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Final wheel drive

The purpose of the ultimate drive gear assembly is to supply the ultimate 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 because of this that the tires never spin as fast as the engine (in virtually all applications) even when 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 are located inside the transmitting/transaxle case. In a typical RWD (rear-wheel drive) program with the engine and transmission mounted in the front, the ultimate drive and differential assembly sit down 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 wheels. The ultimate drive assembly must account for this to drive the trunk wheels. The purpose of the differential is certainly to allow one input to drive 2 wheels and also allow those driven wheels to rotate at different Final wheel drive speeds as a car goes around a corner.
A RWD last drive sits in the rear 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 transferred to the ultimate drive through a drive shaft that runs between the transmission and the final 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. This gives 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 gear drives the ring gear within the housing. When setting up or establishing a final drive, how the pinion gear contacts the ring gear must be considered. Preferably the tooth get in touch with should happen in the exact centre of the band gears teeth, at moderate to complete load. (The gears force away from eachother as load is definitely applied.) Many last drives are of a hypoid style, which implies that the pinion gear sits below the centreline of the ring gear. This allows manufacturers to lower your body of the car (because the drive shaft sits lower) to increase aerodynamics and lower the automobiles center of gravity. Hypoid pinion gear the teeth are curved which causes a sliding action as the pinion equipment drives the ring gear. It also causes multiple pinion equipment teeth to be in contact with the ring gears teeth making the connection more powerful and quieter. The ring equipment drives the differential, which drives the axles or axle shafts which are connected to the rear wheels. (Differential procedure will be explained 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 final 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. Make reference to the provider manual for viscosity and other special requirements.

Note: If you are likely to change your back diff liquid yourself, (or you plan on opening the diff up for support) before you let the fluid out, make sure the fill port can be opened. Absolutely nothing worse than letting fluid out and having no way of getting new fluid back.
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 direction that the wheels must rotate. There is no need to modify/pivot the direction of rotation in the final drive. The final drive pinion equipment will sit on the end of the output shaft. (multiple output shafts and pinion gears are feasible) The pinion gear(s) will mesh with the ultimate drive ring equipment. In almost all cases the pinion and band gear will have helical cut the teeth just like the rest of the tranny/transaxle. The pinion gear will be smaller and have a much lower tooth count than the ring gear. This produces the ultimate drive ratio. The ring equipment will drive the differential. (Differential procedure will be described in the differential section of this article) Rotational torque is delivered 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 usually a simple (cheap) design that uses 4 gears (occasionally 6), that are known as spider gears, to drive the axle shafts but also permit them to rotate at different speeds if required. “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 aspect 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 ride upon this pin and are driven by it. Rotational torpue can be then transferred to the axle part gears and out through the CV shafts/axle shafts to the tires. If the vehicle is traveling in a directly line, there is absolutely no differential action and the differential pinion gears only will drive the axle part gears. If the vehicle enters a switch, the outer 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 outer wheel to increase and the inside 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 road of least level of resistance and the wheel with little traction will spin while the wheel with traction won’t rotate at all. Since 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 compared to the other (more so than durring normal cornering), an LSD will limit the velocity 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 get rotational torque and invite the vehicle to go. 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 possess a separate clutch pack on each one of the axle part gears or axle shafts inside 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 separate the clutch discs. Springs put strain on the axle aspect gears which put pressure 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 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 speed (parking), the clutches provide little resistance. When one drive wheel looses traction and all the torque would go to that whe
el, the clutches resistance becomes much more apparent and the wheel with traction will rotate at (close to) the swiftness of the differential case. This kind of differential will most likely require a special type of liquid or some form of additive. If the fluid isn’t changed at the proper intervals, the clutches can become less effective. Leading to small to no LSD actions. Fluid change intervals differ between applications. There can be 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, just like the name implies, are totally solid and will not really allow any difference in drive wheel quickness. The drive wheels constantly rotate at the same rate, even in a turn. This is not a concern on a drag race vehicle as drag vehicles are generating in a straight line 99% of the time. This may also be an edge for vehicles that are getting set-up for drifting. A welded differential is a regular open differential that has got the spider gears welded to create a solid differential. Solid differentials certainly are a fine modification for vehicles created for track use. For street use, a LSD option will be advisable over a good differential. Every convert a vehicle takes will cause the axles to wind-up and tire slippage. This is most visible when driving through a sluggish turn (parking). The effect is accelerated tire put on along with premature axle failure. One big advantage of the solid differential over the other types is its power. Since torque is used directly to each axle, there is absolutely no spider gears, which are the weak spot of open differentials.


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