Final Drives and Differential Basics

The purpose of the final drive gear 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 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 connected to the differential. In FWD (front-wheel drive) applications, the final drive and differential assembly are located inside the transmission/transaxle case. In a typical RWD (rear-wheel drive) application with the engine and transmission mounted in the front, the final drive and differential assembly sit in the rear of the vehicle and receive rotational torque from the transmission 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 rear wheels. The purpose of the differential is to allow one input to drive 2 wheels as well as allow those driven wheels to rotate at different speeds as a vehicle goes around a corner.

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RWD Final Drives

A RWD final drive sits in the rear of the vehicle, between the two rear wheels. It is located inside a housing which also may also enclose two axle shafts. Rotational torque is transferred to the final drive through a drive shaft that runs between the transmission and the final drive. The final drive gears will consist of 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 gear is much smaller and has a much lower tooth count than the large ring gear. This gives the driveline it’s final drive ratio.The driveshaft delivers 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 gear inside the housing. When installing or setting up a final drive, how the pinion gear contacts the ring gear must be considered. Ideally the tooth contact should happen in the exact centre of the ring gears teeth, at moderate to full load. (The gears push away 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 ring gear. This allows manufacturers to lower the body of the car (because the drive shaft sits lower) to increase aerodynamics and lower the vehicles centre of gravity. Hypoid pinion gear teeth are curved which causes a sliding action as the pinion gear drives the ring gear. It also causes multiple pinion gear teeth to be in contact with the ring gears teeth which makes the connection stronger and quieter. The ring gear drives the differential, which drives the axles or axle shafts which are connected to the rear wheels. (Differential operation will be explained in the differential section of this article) Many final drives house the axle shafts, others use CV shafts like a FWD driveline. Since a RWD final drive is external from the transmission, it requires its own oil for lubrication. This is typically plain gear oil but many hypoid or LSD final drives require a special type of fluid. Refer to the service manual for viscosity and other special requirements.

Note: If you are going to change your rear diff fluid yourself, (or you plan on opening the diff up for service) before you let the fluid out, make sure the fill port can be opened. Nothing worse than letting fluid out and then having no way of getting new fluid back in.

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FWD Final Drives

FWD final drives are very simple compared to RWD set-ups. Almost all FWD engines are transverse mounted, which means that rotational torque is created parallel to the direction that the wheels must rotate. There is no need to change/pivot the direction of rotation in the final drive. The final drive pinion gear 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 final drive ring gear. In almost all cases the pinion and ring gear will have helical cut 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 gear. This produces the final drive ratio. The ring gear will drive the differential. (Differential operation will be explained in the differential section of this article) Rotational torque is delivered to the front wheels through CV shafts. (CV shafts are commonly referred to as axles)

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Open Differentials

An open differential is the most common type of differential found in passenger cars and trucks today. It is a very 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 necessary. “Spider gears” is a slang term that is commonly used to describe all of 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) receives rotational torque through the ring gear and uses it to drive the differential pin. The differential pinion gears ride on this pin and are driven by it. Rotational torpue is then transferred to the axle side gears and out through the CV shafts/axle shafts to the wheels. If the vehicle is travelling in a straight line, there is no differential action and the differential pinion gears will simply drive the axle side gears. If the vehicle enters a turn, the outer wheel must rotate faster than the inside wheel. The differential pinion gears will start to rotate as they drive the axle side gears, allowing the outer wheel to speed up and the inside wheel to slow down. This design works well as long as both of the driven wheels have traction. If one wheel does not have enough traction, rotational torque will follow the path of least resistance and the wheel with little traction will spin while the wheel with traction will not rotate at all. Since the wheel with traction is not rotating, the vehicle cannot move.

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A RWD Final Drive and Open Differential

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Limited-Slip Differentials (LSD)

Limited-slip differentials limit the amount of differential action allowed. If one wheel starts spinning excessively faster than the other (more so than durring normal cornering), an LSD will limit the speed difference. This is an advantage 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 move. There are several different designs currently in use today. Some work better than others depending on the application.

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Clutch Style LSD

Clutch style LSDs are based on a open differential design. They have a separate clutch pack on each of the axle side gears or axle shafts inside 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 separate the clutch discs. Springs put pressure on the axle side gears which put pressure on the clutch. If an axle shaft wants to spin faster or slower than the differential case, it must overcome 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 resist this action. As the speed difference increases, it becomes harder to overcome the clutches. When the vehicle is making a tight turn at low speed (parking), the clutches provide little resistance. When one drive wheel looses traction and all the torque goes to that wheel, the clutches resistance becomes much more apparent and the wheel with traction will rotate at (close to) the speed of the differential case. This type of differential will most likely require a special type of fluid or some form of additive. If the fluid is not changed at the proper intervals, the clutches can become less effective. Resulting in little to no LSD action. Fluid change intervals vary between applications. There is nothing wrong with this design, but keep in mind that they are only as strong as a plain open differential.

