The purpose of the final drive gear Final wheel drive assembly is to provide the final stage of gear reduction to decrease RPM and increase rotational torque. Typical final drive ratios can be between 3:1 and 4.5:1. It really is because of this that the wheels never spin as fast as the engine (in almost 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 final drive and differential assembly are located inside the transmitting/transaxle case. In an average RWD (rear-wheel drive) application with the engine and tranny mounted in leading, the final drive and differential assembly sit down in the rear of the automobile and receive rotational torque from the transmitting through a drive shaft. In RWD applications the final drive assembly receives input at a 90° angle to the drive wheels. The final drive assembly must account for this to drive the trunk wheels. The purpose of the differential can be to permit one input to operate a vehicle 2 wheels in addition to allow those driven tires to rotate at different speeds as a vehicle encircles 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 may also enclose two axle shafts. Rotational torque is used in the ultimate drive through a drive shaft that operates between your transmission and the final drive. The final drive gears will consist of a pinion gear and a ring equipment. The pinion gear receives the rotational torque from the drive shaft and uses it to rotate the ring gear. The pinion equipment is much smaller and has a much lower tooth count than the large ring gear. This gives 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 establishing a final drive, how the pinion equipment contacts the ring gear must be considered. Preferably the tooth get in touch with should happen in the specific centre of the band gears tooth, at moderate to full load. (The gears drive away from eachother as load is usually applied.) Many last drives are of a hypoid design, which means that the pinion gear sits below the centreline of the band gear. This enables manufacturers to lower the body of the automobile (as the drive shaft sits lower) to improve aerodynamics and lower the automobiles center of gravity. Hypoid pinion gear tooth are curved which causes a sliding actions as the pinion equipment drives the ring gear. It also causes multiple pinion equipment teeth to communicate with the band gears teeth making the connection stronger and quieter. The ring equipment 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 portion of this article) Many final drives house the axle shafts, others make use of CV shafts like a FWD driveline. Since a RWD final drive is external from the transmitting, it requires its oil for lubrication. That is typically plain equipment oil but many hypoid or LSD final drives require a special kind of fluid. Make reference to the assistance manual for viscosity and other special requirements.

Note: If you’re going to change your back diff fluid yourself, (or you intend on starting the diff up for services) before you let the fluid out, make sure the fill port can be opened. Nothing worse than letting fluid out and having no way of getting new fluid back.
FWD last drives are extremely simple in comparison to RWD set-ups. Virtually all FWD engines are transverse mounted, which implies that rotational torque is established parallel to the path that the wheels must rotate. You don’t have to modify/pivot the path of rotation in the final drive. The ultimate drive pinion equipment will sit on the finish of the result shaft. (multiple output shafts and pinion gears are possible) The pinion gear(s) will mesh with the final drive ring equipment. In almost all cases the pinion and ring gear will have helical cut the teeth just like the rest of the transmission/transaxle. The pinion gear will be smaller and have a much lower tooth count than 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 generally referred to as axles)
An open up differential is the most common type of differential within passenger vehicles today. It is certainly a simple (cheap) design that uses 4 gears (sometimes 6), that are referred to as spider gears, to drive the axle shafts but also allow them to rotate at different speeds if required. “Spider gears” is usually a slang term that is commonly used to describe 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 casing) gets rotational torque through the band equipment and uses it to operate a vehicle the differential pin. The differential pinion gears ride upon this pin and are driven because of it. Rotational torpue is certainly then transferred to the axle side gears and out through the CV shafts/axle shafts to the tires. If the automobile is traveling in a directly line, there is no differential action and the differential pinion gears only will drive the axle part gears. If the automobile enters a convert, the external wheel must rotate quicker than the inside wheel. The differential pinion gears will start to rotate because they drive the axle side gears, allowing the outer wheel to speed up and the within wheel to decelerate. This design is effective so long as both of the driven wheels possess traction. If one wheel does not have enough traction, rotational torque will observe the road of least level of resistance and the wheel with little traction will spin as the wheel with traction won’t rotate at all. Because the wheel with traction isn’t rotating, the automobile cannot move.
Limited-slide differentials limit the amount of differential action allowed. If one wheel begins spinning excessively faster than the other (more so than durring regular cornering), an LSD will limit the speed difference. That is an advantage over a normal open differential design. If one drive wheel looses traction, the LSD actions allows the wheel with traction to obtain rotational torque and allow the vehicle to go. There are several different designs currently used today. Some work better than others depending on the application.
Clutch style LSDs are based on a open differential design. They possess another clutch pack on each of the axle side 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 the others are splined to the differential case. Friction materials is used to split up the clutch discs. Springs put pressure on the axle side gears which put strain on the clutch. If an axle shaft really wants to spin quicker or slower compared to the differential case, it must get over the clutch to take action. If one axle shaft attempts to rotate quicker than the differential case then your other will attempt to rotate slower. Both clutches will withstand this step. As the quickness difference increases, it turns into harder to overcome the clutches. When the automobile is making a tight turn at low velocity (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 apparent and the wheel with traction will rotate at (near) the quickness of the differential case. This kind of differential will likely require a special type of fluid or some type of additive. If the fluid is not changed at the proper intervals, the clutches may become less effective. Leading to little to no LSD actions. Fluid change intervals differ between applications. There can be 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 used in drag racing. Solid differentials, like the name implies, are totally solid and will not really enable any difference in drive wheel rate. The drive wheels constantly rotate at the same acceleration, 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 that time period. This may also be an edge for cars that are being 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 certainly are a great modification for vehicles created for track use. As for street make use of, a LSD option would be advisable over a solid differential. Every turn a vehicle takes will cause the axles to wind-up and tire slippage. This is most noticeable when traveling through a sluggish turn (parking). The result is accelerated tire wear and also premature axle failure. One big advantage of the solid differential over the other types is its power. Since torque is applied directly to each axle, there is absolutely no spider gears, which will be the weak point of open differentials.