Note: If you’re going to change your back diff liquid 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. Absolutely nothing worse than letting fluid out and having no way to getting new fluid back.
FWD last drives are extremely simple compared to RWD set-ups. Virtually all FWD engines are transverse installed, which implies that rotational torque is created parallel to the path that the tires must rotate. You don’t have to change/pivot the path 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 final drive ring gear. In almost all cases the pinion and band gear will have helical cut teeth just like the rest of the tranny/transaxle. The pinion gear will be smaller and have a lower tooth count than the ring equipment. This produces the ultimate drive ratio. The ring gear will drive the differential. (Final wheel drive differential procedure 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 up differential is the most common type of differential within passenger vehicles today. It is definitely a very simple (cheap) design 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 of the differential gears. There are two different types of spider gears, the differential pinion gears and the axle aspect gears. The differential case (not casing) gets rotational torque through the band gear and uses it to operate a vehicle the differential pin. The differential pinion gears trip upon this pin and so are driven by it. Rotational torpue is usually then used in the axle aspect gears and out through the CV shafts/axle shafts to the wheels. If the automobile is venturing 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 convert, the outer wheel must rotate faster compared to the inside wheel. The differential pinion gears will start to rotate as they drive the axle part gears, allowing the external wheel to increase and the within wheel to decelerate. This design works well so long as both of the driven wheels have got traction. If one wheel doesn’t have enough traction, rotational torque will observe the path of least level of resistance and the wheel with small traction will spin as the wheel with traction won’t rotate at all. Because the wheel with traction isn’t rotating, the vehicle cannot move.
Limited-slip differentials limit the quantity 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 velocity difference. That is an advantage over a normal open differential style. 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 in use today. Some work better than others depending on the application.
Clutch style LSDs derive from a open up differential design. They have a separate clutch pack on each of the axle aspect gears or axle shafts within the final drive housing. Clutch discs sit 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 put strain on the axle aspect 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 tries to rotate quicker compared to the differential case then your other will attempt to rotate slower. Both clutches will withstand this step. As the speed difference increases, it turns into harder to overcome the clutches. When the vehicle is making a good turn at low swiftness (parking), the clutches provide little level of resistance. When one drive wheel looses traction and all of the torque would go to that wheel, the clutches resistance becomes a lot more obvious and the wheel with traction will rotate at (near) the acceleration of the differential case. This type of differential will most likely need a special type of fluid or some type of additive. If the fluid isn’t changed at the correct intervals, the clutches can become less effective. Leading to small to no LSD actions. Fluid change intervals vary between applications. There is certainly 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 found in drag racing. Solid differentials, just like the name implies, are totally solid and will not allow any difference in drive wheel acceleration. The drive wheels always rotate at the same acceleration, even in a turn. This is not a concern on a drag race vehicle as drag vehicles are traveling in a directly line 99% of that time period. This can also be an advantage for cars that are being set-up for drifting. A welded differential is a regular open differential which has got the spider gears welded to make a solid differential. Solid differentials are a great modification for vehicles made for track use. As for street use, a LSD option would be advisable over a good differential. Every change a vehicle takes will cause the axles to wind-up and tire slippage. That is most apparent when driving through a gradual 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 used right to each axle, there is no spider gears, which will be the weak spot of open differentials.
