The purpose of the ultimate drive gear assembly is to provide the final stage of gear reduction to decrease RPM and increase rotational torque. Typical final drive ratios could be between 3:1 and 4.5:1. It is due to this that the wheels never 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 ultimate drive and differential assembly are located inside the transmitting/transaxle case. In an average RWD (rear-wheel drive) program with the engine and transmission mounted in leading, the ultimate drive and differential assembly sit down in the trunk of the vehicle and receive rotational torque from the tranny 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 trunk wheels. The purpose of the differential is to allow one input to drive 2 wheels and also allow those driven wheels to rotate at different speeds as a car goes around a corner.
A RWD last drive sits in the trunk of the vehicle, between the two rear wheels. It really is located inside a housing which also may also enclose two axle shafts. Rotational torque is used in the ultimate drive through a drive shaft that runs between the transmission and the ultimate drive. The ultimate drive gears will consist of a 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 equipment is much smaller and includes a much lower tooth count than the large ring gear. Thus giving the driveline it’s final drive ratio.The driveshaft provides 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 equipment drives the ring gear within the housing. When setting up or establishing a final drive, the way the pinion gear contacts the ring gear must be considered. Preferably the tooth get in touch with should happen in the precise centre of the band gears the teeth, at moderate to full load. (The gears force from eachother as load is definitely applied.) Many final drives are of a hypoid design, which implies that the pinion gear sits below the centreline of the ring gear. This enables manufacturers to lower your body of the car (because the drive shaft sits lower) to improve aerodynamics and lower the vehicles center 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 communicate with the ring gears teeth making the connection more powerful and quieter. The band equipment drives the differential, which drives the axles or axle shafts which are connected to the rear wheels. (Differential procedure will be described in the differential section of this content) Many final drives house the axle shafts, others make use of CV shafts like a FWD driveline. Since a RWD last drive is exterior from the tranny, it requires its oil for lubrication. This is typically plain equipment essential oil but many hypoid or LSD final drives need a special type of fluid. Refer to the service manual for viscosity and other special requirements.

Note: If you’re likely to change your back diff liquid yourself, (or you plan on opening the diff up for services) before you let the fluid out, make sure the fill port could be opened. Nothing worse than letting fluid out and having no way of getting new fluid back in.
FWD final drives are very simple compared to RWD set-ups. Almost all FWD engines are transverse installed, which means that rotational torque is established parallel to the path that the wheels must rotate. There is no need to alter/pivot the direction of rotation in the final drive. The ultimate drive pinion equipment will sit on the end of the output shaft. (multiple result shafts and pinion gears are feasible) The pinion gear(s) will mesh with the ultimate drive ring equipment. In almost all instances the pinion and band gear will have helical cut the teeth just like the rest of the transmitting/transaxle. The pinion gear will be smaller and have a much lower tooth count compared to the ring gear. This produces the ultimate drive ratio. The ring equipment will drive the differential. (Differential procedure will be described in the differential portion of this article) Rotational torque is sent to the front tires through CV shafts. (CV shafts are generally known as axles)
An open differential is the most common type of differential within passenger vehicles today. It is definitely a simple (cheap) style that uses 4 gears (sometimes 6), that are known as spider gears, to operate a vehicle 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 various kinds of spider gears, the differential pinion gears and the axle aspect gears. The differential case (not housing) receives rotational torque through the band gear and uses it to operate a vehicle the differential pin. The differential pinion gears ride upon this pin and are driven because of it. Rotational torpue is certainly then used in the axle part gears and out through the CV shafts/axle shafts to the tires. If the automobile is traveling in a directly line, there is absolutely no differential actions and the differential pinion gears only will drive the axle side gears. If the vehicle enters a convert, the external wheel must rotate faster compared to the inside wheel. The differential pinion gears will start to rotate because they drive the axle side gears, allowing the external wheel to increase and the within wheel to decelerate. This design works well provided that both of the powered wheels have traction. If one wheel doesn’t have enough traction, rotational torque will follow the path of least resistance and the wheel with little traction will spin as 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 actions allowed. If one wheel starts spinning excessively faster compared to the other (way more than durring normal cornering), an LSD will limit the rate difference. This is an benefit over a normal open differential style. If one drive wheel looses traction, the LSD actions allows the wheel with traction to obtain rotational torque and allow the vehicle to go. There are several different designs currently used today. Some work better than others based on the application.
Clutch style LSDs derive from a open up differential design. They possess a separate clutch pack on each one of the axle side gears or axle shafts in the final drive casing. 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 put strain on the axle aspect gears which put strain on the clutch. If an axle shaft wants to spin quicker or slower than the differential case, it must conquer the clutch to take action. If one axle shaft tries to rotate quicker compared to the differential case then your other will try to rotate slower. Both clutches will withstand this step. As the swiftness difference increases, it becomes harder to get over the clutches. When the vehicle is making a good turn at low velocity (Final wheel drive parking), the clutches provide little resistance. When one drive wheel looses traction and all of the torque would go to that wheel, the clutches level of resistance becomes much more apparent and the wheel with traction will rotate at (near) the swiftness of the differential case. This kind of differential will most likely need a special type of liquid or some form of additive. If the liquid isn’t changed at the proper intervals, the clutches may become less effective. Resulting in little to no LSD action. Fluid change intervals vary between applications. There is certainly nothing incorrect with this style, but keep in mind that they are just 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 swiftness. The drive wheels often rotate at the same velocity, even in a convert. This is not an issue on a drag race vehicle as drag automobiles are driving in a straight line 99% of that time period. This may also be an advantage for vehicles that are becoming set-up for drifting. A welded differential is a regular open differential which has got the spider gears welded to create 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 solid differential. Every turn a vehicle takes will cause the axles to wind-up and tire slippage. This is most visible when driving through a slower turn (parking). The result is accelerated tire put on along with 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.