The purpose of the final drive gear assembly is to supply the ultimate stage of gear reduction to diminish RPM and increase rotational torque. Typical final 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 within an overdrive gear. The final drive assembly is connected to the differential. In FWD (front-wheel drive) applications, the ultimate drive and differential assembly can be found inside the transmitting/transaxle case. In a typical RWD (rear-wheel drive) software with the engine and transmission mounted in the front, the final drive and differential assembly sit in the rear 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 wheels. The final drive assembly must account for this to drive the trunk wheels. The purpose of the differential can be to permit one input to drive 2 wheels as well as allow those driven tires to rotate at different speeds as a vehicle goes around a corner.
A RWD final drive sits in the trunk of the vehicle, between the two rear wheels. It is located inside a housing which also could also enclose two axle shafts. Rotational torque is used in the final drive through a drive shaft that operates between your transmission and the ultimate drive. The ultimate drive gears will consist of a pinion equipment and a ring gear. The pinion gear receives the rotational torque from the drive shaft and uses it to rotate the band gear. The pinion gear is a lot smaller and includes a lower tooth count compared to the large ring equipment. 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 gear drives the ring equipment in the housing. When setting up or establishing a final drive, how the pinion gear contacts the ring gear must be Final wheel drive considered. Preferably the tooth get in touch with should happen in the precise centre of the band gears tooth, at moderate to full load. (The gears push away from eachother as load is certainly applied.) Many final drives are of a hypoid design, which implies that the pinion equipment sits below the centreline of the band gear. This allows manufacturers to lower your body of the automobile (because the drive shaft sits lower) to improve aerodynamics and lower the automobiles centre of gravity. Hypoid pinion equipment the teeth are curved which in turn causes a sliding actions as the pinion equipment drives the ring equipment. In addition, it causes multiple pinion gear teeth to be in contact with the band gears teeth which makes the connection stronger and quieter. The band gear drives the differential, which drives the axles or axle shafts which are linked to the rear wheels. (Differential operation will be described in the differential portion of this content) Many final drives house the axle shafts, others make use of CV shafts such as a FWD driveline. Since a RWD final drive is exterior from the transmission, it requires its own oil for lubrication. This is typically plain gear oil but many hypoid or LSD last drives require a special type of fluid. Refer to the assistance manual for viscosity and additional special requirements.

Note: If you are going to change your rear diff fluid yourself, (or you plan on starting the diff up for service) before you let the fluid out, make sure the fill port can be opened. Nothing worse than letting fluid out and having no way of getting new fluid back.
FWD last drives are very simple compared to RWD set-ups. Virtually all FWD engines are transverse mounted, which means that rotational torque is created parallel to the path that the wheels must rotate. There is no need to change/pivot the direction of rotation in the ultimate drive. The ultimate drive pinion gear will sit on the finish of the result shaft. (multiple result shafts and pinion gears are feasible) The pinion gear(s) will mesh with the ultimate drive ring gear. In almost all cases the pinion and ring gear could have helical cut tooth just like the remaining tranny/transaxle. The pinion equipment will be smaller sized and have a 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 section of this content) Rotational torque is sent to the front tires through CV shafts. (CV shafts are generally referred to 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 referred to as spider gears, to drive the axle shafts but also permit them to rotate at different speeds if necessary. “Spider gears” is a slang term that’s commonly used to spell it out all the differential gears. There are two different types of spider gears, the differential pinion gears and the axle aspect gears. The differential case (not casing) receives rotational torque through the band equipment and uses it to drive the differential pin. The differential pinion gears trip upon this pin and so 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 wheels. If the vehicle is traveling in a straight line, there is no differential actions and the differential pinion gears will simply drive the axle part gears. If the vehicle enters a change, the external wheel must rotate quicker compared to the inside wheel. The differential pinion gears will start to rotate because they drive the axle aspect gears, allowing the outer wheel to increase and the inside wheel to slow down. This design works well so long as both of the driven wheels have got traction. If one wheel does not have enough traction, rotational torque will follow the path of least level of resistance and the wheel with little traction will spin while the wheel with traction will not rotate at all. Because the wheel with traction is not rotating, the vehicle cannot move.
Limited-slip differentials limit the amount 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 swiftness difference. That is an benefit over a regular open differential style. 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 several different designs currently used today. Some work better than others depending on the application.
Clutch style LSDs derive from a open up differential design. They have another clutch pack on each one of the axle side gears or axle shafts inside 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 materials is used to separate the clutch discs. Springs put strain on the axle part gears which put strain on the clutch. If an axle shaft really wants to spin quicker or slower than the differential case, it must get over the clutch to take action. If one axle shaft attempts to rotate faster than the differential case then your other will attempt to rotate slower. Both clutches will resist 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 velocity (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 resistance becomes a lot more apparent and the wheel with traction will rotate at (close to) the acceleration of the differential case. This type of differential will most likely require a special type of fluid or some form of additive. If the fluid isn’t changed at the correct intervals, the clutches can become less effective. Leading to small to no LSD actions. Fluid change intervals vary between applications. There is usually nothing wrong with this design, but remember 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 speed. The drive wheels constantly rotate at the same rate, even in a turn. This is not an issue on a drag competition vehicle as drag automobiles are driving in a straight line 99% of that time period. This can also be an edge for cars that are getting set-up for drifting. A welded differential is a regular open differential which has had the spider gears welded to create a solid differential. Solid differentials are a good modification for vehicles designed for track use. For street use, a LSD option would be advisable over a good differential. Every switch a vehicle takes may cause the axles to wind-up and tire slippage. This is most apparent when traveling through a slower turn (parking). The effect is accelerated tire wear as well as premature axle failing. One big benefit of the solid differential over the other types is its strength. Since torque is used right to each axle, there is absolutely no spider gears, which are the weak point of open differentials.