The purpose of the final drive gear Final wheel drive assembly is to provide the ultimate stage of gear reduction to diminish RPM and increase rotational torque. Typical last drive ratios could be between 3:1 and 4.5:1. It is due to this that the tires by no means spin as fast as the engine (in virtually 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 transmitting/transaxle case. In a typical RWD (rear-wheel drive) application with the engine and transmission mounted in the front, the final drive and differential assembly sit in the rear of the automobile 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 wheels. The final drive assembly must account for this to drive the rear wheels. The objective of the differential is certainly to permit one input to drive 2 wheels as well as allow those driven wheels to rotate at different speeds as a car goes around a corner.
A RWD final drive sits in the rear of the automobile, between the two rear wheels. It is located inside a housing which also could also enclose two axle shafts. Rotational torque is transferred to the ultimate drive through a drive shaft that operates between the transmission and the ultimate drive. The final drive gears will consist of a pinion gear and a ring equipment. The pinion gear 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 equipment. Thus giving the driveline it’s last drive ratio.The driveshaft delivers rotational torque at a 90º angle to the direction that the wheels must rotate. The ultimate drive makes up because of this with what sort of pinion gear drives the ring gear in the housing. When setting up or setting up a final drive, the way the pinion equipment contacts the ring gear must be considered. Preferably the tooth contact should happen in the precise centre of the band gears tooth, at moderate to complete load. (The gears push away 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 band gear. This enables manufacturers to lower the body of the automobile (because the drive shaft sits lower) to increase aerodynamics and lower the automobiles centre of gravity. Hypoid pinion gear the teeth are curved which causes a sliding actions as the pinion equipment drives the ring equipment. It also causes multiple pinion equipment teeth to be in contact with the ring gears teeth making the connection more powerful and quieter. The ring equipment drives the differential, which drives the axles or axle shafts which are linked to the trunk 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 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 equipment oil but many hypoid or LSD final drives need a special kind of fluid. Make reference to the service manual for viscosity and additional special requirements.
Note: If you are going to change your rear diff fluid yourself, (or you intend on opening the diff up for assistance) before you allow fluid out, make certain the fill port can be opened. Nothing worse than letting fluid out and then having no way to getting new fluid back.
FWD last drives are very simple compared to RWD set-ups. Almost all FWD engines are transverse installed, which means that rotational torque is created parallel to the direction that the tires must rotate. There is no need to modify/pivot the direction of rotation in the ultimate drive. The final drive pinion equipment will sit on the finish of the output shaft. (multiple result shafts and pinion gears are possible) The pinion gear(s) will mesh with the final drive ring equipment. In almost all cases the pinion and ring gear will have helical cut tooth just like the rest of the transmitting/transaxle. The pinion gear will be smaller sized and have a lower tooth count compared to the ring equipment. This produces the final drive ratio. The ring equipment will drive the differential. (Differential procedure will be described in the differential section of this article) Rotational torque is sent to the front tires through CV shafts. (CV shafts are commonly known as axles)
An open up differential is the most common type of differential within passenger cars and trucks today. It is definitely a simple (cheap) style 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 required. “Spider gears” is usually 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 part gears. The differential case (not casing) receives rotational torque through the ring equipment and uses it to drive the differential pin. The differential pinion gears ride upon this pin and are driven because of it. Rotational torpue can be then used in the axle part gears and out through the CV shafts/axle shafts to the tires. If the vehicle is travelling in a directly line, there is absolutely no differential action and the differential pinion gears only will drive the axle aspect gears. If the automobile enters a turn, the outer wheel must rotate quicker than the inside wheel. The differential pinion gears will begin to rotate because they drive the axle aspect gears, allowing the external wheel to increase and the within wheel to decelerate. This design is effective provided that both of the driven wheels possess traction. If one wheel doesn’t have enough traction, rotational torque will observe the road of least 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 action allowed. If one wheel begins spinning excessively faster compared to the other (way more than durring regular cornering), an LSD will limit the swiftness difference. This is an benefit 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 invite the vehicle to go. There are many 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 possess another clutch pack on each one of the axle part 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 pressure on the axle aspect gears which put pressure on the clutch. If an axle shaft really wants to spin faster or slower compared to the differential case, it must get over the clutch to take action. If one axle shaft attempts to rotate quicker compared to the differential case then your other will try to rotate slower. Both clutches will withstand this action. As the velocity difference increases, it becomes harder to conquer the clutches. When the automobile is making a good turn at low velocity (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 rate of the differential case. This kind of differential will most likely require a special type of liquid or some kind of additive. If the liquid is not changed at the correct intervals, the clutches may become less effective. Leading to little to no LSD actions. Fluid change intervals differ between applications. There is certainly nothing incorrect with this design, but remember that they are only as strong as an ordinary open differential.
Solid/spool differentials are mostly used in drag racing. Solid differentials, like the name implies, are completely solid and will not really allow any difference in drive wheel speed. The drive wheels at all times rotate at the same speed, even in a change. 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 vehicles that are getting set-up for drifting. A welded differential is a normal open differential which has experienced the spider gears welded to create a solid differential. Solid differentials certainly are a great modification for vehicles designed for track use. As for street make use of, 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. That is most obvious when driving through a slower turn (parking). The effect is accelerated tire use in addition to 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 absolutely no spider gears, which are the weak point of open differentials.