Note: If you’re likely to change your rear diff liquid yourself, (or you intend on starting the diff up for service) before you let the fluid out, make sure the fill port could be opened. Nothing worse than letting fluid out and then having no way of getting new fluid back.
FWD last drives are very simple compared to RWD set-ups. Almost all FWD engines are transverse installed, which implies that rotational torque is created parallel to the direction that the tires must rotate. There is no need to modify/pivot the path of rotation in the final drive. The ultimate drive pinion gear will sit on the end of the output shaft. (multiple output Final wheel drive shafts and pinion gears are feasible) The pinion equipment(s) will mesh with the ultimate drive ring gear. In almost all situations the pinion and band gear will have helical cut teeth just like the rest of the transmitting/transaxle. The pinion gear will be smaller sized and have a lower tooth count than the ring equipment. This produces the final drive ratio. The ring equipment will drive the differential. (Differential operation will be explained in the differential section of this content) Rotational torque is delivered to the front tires through CV shafts. (CV shafts are generally referred to as axles)
An open differential is the most typical type of differential found in passenger vehicles today. It is a simple (cheap) style that uses 4 gears (sometimes 6), that are known as spider gears, to drive the axle shafts but also allow them to rotate at different speeds if necessary. “Spider gears” is 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 part gears. The differential case (not casing) gets 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 is definitely then transferred to the axle side gears and out through the CV shafts/axle shafts to the tires. If the automobile is traveling in a straight line, there is no differential action and the differential pinion gears will simply drive the axle side gears. If the automobile enters a turn, the outer wheel must rotate quicker than the inside wheel. The differential pinion gears will start to rotate as they drive the axle side gears, allowing the outer wheel to speed up and the within wheel to slow down. This design is effective so long as both of the driven wheels have traction. If one wheel does not have enough traction, rotational torque will follow the path of least resistance and the wheel with small traction will spin while the wheel with traction will not rotate at all. Because 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 (way more than durring normal cornering), an LSD will limit the rate difference. This is an advantage over a regular open differential design. If one drive wheel looses traction, the LSD action allows the wheel with traction to obtain rotational torque and invite the vehicle to go. There are many 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 possess a separate clutch pack on each of the axle part gears or axle shafts in the final drive casing. Clutch discs sit down between the 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 part gears which put strain on the clutch. If an axle shaft wants to spin faster or slower than the differential case, it must get over the clutch to do so. If one axle shaft tries to rotate faster than the differential case then your other will try to rotate slower. Both clutches will withstand this action. As the quickness difference increases, it turns into harder to conquer the clutches. When the vehicle is making a good turn at low acceleration (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 level of resistance becomes much more apparent and the wheel with traction will rotate at (near) the quickness of the differential case. This type of differential will likely require a special type of fluid or some form of additive. If the fluid isn’t changed at the correct intervals, the clutches may become less effective. Resulting in small to no LSD actions. Fluid change intervals vary between applications. There is definitely 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, like the name implies, are completely solid and will not allow any difference in drive wheel rate. The drive wheels at all times rotate at the same velocity, even in a switch. This is not a concern on a drag race vehicle as drag vehicles are driving in a directly line 99% of that time period. This may also be an edge for vehicles that are becoming set-up for drifting. A welded differential is a regular open differential which has had the spider gears welded to create a solid differential. Solid differentials are a great modification for vehicles made for track use. For street make use of, a LSD option would be advisable over a solid differential. Every change a vehicle takes will cause the axles to wind-up and tire slippage. That is most visible when driving through a slower turn (parking). The effect is accelerated tire wear as well as premature axle failing. One big benefit of the solid differential over the other styles is its strength. Since torque is used directly to each axle, there is no spider gears, which will be the weak point of open differentials.