Controlling rotational differences between drive wheels

●Differential

When a car is turning, the tire on the outside of the car will run a longer distance than the tire on the inside of the corner, so there is a difference in the amount of wheel rotation. Without alleviating the impact of this rotational difference, the tire on the inside will resist turning. The mechanism invented to resolve this issue is called the differential.

Fig. 7-1 While cornering, the tire on the outside of a corner travels a longer distance than the tire on the inside of the corner

The differential gear is located between the drive wheels, together with the final gear. When the car is travelling in a straight line it has no major effect, but while the car is turning, it distributes more engine torque to the wheel on the side that will be travelling the greater distance.

While the differential is an important mechanism on standard roads, it can be a weakness for race cars on the track. This is because if one of the two drive wheels lifts off the ground, the drive wheel that remains on the ground ceases to receive engine torque. The wheel that is not in contact with the ground will spin freely because the differential will keep trying to distribute more drive force to the wheel that is turning more.

Fig. 7-2 Mechanism of the differential gear

●LSD

This is where the LSD (limited slip differential) comes in. This is a mechanism that limits the function of the differential when a certain amount of rotational difference exists between the left and right drive wheels. We will omit the very technical definitions here; simply put, by adding a mechanism that restricts the rotation of the side gears connecting to the left and right wheels, the LSD limits the effect of the differential, so that both wheels will receive drive force.

There are various types of LSD available: multiple plate clutches, electronic controls, those that use the backlash of the gears, those that change the force produced in the axial direction to friction, those that use the sear resistance of viscous fluids… Each performs differently and has its own merits.

While the LSD is also effective when one wheel is stuck and slipping in mud or in snow, remember that in sports cars for sports driving, it is mainly used to secure drive force and to improve control.

Fig. 7-3a The difference between a car without an LSD (Left) and with an LSD (right)

7-3b With an LSD, differential motion between the left and right tires is restricted, transmitting torque to the outer tires