Torque vectoring is a technique used by manufacturers in order to improve the cornering capabilities and stability of their cars.
The basic concept of any torque vectoring system is to redistribute torque between the driven wheels. When cornering, for example, such a system could send more torque to the wheel on the outside of a corner. This extra 'push' from the driven outside wheel generates a yaw moment which helps the turn the car into the corner, tightening its line.
There are two distinct types of torque vectoring. The first is the simple and inexpensive 'torque vectoring by braking'. The second is the more complicated but capable active differential-based torque vectoring, which is far more expensive and often utilised in performance applications.
How does torque vectoring by braking work?
Brake-based torque vectoring makes use of the car's existing braking and stability control systems to enable a cost-effective form of torque vectoring, imitating the far more expensive active differential-based systems.
In a car with an open differential, the driven wheels can rotate at different speeds but receive the same torque. This system limits the torque at the heavily loaded wheel on the outside of the corner to the same as the lightly loaded inside wheel. The vectoring system takes advantage of this to help counter understeer and improve the agility of a vehicle; when a car enters a corner, the vectoring system will apply the brake on the lightly loaded wheel on the inside of the bend.
This generates a braking torque, increasing the total torque at that wheel, allowing the differential to transmit more torque to the heavily loaded wheel on the outside of the corner. This additional push from the wheel with plenty of traction, in conjunction with light braking on the inside wheel, helps yaw the nose of the car in the desired direction. This can help reduce understeer, as well as deliver a more responsive feel.
The obvious drawback, for outright performance applications, is that applying the brake on one wheel will ultimately slow the car. For most, however, this slight negative is often completely outweighed by the improved stability and response offered.
As is the case with the more complex differential-based systems, using brake-based torque vectoring can also reduce the steering lock required to go around a corner.
What about torque vectoring differentials?
Torque vectoring differentials are far more expensive and complicated than the brake-based systems. These setups typically consist of a standard open differential, the outputs of which feed into compact planetary gearsets and electrically or hydraulically actuated multi-disk clutches.
The clutches are electronically controlled and the planetary gearsets are overdrive units - so the output turns quicker than the input. When the car is going in a straight line, the clutches are disengaged and torque is delivered equally from the differential to the axles in the conventional fashion.
When the car begins to corner, the clutch for the wheel on the outside of the bend is engaged. This redirects drive from the differential through the planetary gearset - which steps up the input,increasing the speed of the axle.
This overspeeding of the outside wheel causes a yawing effect, without having to brake the inside wheels, which helps boost the cornering capabilities of the car; it can also be used to help maintain the stability of the car in certain situations.
These complicated differentials are far more unobtrusive than conventional mechanical limited-slip differentials because, when not required, the vectoring feature can simply be disengaged - leaving you with a quiet, smooth and conventional open differential.
Because these active set-ups do not rely on the brakes, they also do not reduce speed and they do not cause additional brake wear.