Final wheel drive

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 last drive ratios could be between 3:1 and 4.5:1. It is because of this that the wheels by no means spin as fast as the engine (in virtually all applications) even though the transmission is in an overdrive gear. The final drive assembly is linked 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) application with the engine and transmission mounted in leading, the ultimate drive and differential assembly sit down in the trunk of the automobile and receive rotational torque from the transmission through a drive shaft. In RWD applications the final drive assembly receives input at a 90° position to the drive wheels. The final drive assembly must account for this to drive the trunk wheels. The objective of the differential is definitely to permit one input to drive 2 wheels as well as allow those driven tires 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 back wheels. It really is located in the housing which also could also enclose two axle shafts. Rotational torque is transferred to the final drive through a drive shaft that operates between the transmission and the final drive. The ultimate drive gears will consist of a pinion equipment and a ring gear. The pinion gear gets the rotational torque from the drive shaft and uses it to rotate the band gear. The pinion equipment is much smaller and includes a lower tooth count than the large ring equipment. This gives the driveline it’s final drive ratio.The driveshaft delivers rotational torque at a 90º angle to the direction that the wheels must rotate. The final drive makes up because of this with the way the pinion gear drives the ring gear within the housing. When installing or establishing a final drive, how the pinion equipment contacts the ring gear must be considered. Ideally the tooth contact should happen in the exact centre of the band gears the teeth, at moderate to complete load. (The gears force from eachother as load can be applied.) Many final drives are of a hypoid style, which means that the pinion gear sits below the centreline of the ring gear. This enables manufacturers to lower your body of the automobile (as the drive shaft sits lower) to increase aerodynamics and lower the vehicles centre of gravity. Hypoid pinion gear 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 communicate with the band gears teeth making the connection stronger and quieter. The band gear drives the differential, which drives the axles or axle shafts which are connected to the trunk wheels. (Differential procedure 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 last drive is external from the transmission, it requires its own oil for lubrication. That is typically plain gear oil but many hypoid or LSD final drives require a special kind of fluid. Make reference to the services manual for viscosity and other special requirements.

Note: If you’re likely to change your back diff liquid yourself, (or you intend on opening the diff up for support) before you let the fluid out, make certain the fill port can be opened. Nothing worse than letting fluid out and having no way to getting new fluid back in.
FWD last drives are very simple compared to RWD set-ups. Virtually all FWD engines are transverse installed, which means that rotational torque is established parallel to the path that the wheels must rotate. You don’t have to alter/pivot the path of rotation in the ultimate drive. The final drive pinion gear will sit on the finish of the output shaft. (multiple output shafts and pinion gears are feasible) The pinion gear(s) will mesh with the ultimate drive ring gear. In almost all instances the pinion and band gear could have helical cut tooth just like the rest of the transmitting/transaxle. The pinion equipment will be smaller and have a lower tooth count than the ring gear. This produces the final drive ratio. The ring gear will drive the differential. (Differential procedure will be explained in the differential section of this article) Rotational torque is delivered to the front wheels through CV shafts. (CV shafts are generally referred to as axles)
An open up differential is the most common type of differential found in passenger cars and trucks today. It is a simple (cheap) style that uses 4 gears (occasionally 6), that are known as spider gears, to operate a vehicle the axle shafts but also permit them to rotate at different speeds if necessary. “Spider gears” can be a slang term that is 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 casing) gets rotational torque through the ring gear and uses it to drive the differential pin. The differential pinion gears ride upon this pin and so are driven by it. Rotational torpue can be then used in the axle side gears and out through the CV shafts/axle shafts to the wheels. If the automobile is venturing in a directly line, there is no differential actions and the differential pinion gears will simply drive the axle side gears. If the automobile enters a switch, 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 external wheel to speed up and the inside wheel to slow down. This design is effective 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 while the wheel with traction won’t rotate at all. Because the wheel with traction is not rotating, the vehicle cannot move.
Limited-slide differentials limit the amount of differential action allowed. If one wheel begins spinning excessively faster than the other (way more than durring normal cornering), an LSD will limit the velocity difference. This is an advantage 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 invite the vehicle to move. There are several different designs currently in use today. Some work better than others based on the application.
Clutch style LSDs derive from a open up differential design. They have a separate clutch pack on each of the axle part gears or axle shafts inside 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 others are splined to the differential case. Friction material is used to split up the clutch discs. Springs place pressure on the axle part 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 than the differential case then the other will try to rotate slower. Both clutches will resist this step. As the velocity difference increases, it becomes harder to conquer the clutches. When the vehicle is making a good turn at low speed (parking), the clutches provide little resistance. When one drive wheel looses traction and all the torque would go to that wheel, the clutches resistance becomes much more obvious and the wheel with traction will rotate at (near) the velocity of the differential case. This type of differential will likely need a special type of liquid or some form of Final wheel drive additive. If the fluid is not changed at the proper intervals, the clutches can become less effective. Leading to little to no LSD action. Fluid change intervals differ between applications. There is nothing wrong with this design, but keep in mind that they are only 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 speed. The drive wheels often rotate at the same velocity, even in a turn. This is not an issue on a drag competition vehicle as drag vehicles are driving in a straight line 99% of the time. This may also be an edge for vehicles that are becoming set-up for drifting. A welded differential is a normal open differential that has acquired the spider gears welded to create a solid differential. Solid differentials certainly are a fine modification for vehicles designed for track use. As 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. That is most obvious when driving through a sluggish turn (parking). The result is accelerated tire use in addition to premature axle failing. One big advantage of the solid differential over the other types is its power. Since torque is used right to each axle, there is absolutely no spider gears, which are the weak point of open differentials.

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