to the axle shaft and others are splined to the differential case. Friction materials is used to split up the clutch discs. Springs put strain on the axle aspect gears which put pressure on the clutch. If an axle shaft 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 your other will try to rotate slower. Both clutches will resist this action. As the quickness difference increases, it becomes harder to overcome the clutches. When the automobile is making a tight turn at low rate (parking), the clutches provide little resistance. When one drive wheel looses traction and all of the torque would go to that wheel, the clutches resistance becomes much more obvious and the wheel with traction will rotate at (close to) the acceleration of the differential case. This type of differential will likely require a special type of fluid or some form of additive. If the fluid isn’t changed at the proper intervals, the clutches can become less effective. Leading to little to no LSD actions. Fluid change intervals Final wheel drive differ between applications. There is definitely nothing incorrect with this design, but keep in mind that they are just as strong as an ordinary open differential.
Solid/spool differentials are mostly found in drag racing. Solid differentials, like the name implies, are totally solid and will not enable any difference in drive wheel rate. The drive wheels constantly rotate at the same swiftness, even in a convert. This is not a concern on a drag race vehicle as drag vehicles are driving in a straight line 99% of the time. This can also be an edge for cars that are getting set-up for drifting. A welded differential is a regular open differential which has had the spider gears welded to create a solid differential. Solid differentials are a fine modification for vehicles created for track use. As for street make use of, a LSD option will be advisable over a solid differential. Every turn a vehicle takes may cause the axles to wind-up and tire slippage. This is most noticeable when traveling through a slower turn (parking). The result is accelerated tire put on as well as premature axle failing. One big advantage of the solid differential over the other styles is its strength. Since torque is applied right to each axle, there is no spider gears, which are the weak point of open differentials.