One of the great bits of tech that remains from the glorious era that was Group B rally racing is the center differential. With it, race beasts from the likes of Audi, Peugeot, Ford, and Lancia all battled for all-wheel-drive supremacy on the greatest rally stages the world over.
The need for multiple differentials exists to this day and these mechanical and electrical bits are what allow a WRC driver to get his car through a turn properly. Here's how the center differential works its magic.
The rear differential sits out back between the rear wheels. It's at the tail end of the driveshaft. Up front you have a front differential that, like the rear unit, meters out horsepower and torque from left to right and vice versa. Set just aft of the front differential is a center differential that uses mechanical and electronic signals to distribute power appropriately.
When scrambling for every bit of traction, the car needs all four wheels sending power to the ground at quite different levels. The center differential reads throttle inputs and steering angle to see what it should do. In this case, the steering angle is minimal as the car is going straight and the throttle is flat, so the center differential is locked to ensure that the power is distributed appropriately.
When approaching a corner, that center diff will be locked but as the driver starts braking and turning it will begin to open up. This allows the car to turn in more easily, as a fully locked car will encounter binding as the wheel turns with greater steering angle. Once past the apex of the corner, the driver will begin to relax the angle while adding power back in with the throttle. The center differential will "know" what's going on and begin to lock back up.
As varied as the center differential can be during a race, so too can the front and rear. A driver may prefer a soft front differential as opposed to a stiff front differential. This allows for easier turn-in on the nose of the car, but there will be a traction deficit on corner exit.
It's all a balancing act that plays to the skills and needs of the driver as well as the terrain at hand.