The purpose of the final drive gear assembly is to supply the final stage of gear reduction to decrease RPM and increase rotational torque. Final wheel drive Typical last drive ratios could be between 3:1 and 4.5:1. It is because of this that the wheels never spin as fast as the engine (in virtually all applications) even though the transmission is in an overdrive gear. The ultimate drive assembly is connected to the differential. In FWD (front-wheel drive) applications, the ultimate drive and differential assembly are located inside the tranny/transaxle case. In an average RWD (rear-wheel drive) software with the engine and tranny mounted in the front, the ultimate drive and differential assembly sit down in the trunk of the automobile and receive rotational torque from the transmitting through a drive shaft. In RWD applications the final drive assembly receives insight at a 90° angle to the drive wheels. The final drive assembly must take into account this to drive the trunk wheels. The purpose of the differential is definitely to allow 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 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 runs between the transmission and the ultimate drive. The final drive gears will contain a pinion equipment and a ring equipment. The pinion equipment 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. Thus giving the driveline it’s last 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 equipment drives the ring gear within the housing. When setting up or setting up a final drive, the way the pinion gear contacts the ring gear must be considered. Preferably the tooth get in touch with should happen in the specific centre of the band gears the teeth, at moderate to complete load. (The gears drive from eachother as load is usually applied.) Many final drives are of a hypoid design, which means that the pinion equipment sits below the centreline of the band gear. This allows manufacturers to lower the body of the car (as the drive shaft sits lower) to improve aerodynamics and lower the vehicles centre of gravity. Hypoid pinion equipment teeth are curved which in turn causes a sliding action as the pinion gear drives the ring equipment. In addition, it causes multiple pinion equipment teeth to communicate with the ring 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 trunk wheels. (Differential procedure will be described in the differential section of this content) Many final drives house the axle shafts, others make use of CV shafts just like a FWD driveline. Since a RWD final drive is exterior from the transmitting, it requires its own oil for lubrication. That is typically plain equipment oil but many hypoid or LSD final drives need a special type of fluid. Make reference to the assistance manual for viscosity and other special requirements.

Note: If you are likely to change your back diff liquid yourself, (or you intend on starting the diff up for service) before you allow fluid out, make sure the fill port can be opened. Nothing worse than letting liquid out and having no way of getting new fluid back.
FWD final 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 alter/pivot the path of rotation in the ultimate drive. The final drive pinion gear will sit on the end of the output shaft. (multiple result shafts and pinion gears are possible) The pinion equipment(s) will mesh with the ultimate drive ring equipment. In almost all instances the pinion and ring gear could have helical cut teeth just like the remaining transmitting/transaxle. The pinion equipment will be smaller sized and have a lower tooth count compared to the ring equipment. This produces the final drive ratio. The band equipment will drive the differential. (Differential operation will be explained in the differential portion of this article) Rotational torque is delivered to the front tires through CV shafts. (CV shafts are commonly known as axles)
An open differential is the most common type of differential within passenger vehicles today. It can be a simple (cheap) style that uses 4 gears (occasionally 6), that are referred to as spider gears, to operate a vehicle the axle shafts but also allow them to rotate at different speeds if necessary. “Spider gears” is usually a slang term that is 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 side gears. The differential case (not housing) gets rotational torque through the band gear and uses it to drive the differential pin. The differential pinion gears trip on this pin and so are driven by it. Rotational torpue is usually then used in the axle aspect gears and out through the CV shafts/axle shafts to the tires. If the automobile is travelling in a directly line, there is absolutely no differential actions and the differential pinion gears will simply drive the axle part gears. If the automobile enters a convert, the external wheel must rotate faster compared to the inside wheel. The differential pinion gears will start to rotate because they drive the axle part gears, allowing the outer wheel to increase and the within wheel to slow down. This design is effective 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 level of resistance and the wheel with little traction will spin while the wheel with traction will not rotate at all. Since the wheel with traction isn’t rotating, the automobile cannot move.
Limited-slide differentials limit the quantity 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. That is an advantage over a normal open differential design. If one drive wheel looses traction, the LSD action allows the wheel with traction to obtain rotational torque and allow the vehicle to go. There are several different designs currently in use today. Some are better than others based on the application.
Clutch style LSDs are based on a open differential design. They have a separate clutch pack on each one of the axle aspect gears or axle shafts within 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 the others are splined to the differential case. Friction material is used to split up the clutch discs. Springs place pressure on the axle aspect gears which put strain on the clutch. If an axle shaft really wants to spin faster or slower than the differential case, it must conquer the clutch to do so. If one axle shaft attempts to rotate quicker compared to the differential case then your other will try to rotate slower. Both clutches will resist this step. As the speed difference increases, it becomes harder to conquer the clutches. When the vehicle is making a tight turn at low rate (parking), the clutches offer 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 obvious and the wheel with traction will rotate at (close to) the swiftness of the differential case. This type of differential will likely require a special type of fluid or some form of additive. If the liquid is not changed at the correct intervals, the clutches can become less effective. Leading to small to no LSD actions. Fluid change intervals differ between applications. There is nothing wrong with this style, but remember that they are just as strong as a plain open differential.
Solid/spool differentials are mostly used in drag racing. Solid differentials, just like the name implies, are totally solid and will not enable any difference in drive wheel quickness. The drive wheels always rotate at the same quickness, even in a change. This is not an issue on a drag race vehicle as drag vehicles are traveling in a straight line 99% of the time. This can also be an edge for cars that are being set-up for drifting. A welded differential is a normal open differential that has experienced the spider gears welded to create a solid differential. Solid differentials certainly are a fine modification for vehicles made for track use. For street use, a LSD option would be advisable over a good differential. Every change a vehicle takes will cause the axles to wind-up and tire slippage. That is most obvious when traveling through a sluggish turn (parking). The result is accelerated tire wear along with premature axle failing. One big advantage of the solid differential over the other styles is its strength. Since torque is used directly to each axle, there is no spider gears, which will be the weak point of open differentials.