Note: If you’re likely to change your back diff fluid yourself, (or you plan on opening the diff up for service) before you allow fluid out, make certain the fill port can be opened. Absolutely nothing worse than letting liquid out and then having no way to getting new fluid back.
FWD last drives are very simple compared to RWD set-ups. Virtually all FWD engines are transverse mounted, which means that rotational torque is established parallel to the 
direction that the tires must rotate. There is no need to modify/pivot the direction of rotation in the ultimate drive. The ultimate drive pinion equipment will sit on the end of the result shaft. (multiple output shafts and pinion gears are feasible) The pinion gear(s) will mesh with the final drive ring gear. In almost all instances the pinion and band gear could have helical cut teeth just like the rest of the transmission/transaxle. The pinion equipment will be smaller sized and have a much lower tooth count than the ring equipment. This produces the final drive ratio. The ring 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 known as axles)
An open up differential is the most common type of differential within passenger vehicles today. It is a 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 necessary. “Spider gears” is certainly 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 side gears. The differential case (not housing) gets rotational torque through the band equipment and uses it to operate a vehicle the differential pin. The differential pinion gears ride on this pin and so are driven by it. Rotational torpue is definitely then transferred to the axle part gears and out through the CV shafts/axle shafts to the tires. If the vehicle is venturing in a directly line, there is absolutely no differential action and the differential pinion gears will simply drive the axle part gears. If the vehicle enters a change, the external wheel must rotate faster compared to the inside wheel. The differential pinion gears will begin to rotate as they drive the axle part gears, allowing the outer wheel to increase and the inside wheel to decelerate. This design works well so long as both of the driven wheels possess traction. If one wheel doesn’t have enough traction, rotational torque will observe the road of least resistance and the wheel with little traction will spin while the wheel with traction will not rotate at all. Since the wheel with traction isn’t rotating, the automobile cannot move.
Limited-slide differentials limit the quantity of differential action allowed. If one wheel begins spinning excessively faster than the other (more so than durring normal cornering), an LSD will limit the speed difference. That is an advantage over a normal open differential design. If one drive wheel looses traction, the LSD action will allow the wheel with traction to get rotational torque and allow the vehicle to move. There are several different designs currently used today. Some work better than others depending on the application.
Clutch style LSDs derive from a open up differential design. They have another clutch pack on each one of the axle side gears or axle shafts in the final drive housing. Clutch discs sit down 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 material is used to separate the clutch discs. Springs put pressure on the axle part gears which put strain 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 faster 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 turns into harder to overcome the clutches. When the automobile is making a tight turn at low quickness (parking), the clutches offer little resistance. When one drive wheel looses traction and all the torque would go to that wheel, the clutches resistance becomes a lot more apparent and the wheel with traction will rotate at (close to) the velocity of the differential case. This kind of differential will most likely require a special type of fluid or some type of additive. If the liquid is not changed at the Final wheel drive correct intervals, the clutches can become less effective. Resulting in small to no LSD action. Fluid change intervals vary between applications. There can be nothing wrong with this design, but remember that they are just as strong as a plain open differential.
Solid/spool differentials are mostly found in drag racing. Solid differentials, like the name implies, are completely solid and will not really allow any difference in drive wheel quickness. The drive wheels often rotate at the same speed, even in a change. This is not an issue on a drag race vehicle as drag automobiles are driving 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 which has acquired the spider gears welded to create a solid differential. Solid differentials are a good modification for vehicles designed for track use. For street use, a LSD option would be advisable over a solid differential. Every convert a vehicle takes may cause the axles to wind-up and tire slippage. That is most obvious when driving through a slow turn (parking). The result is accelerated tire put on in addition to premature axle failing. One big benefit 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.