The purpose of the ultimate drive gear assembly is to supply the final stage of gear reduction to decrease RPM and increase rotational torque. Typical last drive ratios can be between 3:1 and 4.5:1. It is because of 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 ultimate drive and differential assembly are located inside the transmitting/transaxle case. In an average RWD (rear-wheel drive) app with the engine and transmission mounted in leading, the final drive and differential assembly sit in the rear of the vehicle and receive rotational torque from the transmission 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 trunk wheels. The objective of the differential is usually to allow one input to drive 2 wheels as well as 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 back 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 operates between your transmission and the final 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 gear is a lot smaller and has a much lower tooth count compared to the large ring gear. This gives the driveline it’s final drive ratio.The driveshaft delivers rotational torque at a 90º angle to the path that the wheels must rotate. The ultimate drive makes up for this with the way the pinion equipment Final wheel drive drives the ring gear within the housing. When installing or establishing a final drive, how the pinion gear contacts the ring gear must be considered. Ideally the tooth get in touch with should happen in the exact centre of the ring gears tooth, at moderate to full load. (The gears press from eachother as load is definitely applied.) Many last drives are of a hypoid design, which means that the pinion equipment sits below the centreline of the band gear. This enables manufacturers to lower the body of the automobile (because the drive shaft sits lower) to improve aerodynamics and lower the vehicles centre of gravity. Hypoid pinion equipment the teeth are curved which in turn causes a sliding action as the pinion gear drives the ring gear. It also causes multiple pinion gear teeth to be in contact with the band gears teeth which makes the connection more powerful and quieter. The ring equipment 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 make use of CV shafts just like a FWD driveline. Since a RWD final drive is exterior from the tranny, it requires its own oil for lubrication. That is typically plain equipment oil but many hypoid or LSD final drives require a special kind of fluid. Refer to the program manual for viscosity and other special requirements.
Note: If you’re going to change your rear diff fluid yourself, (or you plan on opening the diff up for services) before you allow fluid out, make sure the fill port can be opened. Nothing worse than letting fluid out and having no way to getting new fluid back in.
FWD final 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. You don’t have to change/pivot the path of rotation in the ultimate drive. The final drive pinion gear will sit on the finish of the result shaft. (multiple output shafts and pinion gears are feasible) The pinion gear(s) will mesh with the ultimate drive ring gear. In almost all situations the pinion and ring gear will have helical cut teeth just like the remaining transmission/transaxle. The pinion gear will be smaller sized and have a much lower tooth count than the ring gear. This produces the final drive ratio. The band equipment will drive the differential. (Differential operation will be explained in the differential portion of this article) Rotational torque is sent to the front wheels through CV shafts. (CV shafts are generally referred to as axles)
An open differential is the most common type of differential found in passenger vehicles today. It is usually a very simple (cheap) style that uses 4 gears (sometimes 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 a slang term that is commonly used to spell it out all 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) receives rotational torque through the band gear and uses it to drive the differential pin. The differential pinion gears trip upon this pin and so are driven by it. Rotational torpue is usually then transferred to the axle side gears and out through the CV shafts/axle shafts to the wheels. If the vehicle is venturing in a straight line, there is absolutely no differential action and the differential pinion gears only will drive the axle side gears. If the vehicle enters a change, the outer wheel must rotate quicker than the inside wheel. The differential pinion gears will begin to rotate because they drive the axle part gears, allowing the outer wheel to increase and the inside wheel to slow down. This design is effective provided that both of the driven wheels possess traction. If one wheel does not have enough traction, rotational torque will observe the path of least level of resistance and the wheel with little traction will spin while the wheel with traction won’t rotate at all. Since the wheel with traction is not rotating, the vehicle cannot move.
Limited-slide differentials limit the amount of differential action allowed. If one wheel starts spinning excessively faster compared to the other (way more than durring regular cornering), an LSD will limit the rate difference. That is an benefit over a normal open differential design. If one drive wheel looses traction, the LSD action will allow the wheel with traction to obtain rotational torque and invite the vehicle to move. There are several different designs currently in use today. Some are better than others depending on the application.
Clutch style LSDs are based on a open up differential design. They have a separate clutch pack on each one of the axle part gears or axle shafts within the final drive casing. 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 place pressure on the axle side gears which put strain on the clutch. If an axle shaft really wants to spin quicker or slower compared to the differential case, it must get over the clutch to do so. If one axle shaft tries to rotate quicker than the differential case then your other will attempt to rotate slower. Both clutches will resist this step. As the swiftness difference increases, it becomes harder to conquer the clutches. When the automobile is making a tight turn at low velocity (parking), the clutches offer little resistance. When one drive wheel looses traction and all the torque would go to that wheel, the clutches 
resistance becomes a lot more apparent and the wheel with traction will rotate at (close to) the quickness of the differential case. This type of differential will likely need a special type of liquid or some kind of additive. If the fluid isn’t changed at the proper intervals, the clutches can become less effective. Leading to little to no LSD action. Fluid change intervals differ between applications. There is nothing incorrect with this design, but remember that they are only as strong as a plain open differential.
Solid/spool differentials are mostly used in drag racing. Solid differentials, just like the name implies, are completely solid and will not allow any difference in drive wheel speed. The drive wheels generally rotate at the same quickness, even in a convert. This is not a concern on a drag race vehicle as drag vehicles are traveling in a directly line 99% of the time. This may also be an edge for vehicles that are getting 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 are a fine modification for vehicles made for track use. For street use, a LSD option will be advisable over a good differential. Every turn a vehicle takes will cause the axles to wind-up and tire slippage. That is most noticeable when traveling through a sluggish turn (parking). The result is accelerated tire wear and also premature axle failing. One big advantage of the solid differential over the other styles is its strength. Since torque is used directly to each axle, there is absolutely no spider gears, which will be the weak spot of open differentials.