Note: If you’re likely to change your back diff liquid yourself, (or you plan on starting the diff up for program) before you let the fluid out, make sure the fill port could be opened. Nothing worse than letting fluid out and having no way of getting new fluid back.
FWD final drives are extremely simple in comparison to RWD set-ups. Virtually all FWD engines are transverse installed, which implies that rotational torque is established parallel to the path that the wheels must rotate. You don’t have to change/pivot the direction of rotation in the final drive. The final drive pinion equipment will sit on the finish of the output shaft. (multiple output 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 band gear will have helical cut tooth just like the remaining tranny/transaxle. The pinion gear will be smaller sized and have a lower tooth count compared to the ring gear. This produces the ultimate drive ratio. The band equipment will drive the differential. (Differential operation will be described in the differential portion of this content) Rotational torque is sent to the front tires through CV shafts. (CV shafts are generally referred to as axles)
An open up differential is the most common type of differential found in passenger cars and trucks today. It is certainly a very simple (cheap) design that uses 4 gears (occasionally 6), that are referred to as spider gears, to drive the axle shafts but also allow them to rotate at different speeds if required. “Spider gears” can be a slang term that is commonly used to describe all of the differential gears. There are two various kinds of spider gears, the differential pinion gears and the axle aspect gears. The differential case (not casing) gets rotational torque through the band equipment and uses it to drive the differential pin. The differential pinion gears ride upon this pin and so are driven by it. Rotational torpue is then used in the axle side gears and out through the CV shafts/axle shafts to the wheels. If the automobile is traveling in a directly line, there is absolutely no differential action and the differential pinion gears will simply drive the axle part gears. If the automobile enters a convert, the external wheel must rotate faster compared to the inside wheel. The differential pinion gears will start to rotate as they drive the axle aspect gears, allowing the external 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 doesn’t have enough traction, rotational torque will follow the road of least level of resistance and the wheel with small traction will spin while the wheel with traction will not rotate at all. Since the wheel with traction is not rotating, the automobile cannot move.
Limited-slide differentials limit the quantity of differential action allowed. If one wheel starts spinning excessively faster than the other (way more than durring normal cornering), an LSD will limit the velocity difference. This is an benefit over a regular open differential design. If one drive wheel looses traction, the LSD action allows the wheel with traction to get rotational torque and invite the vehicle to go. There are several different designs currently used today. Some work better than Final wheel drive others based on the application.
Clutch style LSDs are based on a open up differential design. They have a separate clutch pack on each of the axle part gears or axle shafts within the final drive casing. 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 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 faster or slower than 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 resist this step. As the acceleration difference increases, it turns into harder to conquer the clutches. When the vehicle is making a tight turn at low rate (parking), the clutches offer little resistance. When one drive wheel looses traction and all the torque goes to that wheel, the clutches level of resistance becomes a lot more apparent 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 fluid or some kind of additive. If the fluid isn’t changed at the correct intervals, the clutches may become less effective. Resulting in small to no LSD action. Fluid change intervals vary between applications. There can be nothing incorrect with this design, 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 enable any difference in drive wheel rate. The drive wheels at all times rotate at the same acceleration, even in a convert. This is not a concern on a drag competition vehicle as drag automobiles are generating in a straight line 99% of the time. This may also be an edge for vehicles that are being set-up for drifting. A welded differential is a normal 
open differential that has acquired the spider gears welded to create a solid differential. Solid differentials certainly are a great modification for vehicles designed for track use. As for street use, a LSD option will be advisable over a good differential. Every switch a vehicle takes may cause the axles to wind-up and tire slippage. That is most apparent when traveling through a slower turn (parking). The effect is accelerated tire put on and also premature axle failing. One big benefit of the solid differential over the other styles is its power. Since torque is used right to each axle, there is no spider gears, which are the weak spot of open differentials.