The purpose of the ultimate drive gear assembly is to supply 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 really is because of this that the wheels by no means spin as fast as the engine (in almost all applications) even when the transmission is within an overdrive gear. The final 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 an average RWD (rear-wheel drive) program with the engine and tranny mounted in leading, the ultimate drive and differential assembly sit in the trunk of the vehicle and receive rotational torque from the transmission through a drive shaft. In RWD applications the final drive assembly receives input at a 90° angle to the drive tires. The final drive assembly must account for this to drive the rear wheels. The objective of the differential is definitely to allow one input to drive 2 wheels along with allow those driven wheels to rotate at different speeds as a vehicle goes around 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 could also enclose two axle shafts. Rotational torque is used in the ultimate drive through a drive shaft that runs between your transmission and the ultimate drive. The ultimate drive gears will contain a Final wheel drive pinion equipment and a ring equipment. The pinion gear gets the rotational torque from the drive shaft and uses it to rotate the ring gear. The pinion gear is a lot smaller and includes a much lower tooth count compared to the large ring equipment. 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 what sort of pinion gear drives the ring gear in the housing. When setting up or establishing a final drive, the way the pinion equipment contacts the ring gear must be considered. Preferably the tooth get in touch with should happen in the specific centre of the ring gears teeth, at moderate to complete load. (The gears force away from eachother as load is usually applied.) Many final drives are of a hypoid design, which implies that the pinion gear sits below the centreline of the ring gear. This allows manufacturers to lower the body of the automobile (as the drive shaft sits lower) to increase aerodynamics and lower the vehicles center of gravity. Hypoid pinion gear tooth 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 making the connection stronger and quieter. The band equipment drives the differential, which drives the axles or axle shafts which are linked to the rear wheels. (Differential operation will be explained in the differential section of this content) Many final drives home the axle shafts, others make use of CV shafts just like a FWD driveline. Since a RWD final drive is external from the tranny, it requires its own oil for lubrication. This is typically plain gear oil but many hypoid or LSD final drives require a special kind of fluid. Make reference to the support manual for viscosity and additional special requirements.

Note: If you’re likely to change your rear diff liquid yourself, (or you plan on opening the diff up for support) before you let the fluid out, make sure the fill port could be opened. Nothing worse than letting fluid out and then 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 means 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 ultimate drive. The ultimate drive pinion equipment will sit on the end of the result shaft. (multiple result shafts and pinion gears are feasible) The pinion equipment(s) will mesh with the ultimate drive ring equipment. In almost all cases the pinion and band gear will have helical cut teeth just like the remaining transmitting/transaxle. The pinion gear will be smaller sized and have a lower tooth count than the ring gear. This produces the ultimate drive ratio. The band equipment will drive the differential. (Differential operation will be described in the differential section of this article) Rotational torque is sent to the front tires through CV shafts. (CV shafts are generally known as axles)
An open up differential is the most common type of differential within passenger vehicles today. It is certainly a very simple (cheap) design that uses 4 gears (occasionally 6), that are known as spider gears, to drive the axle shafts but also allow them to rotate at different speeds if necessary. “Spider gears” is definitely 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 part gears. The differential case (not casing) gets rotational torque through the ring gear and uses it to operate a vehicle 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 wheels. If the vehicle is venturing in a directly line, there is no differential actions and the differential pinion gears only will drive the axle aspect gears. If the vehicle enters a turn, the outer wheel must rotate faster compared to the inside wheel. The differential pinion gears will begin to rotate because they drive the axle part gears, allowing the outer wheel to speed up and the within wheel to slow down. This design works well so long as both of the driven wheels have traction. If one wheel does not have enough traction, rotational torque will observe the road of least resistance and the wheel with little traction will spin as the wheel with traction will not rotate at all. Because the wheel with traction is not rotating, the automobile cannot move.
Limited-slip differentials limit the quantity of differential action allowed. If one wheel starts spinning excessively faster than the other (more so than durring normal cornering), an LSD will limit the acceleration difference. This is an advantage over a normal open differential style. If one drive wheel looses traction, the LSD actions will allow the wheel with traction to get rotational torque and invite the vehicle to go. There are many different designs currently in use today. Some work better than others based on the application.
Clutch style LSDs derive from a open differential design. They have another clutch pack on each one of the axle aspect gears or axle shafts within the final drive housing. Clutch discs sit between the 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 strain on the axle side gears which put pressure on the clutch. If an axle shaft wants to spin faster or slower than the differential case, it must overcome the clutch to take action. If one axle shaft tries to rotate quicker compared to the differential case then the other will try to rotate slower. Both clutches will withstand this action. As the velocity difference increases, it becomes harder to get over the clutches. When the vehicle is making a tight turn at low speed (parking), the clutches provide little level of resistance. When one drive wheel looses traction and all of the torque goes to that wheel, the clutches level of resistance becomes a lot 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 fluid or some type of additive. If the fluid isn’t changed at the correct intervals, the clutches may become less effective. Leading to small to no LSD action. Fluid change intervals vary between applications. There is definitely nothing incorrect 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, like the name implies, are totally solid and will not really allow any difference in drive wheel velocity. The drive wheels always rotate at the same swiftness, even in a switch. This is not a concern on a drag competition vehicle as drag automobiles are traveling in a straight line 99% of the time. This may also be an advantage for cars that are becoming 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 certainly are a fine modification for vehicles made for track use. For street use, a LSD option will be advisable over a good differential. Every switch a vehicle takes may cause the axles to wind-up and tire slippage. That is most visible when traveling through a sluggish turn (parking). The result is accelerated tire use as well as premature axle failure. One big benefit of the solid differential over the other types is its power. Since torque is applied directly to each axle, there is absolutely no spider gears, which will be the weak spot of open differentials.