The purpose of the final drive gear assembly is to provide the ultimate stage of gear reduction to diminish RPM and increase rotational torque. Typical last drive ratios can 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 in 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 transmitting/transaxle case. In a typical RWD (rear-wheel drive) application with the engine and transmitting mounted in the front, the final drive and differential assembly sit in the rear of the automobile and receive rotational torque from the tranny through a drive shaft. In RWD applications the final drive assembly receives insight at a 90° position to the drive wheels. The ultimate 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 in addition to allow those driven wheels to rotate at different speeds as a vehicle encircles a corner.
A RWD final drive sits in the trunk of the automobile, between the two back wheels. It really is located in the housing which also may also enclose two axle shafts. Rotational torque is used in the ultimate drive through a drive shaft that operates between the transmission and the ultimate drive. The final drive gears will contain a Final wheel drive 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 gear is much smaller and has a much lower tooth count than the large ring gear. This gives 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 ultimate drive makes up for this with the way the pinion gear drives the ring gear in the housing. When installing or establishing a final drive, the way the pinion gear contacts the ring gear must be considered. Preferably the tooth contact should happen in the exact centre of the band gears the teeth, at moderate to full load. (The gears drive away from eachother as load is certainly applied.) Many final drives are of a hypoid style, which implies that the pinion gear sits below the centreline of the ring gear. This allows manufacturers to lower the body of the car (as the drive shaft sits lower) to increase aerodynamics and lower the vehicles centre of gravity. Hypoid pinion equipment tooth are curved which causes a sliding action as the pinion equipment drives the ring equipment. In addition, it causes multiple pinion equipment teeth to be in contact with the band gears teeth making 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 procedure will be explained in the differential portion of this content) Many final drives house the axle shafts, others make use of CV shafts just like a FWD driveline. Since a RWD last drive is exterior from the tranny, it requires its oil for lubrication. This is typically plain gear oil but many hypoid or LSD final drives require a special type of fluid. Make reference to the assistance manual for viscosity and various other special requirements.

Note: If you are going to change your rear diff fluid yourself, (or you intend on starting the diff up for service) before you allow fluid out, make certain the fill port can be opened. Absolutely nothing worse than letting fluid out and 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 mounted, which implies that rotational torque is created parallel to the path that the tires must rotate. You don’t have to alter/pivot the path of rotation in the final drive. The ultimate drive pinion equipment will sit on the finish of the output shaft. (multiple result shafts and pinion gears are possible) The pinion equipment(s) will mesh with the final drive ring equipment. In almost all situations the pinion and band gear could have helical cut teeth just like the rest of the tranny/transaxle. The pinion gear will be smaller and have a much lower tooth count than the ring gear. This produces the ultimate drive ratio. The band equipment will drive the differential. (Differential operation will be explained in the differential section of this article) 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 within passenger cars and trucks today. It is usually a simple (cheap) style that uses 4 gears (sometimes 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 certainly a slang term that’s commonly used to describe all of the differential gears. There are two different types of spider gears, the differential pinion gears and the axle side gears. The differential case (not housing) gets rotational torque through the ring gear and uses it to drive the differential pin. The differential pinion gears trip upon this pin and are driven by it. Rotational torpue is certainly then used in the axle aspect gears and out through the CV shafts/axle shafts to the tires. If the vehicle is traveling in a directly line, there is absolutely no differential actions and the differential pinion gears will simply drive the axle side gears. If the automobile enters a convert, the outer wheel must rotate quicker compared to the inside wheel. The differential pinion gears will start to rotate because they drive the axle part gears, allowing the external wheel to increase and the within wheel to decelerate. This design works well as long as both of the driven wheels have traction. If one wheel does not have enough traction, rotational torque will follow the road of least level of 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 (way more than durring regular cornering), an LSD will limit the rate difference. That is an advantage over a regular open differential style. If one drive wheel looses traction, the LSD actions will allow the wheel with traction to obtain rotational torque and allow the vehicle to go. There are several different designs currently in use today. Some are better than others depending on the application.
Clutch style LSDs derive from a open up 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 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 place pressure on the axle part 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 tries to rotate faster compared to the differential case then your other will attempt to rotate slower. Both clutches will withstand this step. As the acceleration difference increases, it turns into harder to get over the clutches. When the vehicle is making a good turn at low acceleration (parking), the clutches offer little level of resistance. When one drive wheel looses traction and all of 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 kind of differential will most likely need a special type of liquid or some kind of additive. If the fluid isn’t changed at the correct intervals, the clutches may become less effective. Leading to little to no LSD action. Fluid change intervals vary between applications. There is nothing wrong with this style, but keep in mind that they are just as strong as a plain open differential.
Solid/spool differentials are mostly found in drag racing. Solid differentials, just like the name implies, are totally solid and will not really enable any difference in drive wheel rate. The drive wheels always rotate at the same velocity, even in a change. This is not an issue on a drag competition vehicle as drag automobiles are generating in a straight line 99% of the time. This can also be an edge for vehicles that are being set-up for drifting. A welded differential is a regular open differential that has had the spider gears welded to make a solid differential. Solid differentials certainly are a good modification for vehicles designed for track use. As for street use, a LSD option would be advisable over a solid differential. Every convert a vehicle takes may cause the axles to wind-up and tire slippage. That is most apparent when generating through a slower turn (parking). The effect is accelerated tire wear as well as premature axle failing. One big advantage of the solid differential over the other styles is its power. Since torque is applied directly to each axle, there is absolutely no spider gears, which are the weak point of open differentials.