The purpose of the final 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 wheels by no means spin as fast as the engine (in almost all applications) even though the transmission is within an overdrive gear. The ultimate drive assembly is connected to the differential. In FWD (front-wheel drive) applications, the ultimate drive and differential assembly can be found inside the tranny/transaxle case. In an average RWD (rear-wheel drive) app with the engine and tranny mounted in leading, the final drive and differential assembly sit in the trunk of the automobile and receive rotational torque from the transmission through a drive shaft. In RWD applications the ultimate drive assembly receives input at a 90° position to the drive wheels. The ultimate drive assembly must take into account this to drive the trunk wheels. The purpose of the differential is usually to permit one input to operate a vehicle 2 wheels as well as allow those driven tires to rotate at different speeds as a car goes around a corner.
A RWD final drive sits in the rear of the automobile, 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 final drive through a drive shaft that operates between the transmission and the ultimate drive. The ultimate drive gears will consist of a pinion gear and a ring gear. The pinion equipment gets the rotational torque from the drive shaft and uses it to rotate the band gear. The pinion equipment is a lot smaller and includes a much lower tooth count compared to the large ring gear. Thus giving the driveline it’s final drive ratio.The driveshaft provides rotational torque at a 90º angle to the path that the wheels must rotate. The ultimate drive makes up because of this with the way the pinion equipment drives the ring gear in the housing. When setting up or establishing a final drive, how the pinion equipment contacts the ring equipment must be considered. Preferably the tooth contact should happen in the specific centre of the band gears tooth, at moderate to full load. (The gears force away from eachother as load is definitely applied.) Many last 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 (as the drive shaft sits lower) to increase aerodynamics and lower the automobiles centre of gravity. Hypoid pinion equipment teeth are curved which causes a sliding action 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 making the connection more powerful and quieter. The ring gear drives the differential, which drives the axles or axle shafts which are linked to the rear wheels. (Differential procedure will be described in the differential section of this content) Many final drives house the axle shafts, others use CV shafts like a FWD driveline. Since a RWD final drive is exterior from the transmitting, it requires its oil for lubrication. This is typically plain equipment essential oil but many hypoid or LSD final drives require a special type of fluid. Make reference to the services manual for viscosity and various other special requirements.
Note: If you’re going to change your rear diff liquid yourself, (or you intend on opening the diff up for program) before you let the fluid out, make certain the fill port can be opened. Absolutely nothing worse than letting liquid out and having no way of getting new fluid back in.
FWD last drives are extremely simple compared to RWD set-ups. Virtually all FWD engines are transverse installed, which means that rotational torque is created parallel to the direction that the tires must rotate. You don’t have to change/pivot the direction of rotation in the final drive. The ultimate drive pinion gear will sit on the finish of the output shaft. (multiple result shafts and pinion gears are feasible) The pinion gear(s) will mesh with the final drive ring gear. In almost all situations the pinion and band gear could have helical cut the teeth just like the remaining tranny/transaxle. The pinion gear will be smaller sized and have a much lower tooth count compared to the ring gear. This produces the ultimate drive ratio. The band equipment will drive the differential. (Differential procedure will be explained in the differential portion of this article) Rotational torque is sent to the front tires through CV shafts. (CV shafts are generally referred to as axles)
An open up differential is the most typical type of differential within passenger cars and trucks today. It is certainly a simple (cheap) style 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 required. “Spider gears” is definitely 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) gets rotational torque through the ring equipment and uses it to drive the differential pin. The differential pinion gears trip on this pin and are driven by it. Rotational torpue is definitely then used in the axle aspect gears and out through the CV shafts/axle shafts to the wheels. If the vehicle is traveling in a directly line, there is absolutely no differential action and the differential pinion gears will simply drive the axle aspect gears. If the vehicle enters a switch, the external wheel must rotate faster compared to the inside wheel. The differential pinion gears will start to rotate as they drive the axle side gears, allowing the outer 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 does not have enough traction, rotational torque will follow the road of least Final wheel drive resistance and the wheel with small traction will spin as the wheel with traction will not rotate at all. Because the wheel with traction isn’t rotating, the vehicle cannot move.
Limited-slide differentials limit the amount of differential action allowed. If one wheel starts spinning excessively faster compared to the other (more so than durring regular cornering), an LSD will limit the acceleration difference. This is an benefit over a normal open differential design. If one drive wheel looses traction, the LSD actions will allow the wheel with traction to obtain rotational torque and invite the vehicle to move. There are many different designs currently in use today. Some are better than others depending on the application.
Clutch style LSDs are based on a open up differential design. They have another clutch pack on each of the axle part gears or axle shafts within 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 the others are splined to the differential case. Friction materials is used to separate the clutch discs. Springs place strain on the axle part gears which put pressure on the clutch. If an axle shaft really wants to spin quicker or slower compared to the differential case, it must conquer the clutch to do so. If one axle shaft tries to rotate faster than the differential case then the other will attempt to rotate slower. Both clutches will resist this action. As the swiftness difference increases, it becomes harder to overcome the clutches. When the vehicle is making a tight turn at low rate (parking), the clutches provide little level of resistance. When one drive wheel looses traction and all 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 velocity of the differential case. This type of differential will likely need a special type of fluid or some type of additive. If the liquid isn’t changed at the proper intervals, the clutches may become less effective. Leading to little to no LSD actions. Fluid change intervals vary between applications. There is certainly nothing wrong with this style, 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 enable any difference in drive wheel swiftness. The drive wheels generally rotate at the same quickness, even in a change. This is not a concern on a drag competition vehicle as drag automobiles are generating in a straight line 99% of the time. This may also be an edge for cars that are becoming set-up for drifting. A welded differential is a normal open differential that has acquired the spider gears welded to make a solid differential. Solid differentials are a good modification for vehicles created for track use. For street use, a LSD option would 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 traveling through a sluggish turn (parking). The result is accelerated tire use and also premature axle failing. One big benefit of the solid differential over the other styles is its strength. Since torque is used right to each axle, there is no spider gears, which will be the weak spot of open differentials.