Hello all, I just finished writing my last Final Exam for the semester and thought it would be a good time to write another tech article.
Being the winter season I thought a good place to start off would be in maximizing traction seeing as we have so little of it when there is snow to be found.
This article will be split into 3 sections. In the first section I will be describing the difference between the 3 major types of differentials (locked, open, and limited slip/locking). The second section will consist of more details of specific types of differentials such as the Torsen and clutch plate differentials. Finally, in the third section I will cover a couple ways of automatically controlling torque biasing.
So what does a differential do? Quite simply, it is what divides the power from the engine among the driven wheels. A good differential will maximize amount of traction available by supplying varying amounts of torque to each of the driven wheels. While a not very good differential will lose torque by causing the lesser laden wheel to spin.
The first differentials were locked differentials. Meaning that for every one revolution of the engine, the wheels would each spin exactly the same amount. In essence, the wheels were locked together and were not free to spin relative to each other. A locked differentials main advantage is the fact that both wheels receive a fixed rotation speed. In the case that one wheel has less traction, the wheel with more traction takes up the slack. The main drawback with locked differentials however results when cornering. When you go around a corner, the outside wheels will naturally want to rotate more than the inside wheels. If your wheels are incapable of spinning at different speeds relative to each other, you lose valuable traction in the corner as some of it is used up with each tire fighting each other.
Due to this issue, the very first open differentials were brought to market. By having a simple three gear setup within a housing engineers were finally able to drive both rear wheels and allow each wheel to rotate independently of each other. This dramatically helped cornering. However, it soon became evident that this new open differential had one clear drawback. This drawback is what happens when one wheel loses traction. If one wheel loses traction, all of the engine torque is lost in spinning the wheel with no traction.
This is why the limited slip and locking differentials came into play. Limited slip and locking differentials are a hybrids between locked and free differentials. In limited slip differentials there is a mechanism (usually clutch plates or a Torsen) that resist the wheels spinning independently of each other. By providing resistance versus actually locking the wheels, the car is still able to provide torque to the laden wheel even if the lesser laden wheel is spinning. Unfortunately however this is still a compromise, the resistance created will still allow for excess torque to go to the unladen wheel and it will also provide some resistance to turning again causing traction to be lost.
In locking differentials the differential stays as a fully open differential until a driving torque is applied to the wheels. This allows for a perfect corner with the wheels rotating independently of each other, and allows the wheels to be fixed under hard acceleration. However, the negative aspect to a locking differential can best be seen when accelerating out of a corner. When you step on the gas pedal in a corner with a locking differential, it will lock up to evenly drive the wheels. This means all that nice open differential free wheeling you were doing earlier no longer has any effect.
Now a couple more details about Torsen differentials and clutch plates differentials. Due to the layout of the gears in the Torsen differential, the differential is able to create something known as torque biasing. A standard Torsen will usually be able to give a 3:1 torque bias ratio, this meaning that a standard Torsen will be able to deliver up to three times as much torque to the laden wheel compared to the lesser laden wheel. This is a great thing! It allows torque to be shifted mechanically all without any input other then the amount of traction the differential feels on either side. However, if you exceed that 3:1 ratio your vehicle will still end up wasting torque on the lesser laden wheel. If one wheel of a car is suspended in the air, be it as the vehicle is moving very hard around a corner or if the vehicle is parked on a very awkward slope, then a most of the torque available will be lost.
With Clutch plate differentials you can adjust just how "locked" or "open" a differential is by varying a set of springs that are pre-loading the clutch plates. Stiffer springs will cause the differential to act more like a locked differential, and softer springs will cause it to act more like a open differential. Again with a clutch differential you are still playing the non ideal compromise between a fully fixed and a fully open setup.
Finally, active differentials. There are two main types of active differentials to my knowledge. The first relies on using the vehicles brakes, and is usually mated to a Torsen differential setup. The second is by actively changing the pre-load on a clutch plate setup.
For active Torsen differential setups, the vehicles computer will calculate when the Torsen is exceeding its maximum torque bias ratio and selectively apply the brakes to the unladen or spinning wheel. By applying brakes to this wheel, the vehicle imitates traction and is able to once again load the laden wheel. The only drawback to this system is that in using the brakes under power, some of the engine power will be used up by the brakes rather then where it needs to be... on the laden wheel.
The most superior differential setup in my opinion is an active clutch plate differential. When the vehicle with this system recognizes torque losses to the free or lesser laden wheel, it selectively engages the clutch so that the laden wheel can receive anywhere from a half to all of the engines torque. No energy is wasted, and what is more if coupled to other sensors such as accelerometers, this differential can even be used to help the car turn by automatically applying more force on one side.
Hope you enjoyed this read. Leave a shout out if you really want to see another topic on whatever you want, I might get around to writing another this Christmas Season ;)