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Locking LSD

There are several different kinds of locking differentials. A locking differential has the ability to “lock” if the driver or conditions demand it. When the differential is locked, there is no differential action and both drive wheels must turn at the same speed as the case. Some systems will allow the driver to manually lock or unlock the differential. Other systems mechanically monitor the difference in axle speed and will lock the differential when one axle/wheel starts to rotate a set percentage faster than the other.

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Torque Bias LSD

Torque bias LSDs transfer torque to the slower rotating drive wheel at all times. The maximum amount of torque transfer is determined by the differentials’ torque bias ratio. If a diff has a torque bias ratio of 3:1 that means that 3 times as much torque can be transferred to the slower rotating wheel. They will use worm gears to transfer torque to the two sun gears which are connected to the drive wheels. The worm gears sit on shafts that are connected to the differential case. The worm gears have bevel gears on each end to mesh with the opposing sun gears’ worm gear. Both sun gears’ teeth are cut in the same direction and the worm gears’ teeth are cut to mesh with them. When the case starts to rotate, the worm gears are driven by the shaft that they ride on. The worm gears would rather rotate around the sun gears than drive them. Since both sun gears’ teeth are cut in the same direction, for the worm gears to simply run around the sun gears, they would have to rotate in the same direction. This cannot happen because the worm gears are meshed to each other. The worm gears have no choice but to drive the sun gears, without rotating themselves. This continues as long as the vehicle continues to drive in a straight line. When a vehicle enters a turn, the inside wheel must rotate slower than the differential case and the outside wheel must rotate faster than the case. Since it is difficult for a helical gear to drive a worm gear, there is resistance in the differential to drive wheels rotating at different speeds. As long as both drive wheels have equal traction they will be allowed to rotate at slightly different speeds. The worm gears will start to rotate to slightly overdrive the outside wheels’ sun gear and rotate around the inside wheels’ sun gear. This difference in speed is allowed by the differential as long as both drive wheels have relatively equal traction. More torque is always sent to the wheel that is rotating slower. This means that in a turn, the inside wheel will receive slightly more torque than the outside wheel. The difference in wheel speed is minimal in a normal turn compared to when a wheel starts to slip. In a hard turn, most of the vehicles weight is transferred to the outside wheels. If the inside wheel looses traction because of this, it will start to slip and momentarily become the faster spinning wheel. The moment this starts to happen, the differential will transfer torque to the outside wheel. This happens almost instantly, and without internal differential slippage which is what makes this type of differential so useful. The same thing will happen if one wheel is on pavement and the other is on ice. Most of the torque will be transferred to the slower spinning wheel with traction, and the vehicle moves. The one drawback with this type of differential is that if one drive wheel leaves the ground, (zero traction) no torque can be transferred to the other wheel. Some newer designs use this type of differential with clutch-packs or electronic differentials to work around this problem. These differentials are much stronger than an open differential, do not allow any internal differential slippage (like a clutch style LSD) and react instantly to changes in traction making torque bias differentials highly desirable. There are several variations to this design, but all torque bias LSDs work on similar principles. It is also worth noting that this style of LSD does not need clutches or control devices that can wear out or fail over time. (Although some do use clutches in case one drive wheel is exposed to a zero traction situation.) These differentials can also be used alongside an electronic differential system.

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Viscous Coupling

Viscous coupling units are common in 4WD/AWD applications a the centre differential, but they are also sometimes used as a final drive differential in 2WD applications. A viscous coupling has two sets of plates submerged in a silicone based fluid. Half of the plates will be meshed with the inner shaft, the rest will be meshed with the outer drum. When the two sets of plates rotate at the same speed, the fluid remains cool. If there is a significant difference in speed, the plates will shear the fluid causing the fluid to heat up. When the special fluid reaches a set temperature, it solidifies and locks the plates together. Both the inner shaft and the outer drum begin to rotate at the same speed. (or at least very close) Once the fluid cools down a few degrees, it will return to a liquid state and allow slight differences in plate speed.

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Electronically Controlled Differentials

Electronically controlled differentials use the ABS wheel speed sensors to monitor the speed of all 4 wheels. If the system determines that one or both drive wheels (2WD) are slipping, the system will grab the slipping wheel(s) brake to slow it down and transfer torque to the other drive wheel through (usually) a regular open differential. This system works well and is relatively cheap, considering the ABS system monitors all 4 wheels continuously anyway and an expensive LSD is not necessary. These systems are installed from the factory and cannot be bought after-market. The disadvantage to this system is the fact that slip must occur before the system can compensate whereas some mechanical LSDs can prevent the slip in the first place. Also, the system still relies on a weak open differential design compared to some LDSs. Some more complicated systems will detect when the vehicle is sliding and grab the appropriate individual brake or brakes to bring the vehicle back under control.

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Solid Differential/Spool Differential/Welded 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 always rotate at the same speed, even in a turn. This is not an issue on a drag race vehicle as drag vehicles are driving in a straight line 99% of the time. This can also be an advantage 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 create a solid differential. Solid differentials are a fine modification for vehicles designed for track use. As for street use, 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 driving through a slow turn (parking). The result is accelerated tire wear as well as premature axle failure. One big advantage of the solid differential over the other types is its strength. Since torque is applied directly to each axle, there is no spider gears, which are the weak point of open differentials.

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