The purpose of the ultimate drive gear assembly is to provide 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 due to this that the wheels never spin as fast as the engine (in virtually all applications) even when 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 Final wheel drive differential assembly are located inside the tranny/transaxle case. In an average RWD (rear-wheel drive) application with the engine and transmission mounted in leading, the final drive and differential assembly sit in the rear of the vehicle and receive rotational torque from the transmitting through a drive shaft. In RWD applications the ultimate drive assembly receives insight at a 90° position to the drive wheels. The final drive assembly must take into account 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 wheels to rotate at different speeds as a car goes around a corner.
A RWD last drive sits in the rear of the vehicle, between the two back wheels. It really is located inside a housing which also could also enclose two axle shafts. Rotational torque is used in the final drive through a drive shaft that runs between your transmission and the final drive. The ultimate drive gears will contain a pinion gear and a ring gear. 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 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 direction that the wheels must rotate. The final drive makes up for this with the way the pinion gear drives the ring equipment in the housing. When installing or setting up a final drive, how the pinion gear contacts the ring equipment must be considered. Preferably the tooth contact should happen in the precise centre of the ring gears teeth, at moderate to complete load. (The gears press from eachother as load is applied.) Many final 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 car (as the drive shaft sits lower) to improve aerodynamics and lower the vehicles centre of gravity. Hypoid pinion gear the teeth are curved which causes a sliding actions as the pinion equipment drives the ring equipment. It also causes multiple pinion equipment teeth to be in contact with the ring gears teeth making the connection stronger and quieter. The ring gear drives the differential, which drives the axles or axle shafts which are connected to the trunk wheels. (Differential procedure will be explained in the differential portion of this content) Many final drives house the axle shafts, others make use of CV shafts like a FWD driveline. Since a RWD last drive is external from the tranny, it requires its own oil for lubrication. This is typically plain equipment oil but many hypoid or LSD final drives need a special type of fluid. Make reference to the program manual for viscosity and additional special requirements.
Note: If you’re going to change your rear diff fluid yourself, (or you plan on starting the diff up for provider) before you allow fluid out, make certain the fill port can be opened. Nothing worse than letting liquid out and having no way of getting new fluid back.
FWD last drives are extremely simple in comparison 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. You don’t have to alter/pivot the direction of rotation in the ultimate drive. The ultimate drive pinion gear will sit on the end of the result shaft. (multiple output shafts and pinion gears are feasible) The pinion equipment(s) will mesh with the final drive ring gear. In almost all instances the pinion and ring gear will have helical cut the teeth just like the rest of the transmitting/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 ring gear will drive the differential. (Differential operation will be explained in the differential section of this content) Rotational torque is sent to the front wheels through CV shafts. (CV shafts are commonly referred to as axles)
An open differential is the most typical type of differential within passenger cars and trucks today. It is definitely a very simple (cheap) design that uses 4 gears (occasionally 6), that are known as spider gears, to operate a vehicle the axle shafts but also allow them to rotate at different speeds if necessary. “Spider gears” is 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 housing) gets rotational torque through the band gear and uses it to drive the differential pin. The differential pinion gears ride upon this pin and are driven because of it. Rotational torpue is definitely then used in the axle side gears and out through the CV shafts/axle shafts to the tires. If the vehicle is travelling in a directly line, there is no differential action and the differential pinion gears will simply drive the axle side gears. If the automobile enters a convert, the outer wheel must rotate quicker compared to the inside wheel. The differential pinion gears will start to rotate because they drive the axle side gears, allowing the external wheel to speed up and the inside wheel to slow down. This design works well provided that both of the driven 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 small traction will spin as 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 amount of differential actions allowed. If one wheel starts spinning excessively faster compared to the other (more so than durring regular cornering), an LSD will limit the quickness difference. This is an benefit over a regular open differential design. If one drive wheel looses traction, the LSD action will allow the wheel with traction to get rotational torque and allow the vehicle to go. There are several different designs currently used today. Some work 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 side gears or axle shafts in the final drive casing. 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 separate the clutch discs. Springs place strain on the axle part gears which put pressure on the clutch. If an axle shaft wants to spin quicker or slower than the differential case, it must get over the clutch to do so. If one axle shaft tries to rotate faster than the differential case then the other will try to rotate slower. Both clutches will withstand this action. As the rate difference increases, it becomes harder to conquer the clutches. When the automobile is making a good turn at low rate (parking), the clutches offer little level of resistance. When one drive wheel looses traction and all of the torque goes to that wheel, the clutches resistance becomes much more obvious and the wheel with traction will rotate at (near) the speed of the differential case. This type of differential will most likely need a special type of fluid or some type of additive. If the liquid is not changed at the proper intervals, the clutches may become less effective. Resulting in small 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, just like the name implies, are totally solid and will not really enable any difference in drive wheel speed. The drive wheels usually rotate at the same velocity, even in a convert. This is not a concern on a drag competition vehicle as drag automobiles are traveling in a directly line 99% of that time period. This can also be an advantage for vehicles that are becoming set-up for drifting. A welded differential is a regular open differential which has had the spider gears welded to create a solid differential. Solid differentials certainly are a fine modification for vehicles created for track use. As for street use, a LSD option will be advisable over a solid differential. Every switch a vehicle takes may cause the axles to wind-up and tire slippage. That is most obvious when driving through a slower turn (parking). The result is accelerated tire wear in addition to premature axle failure. One big benefit of the solid differential over the other styles is its strength. Since torque is used directly to each axle, there is absolutely no spider gears, which will be the weak point of open differentials.