Note: If you’re going to change your back diff liquid yourself, (or you plan on starting the diff up for support) before you allow fluid out, make sure the fill port could be opened. Nothing worse than letting fluid out and then having no way to getting new fluid back in.
FWD last drives are very simple in comparison to RWD set-ups. Virtually all FWD engines are transverse installed, which implies that rotational torque is created 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 Final wheel drive pinion equipment will sit on the end of the result shaft. (multiple result shafts and pinion gears are possible) The pinion equipment(s) will mesh with the final drive ring gear. In almost all instances the pinion and ring gear will have helical cut teeth just like the rest of the tranny/transaxle. The pinion equipment will be smaller and have a much lower tooth count compared to the ring gear. This produces the final drive ratio. The ring equipment will drive the differential. (Differential operation will be described in the differential portion of this article) Rotational torque is delivered to the front tires through CV shafts. (CV shafts are commonly referred to as axles)
An open up differential is the most typical type of differential found in passenger cars and trucks today. It is a very simple (cheap) style that uses 4 gears (occasionally 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 required. “Spider gears” is a slang term that is 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 side gears. The differential case (not casing) gets 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 are driven by it. Rotational torpue is definitely then used in the axle aspect gears and out through the CV shafts/axle shafts to the wheels. If the automobile is travelling in a directly line, there is no differential actions and the differential pinion gears only will drive the axle side gears. If the vehicle enters a turn, the external wheel must rotate quicker compared to the inside wheel. The differential pinion gears will begin to rotate because they drive the axle part gears, allowing the outer wheel to speed up and the inside wheel to decelerate. This design works well so long as both of the powered wheels have got traction. If one wheel doesn’t have enough traction, rotational torque will follow the road of least 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 automobile cannot move.
Limited-slide differentials limit the amount of differential actions allowed. If one wheel begins spinning excessively faster than the other (more so than durring regular cornering), an LSD will limit the velocity difference. This is an benefit over a regular open differential style. If one drive wheel looses traction, the LSD action will allow the wheel with traction to obtain rotational torque and invite the vehicle to go. There are many different designs currently used today. Some are better than others depending on the application.
Clutch style LSDs derive from a open differential design. They have a separate clutch pack on each of the axle part gears or axle shafts in 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 others are splined to the differential case. Friction materials is used to separate the clutch discs. Springs put strain on the axle side gears which put strain on the clutch. If an axle shaft 
wants to spin faster or slower compared to the differential case, it must get over the clutch to take action. If one axle shaft attempts to rotate faster compared to the differential case then your other will attempt to rotate slower. Both clutches will resist this step. As the acceleration difference increases, it turns into harder to get over the clutches. When the vehicle is making a good turn at low rate (parking), the clutches offer little resistance. When one drive wheel looses traction and all the torque would go to that wheel, the clutches level of resistance becomes much more obvious and the wheel with traction will rotate at (near) the velocity of the differential case. This type of differential will likely require a special type of liquid or some type of additive. If the fluid is not changed at the proper intervals, the clutches can become less effective. Leading to small to no LSD actions. Fluid change intervals differ between applications. There is usually nothing incorrect with this design, but remember that they are only as strong as a plain open differential.
Solid/spool differentials are mostly found in drag racing. Solid differentials, like the name implies, are totally solid and will not allow any difference in drive wheel acceleration. The drive wheels at all times rotate at the same speed, even in a switch. This is not an issue on a drag race vehicle as drag automobiles are generating in a straight line 99% of the time. This can also be an advantage for vehicles that are getting set-up for drifting. A welded differential is a normal open differential that has experienced the spider gears welded to create a solid differential. Solid differentials are a good modification for vehicles made for track use. For street use, 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. This is most apparent when driving through a slower turn (parking). The effect is accelerated tire use along with premature axle failure. 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 are the weak point of open differentials.