The purpose of the final drive gear assembly is to provide the ultimate stage of gear reduction to decrease 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 wheels by no means spin as fast as the engine (in almost all applications) even though the transmission is in an overdrive gear. The final drive assembly is connected 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) app with the engine and transmitting mounted in leading, the final drive and differential assembly sit in the rear of the automobile and receive rotational torque from the transmitting through a drive shaft. In RWD applications the ultimate drive assembly receives input at a 90° angle to the drive tires. The ultimate drive assembly must account for this to drive the trunk wheels. The purpose of the differential is certainly to permit one input to drive 2 wheels along with allow those driven tires to rotate at different speeds as a car encircles a corner.
A RWD final drive sits in the rear of the vehicle, between the two back wheels. It 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 your transmission and the final drive. The final drive gears will contain a pinion gear and a ring gear. The pinion equipment receives the rotational torque from the drive shaft and uses it to rotate the ring gear. The pinion equipment is a lot smaller and has a lower tooth count than the large ring gear. Thus giving 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 final drive makes up because of this with the way the pinion gear drives the ring equipment within 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 specific centre of the band gears teeth, at moderate to full load. (The gears press away from eachother as load is certainly applied.) Many final 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 car (because the drive shaft sits lower) to increase aerodynamics and lower the automobiles center of gravity. Hypoid pinion equipment the teeth are curved which in turn causes a sliding action as the pinion equipment drives the ring equipment. It also causes multiple pinion gear 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 linked to the rear wheels. (Differential operation will be described in the differential portion of this article) Many final drives home the axle shafts, others make use of CV shafts like a FWD driveline. Since a RWD last drive is exterior from the transmitting, it requires its own oil for lubrication. This is typically plain equipment oil but many hypoid or LSD final drives need a special type of fluid. Make reference to the support manual for viscosity and additional special requirements.

Note: If you’re going to change your rear diff fluid yourself, (or you intend on starting the diff up for provider) before you let the fluid out, make sure the fill port could be opened. Absolutely nothing worse than letting fluid out and having no way of getting new fluid back.
FWD last drives are very simple in comparison to RWD set-ups. Virtually all FWD engines are transverse installed, which implies that rotational torque is created parallel to the direction that the wheels must rotate. You don’t have to change/pivot the direction of rotation in the final drive. The ultimate drive pinion equipment will sit on the end of the result shaft. (multiple output shafts and pinion gears are possible) The pinion equipment(s) will mesh with the final drive ring gear. In almost all situations the pinion and ring gear will have helical cut the teeth just like the remaining tranny/transaxle. The pinion equipment will be smaller and have a much lower tooth count compared to the ring gear. This produces the final drive ratio. The band gear will drive the differential. (Differential procedure will be explained in the differential section of this content) Rotational torque is delivered 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 certainly a simple (cheap) design that uses 4 gears (sometimes 6), that are known as spider gears, to drive the axle shafts but also permit them to rotate at different speeds if necessary. “Spider gears” is definitely 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 side gears. The differential case (not housing) receives rotational torque through the ring equipment and uses it to operate a vehicle the differential pin. The differential pinion gears trip upon this pin and so are driven by it. Rotational torpue is definitely then used in 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 absolutely no differential actions and the differential pinion gears will simply drive the axle part gears. If the vehicle enters a turn, the external wheel must rotate quicker than the inside wheel. The differential pinion gears will begin to rotate because they drive the axle side gears, allowing the external wheel to increase and the within wheel to slow down. This design works well provided that both of the driven wheels possess traction. If one wheel doesn’t have enough traction, rotational torque will follow the road of least resistance and the wheel with small traction will spin while the wheel with traction won’t rotate at all. Because the wheel with traction isn’t rotating, the vehicle cannot move.
Limited-slide differentials limit the amount of differential actions allowed. If one wheel starts spinning excessively faster than the other (more so than durring normal cornering), an LSD will limit the swiftness difference. That is an advantage over a regular open differential design. If one drive wheel looses traction, the LSD actions allows the wheel with traction to get 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 one of the axle aspect gears or axle shafts in the final drive casing. Clutch discs sit between your 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 material is used to split up the clutch discs. Springs place strain on the axle side gears which put strain on the clutch. If an axle shaft wants to spin faster or slower than the differential case, it must get over the clutch to do so. If one axle shaft tries to rotate quicker than the differential case then the other will attempt to rotate slower. Both clutches will withstand this step. As the swiftness difference increases, it becomes harder to conquer the clutches. When the automobile is making a tight turn at low acceleration (Final wheel drive 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 speed of the differential case. This kind of differential will most likely require a special type of liquid or some type of additive. If the liquid is not changed at the proper intervals, the clutches can become less effective. Resulting in small to no LSD action. Fluid change intervals differ between applications. There is usually nothing incorrect with this design, but keep in mind that they are just as strong as an ordinary open differential.
Solid/spool differentials are mostly found in drag racing. Solid differentials, like the name implies, are completely solid and will not allow any difference in drive wheel velocity. The drive wheels always rotate at the same quickness, even in a turn. This is not a concern on a drag race vehicle as drag automobiles are driving in a straight line 99% of the time. This can also be an edge for cars that are getting 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 are a fine modification for vehicles created for track use. For street use, a LSD option would be advisable over a solid differential. Every convert a vehicle takes will cause the axles to wind-up and tire slippage. That is most obvious when driving through a gradual turn (parking). The result is accelerated tire use as well as premature axle failing. One big advantage of the solid differential over the other types is its strength. Since torque is applied directly to each axle, there is absolutely no spider gears, which are the weak spot of open differentials.