Note: If you’re likely to change your back diff liquid yourself, (or you plan on opening the diff up for assistance) before you let the fluid out, make certain the fill port can be opened. Absolutely nothing worse than letting liquid out and then Final wheel drive having no way to getting new fluid back.
FWD last drives are very simple compared to RWD set-ups. Almost all FWD engines are transverse mounted, which means that rotational torque is established parallel to the direction that the tires must rotate. You don’t have to alter/pivot the direction of rotation in the final drive. The ultimate drive pinion gear will sit on the finish of the output shaft. (multiple result shafts and pinion gears are possible) The pinion gear(s) will mesh with the ultimate drive ring equipment. In almost all instances the pinion and ring gear will have helical cut teeth just like the rest of the transmission/transaxle. The pinion equipment will be smaller sized and have a 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 portion of this article) 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 common type of differential found in 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 drive 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 various kinds of spider gears, the differential pinion gears and the axle part 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 trip on this pin and so are driven because of it. Rotational torpue can be then used in the axle side gears and out through the CV shafts/axle shafts to the wheels. If the automobile is venturing in a directly line, there is no differential action and the differential pinion gears only will 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 start to rotate as they drive the axle part gears, allowing the outer wheel to increase and the inside wheel to slow down. This design works well provided that both of the driven wheels have traction. If one wheel does not have enough traction, rotational torque will follow the road of least resistance and the wheel with small traction will spin while the wheel with traction won’t rotate at all. Because the wheel with traction is not rotating, the vehicle cannot move.
Limited-slide differentials limit the amount of differential action allowed. If one wheel begins spinning excessively faster compared to the other (more so than durring regular cornering), an LSD will limit the velocity difference. This is an benefit over a normal open differential style. If one drive wheel looses traction, the LSD actions will allow the wheel with traction to get 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 derive from a open up differential design. They have another clutch pack on each of the axle aspect gears or axle shafts in 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 the others are splined to the differential case. Friction materials is used to split up the clutch discs. Springs put pressure on the axle aspect gears which put strain on the clutch. If an axle shaft really wants to spin quicker or slower than the differential case, it must overcome the clutch to do so. If one axle shaft attempts to rotate quicker compared to the differential case then your other will attempt to rotate slower. Both clutches will resist this action. As the acceleration difference increases, it becomes harder to conquer the clutches. When the vehicle is making a good turn at low swiftness (parking), the clutches offer little level of resistance. When one drive wheel looses traction and all 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 kind of differential will likely need a special type of liquid or some type of additive. If the liquid is not changed at the proper intervals, the clutches may become less effective. Resulting in little to no LSD actions. Fluid change intervals vary between applications. There is certainly nothing wrong with this style, but keep in mind that they are just as strong 
as an ordinary open differential.
Solid/spool differentials are mostly found in drag racing. Solid differentials, like the name implies, are totally solid and will not enable any difference in drive wheel speed. The drive wheels usually rotate at the same speed, even in a change. This is not a concern on a drag competition vehicle as drag vehicles are driving in a straight line 99% of that time period. This may also be an advantage for cars that are getting set-up for drifting. A welded differential is a regular open differential that has got the spider gears welded to create a solid differential. Solid differentials certainly are a great modification for vehicles made for track use. For street make use of, a LSD option will be advisable over a good differential. Every convert a vehicle takes will cause the axles to wind-up and tire slippage. That is most visible when driving through a gradual turn (parking). The effect is accelerated tire use as well as premature axle failing. One big advantage of the solid differential over the other types is its power. Since torque is used directly to each axle, there is absolutely no spider gears, which will be the weak point of open differentials.