Note: If you’re going to change your rear diff liquid yourself, (or you plan on starting 
the diff up for services) before you allow fluid out, make certain the fill port can be opened. Absolutely nothing worse than letting fluid out and having no way to getting new fluid back in.
FWD final drives are extremely simple compared to RWD set-ups. Virtually all FWD Final wheel drive engines are transverse mounted, which means that rotational torque is created parallel to the path that the tires must rotate. You don’t have to alter/pivot the direction of rotation in the final drive. The final drive pinion equipment will sit on the finish of the result shaft. (multiple result shafts and pinion gears are feasible) 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 remaining transmitting/transaxle. The pinion equipment will be smaller and have a much lower tooth count than the ring gear. This produces the ultimate drive ratio. The band gear will drive the differential. (Differential procedure will be described in the differential section of this content) Rotational torque is sent to the front tires through CV shafts. (CV shafts are commonly referred to as axles)
An open up differential is the most common type of differential found in passenger vehicles today. It is usually a simple (cheap) design that uses 4 gears (occasionally 6), that are referred to as spider gears, to operate a vehicle the axle shafts but also permit them to rotate at different speeds if required. “Spider gears” is a slang term that’s commonly used to describe 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) receives rotational torque through the band equipment and uses it to drive the differential pin. The differential pinion gears ride on this pin and so are driven because of it. Rotational torpue is usually then transferred to the axle aspect gears and out through the CV shafts/axle shafts to the wheels. If the vehicle is traveling in a straight line, there is absolutely no differential actions and the differential pinion gears only will drive the axle aspect gears. If the automobile enters a switch, the external wheel must rotate quicker than the inside wheel. The differential pinion gears will begin to rotate because they drive the axle side gears, allowing the external wheel to increase and the within wheel to slow down. This design is effective so long as both of the driven wheels have got 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 won’t rotate at all. Because the wheel with traction isn’t rotating, the automobile cannot move.
Limited-slip 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 normal open differential style. If one drive wheel looses traction, the LSD action will allow the wheel with traction to obtain rotational torque and allow the vehicle to move. There are many different designs currently in use today. Some are better than others depending on the application.
Clutch style LSDs are based on a open differential design. They have another clutch pack on each of the axle aspect gears or axle shafts inside 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 materials is used to split up the clutch discs. Springs put pressure on the axle side gears which put pressure on the clutch. If an axle shaft wants to spin quicker or slower compared to the differential case, it must get over the clutch to take action. If one axle shaft tries to rotate quicker than the differential case then the other will try to rotate slower. Both clutches will resist this step. As the speed difference increases, it turns into harder to conquer the clutches. When the vehicle is making a tight turn at low rate (parking), the clutches provide little resistance. When one drive wheel looses traction and all the torque would go to that wheel, the clutches resistance becomes much more apparent and the wheel with traction will rotate at (near) the velocity of the differential case. This kind of differential will likely require a special type of liquid or some form of additive. If the liquid isn’t 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 certainly nothing wrong with this style, but remember that they are just as strong as an ordinary 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 speed. The drive wheels always rotate at the same acceleration, even in a convert. This is not an issue on a drag competition vehicle as drag vehicles are generating in a directly line 99% of the time. This may also be an edge for cars that are becoming set-up for drifting. A welded differential is a regular open differential that has experienced the spider gears welded to make a solid differential. Solid differentials are a fine modification for vehicles created for track use. As for street make use of, a LSD option would be advisable over a good differential. Every turn a vehicle takes will cause the axles to wind-up and tire slippage. That is most apparent when traveling through a gradual turn (parking). The result is accelerated tire put on along with premature axle failing. One big advantage of the solid differential over the other styles is its strength. Since torque is applied right to each axle, there is no spider gears, which will be the weak spot of open differentials.
