What is a differential? A differential in this context is essentially a small gearbox that receives power from the transmission in the vehicle and then sends that power in 2 directions out to each of the wheels on either side of it, but it also allows the 2 wheels to turn at different speeds when needed. The key upside to a differential is when you make a turn, the tire that is on the outside of the turn, say, if you turn left, the right tire, must travel further than the left tire, on the inside of the turn in this scenario. A differential allows both tires to roll at the speed needed without one tire scooting or spinning. The downside is this when you encounter a situation where one tire, for example, the left is on ice, and the right is on dry pavement, it is hard for the vehicle to move because the differential sends equal torque to both wheels, but the amount of torque sent is the amount required to spin the tire with the least amount of traction. Torque is twisting force, and it's important to get enough torque to the rear tires. Enough torque means the rear tires can spin/twist and the vehicle can move forward. Now, back to the ice patch scenario. If the torque required for the left tire to spin on ice is not enough torque to make the vehicle move forward, you are stuck. Because, the right-side tire is only getting the same amount of torque as the left-side tire, no more. This is the scenario that all the traction aids have been created to deal with. Let's look at them now.
Locking differential: To understand a locker, it helps to understand a spool. A spool does the same thing as a differential in that it takes power from the transmission and sends it out to both rear tires. However, it mechanically locks the rear tires together. The downside is, when you turn, 1 tire will either spin or scoot. The upside is, that it sends equal torque to both wheels as well, but it sends the amount required to spin the tire with the most traction. Back to the ice patch scenario. The right side tire on the pavement gets enough torque to turn on the pavement. The left side tire on the ice is mechanically locked to the right side and turns at the same speed, with no excess wheel spin. Now, a locking differential is essentially a differential that can switch from being a differential to being a spool. Manual lockers are ones where the operator of the vehicle decides when the switch happens. Automatic lockers are ones where it goes into spool mode whenever it thinks it needs to, based on circumstances, or vice versa. Auto lockers can be pretty rough and unpredictable, so I prefer manual lockers.
Limited-Slip differential: A limited-slip differential will have a clutch pack or 2 inside the case that pulls together on the 2 sides of the differential to gradually connect the 2 rear wheels. It will vary this connection somewhere in between a differential and a spool. If the clutching is strong enough, it can function like a spool but is prone to overheat if this is done too long. This style differential is nicer than an automatic locker in many cases because it has the same convenience benefits, but is usually smoother, and some can manually lock on request as well.
Torsen differential: These are often called Torsen, named after the company who invented them, but they are a type of limited-slip that uses gears to multiply torque. In the ice patch scenario again, if it required 10 lbs of torque to spin the left tire on ice, a Torsen would within the blink of an eye send about 25 lbs (varies based on design) to the other tire on pavement. If 25lbs is enough, away you go. If not, you are still stuck unless you know a trick. That trick is to push on the brake pedal and the gas pedal at the same time. Now with the brakes applied, it might take 1'000 lbs of torque to turn the tire on ice, and so the Torsen sends 2'500 lbs to the tire on pavement, and since that's probably going to be enough, away you go. Having to drive with 2 feet on occasion is the only real downside to a Torsen that I can see.
Traction Control: Traction control is an electronic aid that is not in the differential like the other types are. Traction control does 2 things. 1. Brake the wheel on the ice patch. In a standard differential, equal torque is sent to both wheels, and it's the amount equal to the wheel with the least traction like we see in the ice patch scenario. This braking action by traction control forces more torque onto the wheel on the pavement. 2. Cut engine power to drastically reduce wheelspin.
Brake Limited Slip: This is a feature inside the logic of traction control. Most car makers can do it but only a few have truly mastered it in my opinion. When brake limited-slip is activated, it measures the speeds on the tires on both sides of the differential. If one is faster, it brakes that one. It does not reduce engine power. So, the tires can spin as fast as they want as long as both tires are spinning at the same speed. In the ice patch scenario. Brake limited slip would brake the tire on the ice and force torque to go to the tire on pavement. The vehicle would carry on with only a quick interruption or pause. The reason this is called brake limited-slip is that it does almost the same thing as a conventional limited-slip differential but does so with the wheel brakes. Downsides to this? Well, it could be hard on your brake pads, also, since it isn't actually mechanically in the differential, you'd hope your specific brand with this feature is one of the few that knows how to program it properly.
Concluding thoughts:
For performance vehicles, it is hard to argue with Torsen or limited-slip differentials for overall track cornering ability. You have the easy turning of a differential when entering a corner, and then equalized power across the car when trying to power out of the corner again.
For normal road driving when nothing is going wrong in your word, traction control is a nice safety net against sliding on wet or snowy roads, but if you actually are stuck, its abilities are very limited.
For off-roading, it's really hard to argue against a spool when it comes to traction. Manual lockers give you the ability to have a spool off-road and a differential on-road or when making turns, so, ultimately, that's pretty sweet for something like a Bronco or a Wrangler to have.
For vehicles that are all-rounders, like pickup trucks and utility SUVs, the 3 different kinds of limited-slip differentials have their place.
Brake Limited Slip is amazing when it's programmed correctly. It can outperform mechanical limited-slip on rock crawling situations if it's programmed correctly.
