The purpose of the ultimate drive gear assembly is to supply the ultimate stage of gear reduction to diminish RPM and increase rotational torque. Typical final drive ratios can be between 3:1 and 4.5:1. It really is due to this that the tires never spin as fast as the engine (in almost all applications) even though the transmission is within an overdrive gear. The final drive assembly is linked to the differential. In FWD (front-wheel drive) applications, the ultimate drive and differential assembly can be found inside the transmitting/transaxle case. In a typical RWD (rear-wheel drive) software with the engine and tranny mounted in the front, the ultimate 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 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 permit one input to drive 2 Final wheel drive wheels and also allow those driven tires to rotate at different speeds as a vehicle goes around a corner.
A RWD last drive sits in the trunk of the vehicle, between the two back wheels. It is located inside a housing which also could also enclose two axle shafts. Rotational torque is used in the ultimate drive through a drive shaft that runs between the transmission and the final drive. The final drive gears will consist of a pinion equipment and a ring gear. The pinion gear receives the rotational torque from the drive shaft and uses it to rotate the band gear. The pinion equipment is much smaller and includes a much lower tooth count compared to the large ring equipment. 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 the way the pinion gear drives the ring equipment in 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 specific centre of the band gears the teeth, at moderate to full load. (The gears drive from eachother as load is applied.) Many final drives are of a hypoid design, which implies that the pinion equipment sits below the centreline of the band gear. This enables manufacturers to lower the body of the car (because the drive shaft sits lower) to improve aerodynamics and lower the automobiles center of gravity. Hypoid pinion gear tooth are curved which in turn causes a sliding actions 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 which makes the connection stronger and quieter. The band gear drives the differential, which drives the axles or axle shafts which are linked to the rear wheels. (Differential procedure will be explained in the differential portion of this content) Many final drives home the axle shafts, others use CV shafts such as a FWD driveline. Since a RWD final drive is exterior from the transmission, it requires its own oil for lubrication. This is typically plain gear essential oil but many hypoid or LSD last drives require a special type of fluid. Refer to the program manual for viscosity and additional special requirements.
Note: If you are likely to change your rear diff fluid yourself, (or you plan on starting the diff up for service) 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 last drives are very simple compared to RWD set-ups. Virtually all FWD engines are transverse mounted, which means that rotational torque is established parallel to the path that the wheels must rotate. You don’t have to change/pivot the direction of rotation in the ultimate drive. The ultimate drive pinion equipment will sit on the end of the output shaft. (multiple output shafts and pinion gears are possible) The pinion gear(s) will mesh with the ultimate drive ring gear. In almost all instances the pinion and ring gear could have helical cut the teeth just like the rest of the tranny/transaxle. The pinion gear will be smaller sized and have a lower tooth count than the ring gear. This produces the ultimate drive ratio. The ring equipment will drive the differential. (Differential procedure will be explained in the differential section of this content) Rotational torque is sent 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 can be a simple (cheap) style that uses 4 gears (sometimes 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’s commonly used to spell it out 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) receives 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 so are driven because of it. Rotational torpue is certainly then used in the axle aspect gears and out through the CV shafts/axle shafts to the wheels. If the vehicle is venturing in a straight line, there is no differential actions and the differential pinion gears will simply drive the axle aspect gears. If the vehicle enters a switch, the outer wheel must rotate quicker compared to the inside wheel. The differential pinion gears will begin to rotate as they drive the axle aspect gears, allowing the outer wheel to increase and the inside wheel to slow down. This design works well as long as both of the powered wheels have got traction. If one wheel does not have enough traction, rotational torque will follow the path of least level of resistance and the wheel with small traction will spin while 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 actions allowed. If one wheel begins spinning excessively faster compared to the other (way more than durring regular cornering), an LSD will limit the acceleration difference. That is an benefit over a regular open differential style. If one drive wheel looses traction, the LSD actions will allow the wheel with traction to obtain rotational torque and invite the vehicle to go. There are several different designs currently used today. Some work better than others depending on the application.
Clutch style LSDs derive from a open up differential design. They have another clutch pack on each one of the axle side gears or axle shafts inside the final drive casing. Clutch discs sit down between your 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 pressure on the axle part gears which put pressure on the clutch. If an axle shaft really wants to spin quicker or slower than the differential case, it must get over the clutch to take action. If one axle shaft attempts to rotate faster than the differential case then the other will try to rotate slower. Both clutches will resist this action. As the rate difference increases, it turns into harder to get over the clutches. When the automobile is making a tight turn at low rate (parking), the clutches offer little resistance. When one drive wheel looses traction and all of the torque goes 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 most likely require a special type of fluid or some form of additive. If the fluid isn’t changed at the correct intervals, the clutches may become less effective. Leading to little to no LSD actions. Fluid change intervals vary between applications. There is nothing wrong with this design, but remember that they are only as strong as an ordinary open differential.
Solid/spool differentials are mostly used in drag racing. Solid differentials, like the name implies, are completely solid and will not allow any difference in drive wheel speed. The drive wheels constantly rotate at the same acceleration, even in a turn. This is not an issue on a drag competition vehicle as drag automobiles are driving in a directly line 99% of that time period. This can also be an edge for cars that are getting set-up for drifting. A welded differential is a normal open differential that has had the spider gears welded to create a solid differential. Solid differentials are a good modification for vehicles created for track use. For street use, a LSD option will be advisable over a good differential. Every switch a vehicle takes may cause the axles to wind-up and tire slippage. This is most apparent when generating through a slower turn (parking). The effect is accelerated tire wear as well as premature axle failing. One big benefit of the solid differential over the other types is its power. Since torque is used right to each axle, there is absolutely no spider gears, which are the weak point of open differentials.