WHAT IS DIFFERENTIAL?
A differential is nothing but a mechanical device used to drive a pair of wheels while allowing them to rotate at different speeds.Vehicles which don't have differential have same wheel speed on both ends of the axle, driven by a simple chain-drive mechanism.While cornering, the inner wheel travels a shorter distance than the outer wheel, so without a differential either the inner wheel will rotate too quickly or the outer wheel will rotate too slowly, which results in difficult and unpredictable handling, damage to tires and roads, and strain on (or possible failure of) the drivetrain. For example, if the car is making a turn to the right, the main ring gear may make 10 full rotations. During that time, the left wheel will make more rotations because it has farther to travel, and the right wheel will make fewer rotations as it has less distance to travel. The sun gears (which drive the axle half-shafts) will rotate at different speeds relative to the ring gear (one faster, one slower) by, say, 2 full turns each (4 full turns relative to each other), resulting in the left wheel making 12 rotations, and the right wheel making 8 rotations.
Source: www.mathworks.com
ACTIVE DIFFERENTIALS
Active differential is an electronically controlled, relatively new technology . An electronic control unit (ECU) uses inputs from multiple sensors, including yaw rate, steering input angle, and lateral acceleration, and adjusts the distribution of torque to compensate for undesirable handling behaviours such as understeer.Fully integrated active differentials are used on the likes of Ferrari F430, Mitsubishi Lancer Evolution, and on the rear wheels in the Acura RL.
LOCKING DIFFERENTIALS
One undesirable side effect of an open differential is that it can limit traction under less than ideal conditions. It(open diff.) basically provides same torque to both the wheels in lower gears, and thus at lower speeds, and unless the load is exceptionally high, the drivetrain can supply as much torque as necessary, so the limiting factor becomes the traction under each wheel. If the torque applied to one of the drive wheels is in excess to the threshold of traction, then that wheel will spin.For example, lets imagine a simple rwd vehicle, with one rear wheel on asphalt with good grip, and the other on a patch of mud/slush. It takes very little torque to spin the side on slippery surface, and because a differential splits torque equally to each side, the torque that is applied to the side that is on asphalt is limited to this amount.
In order to overcome this limitation, the concept of locking differential was introduced. It essentially "locks" both wheels on an axle together as if on a common shaft. This forces both wheels to turn in unison, regardless of the traction available to either wheel individually. When the differential is unlocked (open differential), it allows each wheel to rotate at different speeds (such as when negotiating a turn), thus avoiding tire scuffing. An open (or unlocked) differential always provides the same torque (rotational force) to each of the two wheels, on that axle. So although the wheels can rotate at different speeds, they apply the same rotational force, even if one is entirely stationary, and the other spinning. (Equal torque, unequal rotational speed).
source:https://www.topgear.com.ph/
Full time four wheel drive vehicles have three differentials:
- In front axle
- Central diff between both axles(transfer case)
- In rear axle
Part-time four-wheel-drive systems don't have a differential between the front and rear wheels; instead, they are locked together so that the front and rear wheels have to turn at the same average speed. This is why these vehicles are hard to turn on concrete when the four-wheel-drive system is engaged.
CONCLUSION
So, lets see the pros of locking diffs:
- Distributes power evenly to the wheels
- Enhanced off-road performance
- Improves traction in rough terrain
- Selectable lockers are great for daily drivers
- Some can be activated with the push of a button
Some cons:
- Higher cost for selectable lockers
- Clunky engage and disengage for automatic lockers
- Can put added stress on driveline components if misused
- Selectable lockers can be complicated to fix if they break down
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