When straight roads become curvy, torque vectoring makes all the difference.
We live an era where the motor car is constantly evolving. In the search of new ways to go faster, eking out micro-seconds around each corner, we rehash 'old' technologies and develop them to new limits. One such technology that's found prominence of late is torque vectoring – finding use in everything from the hybrid Acura NSX to the rather humdrum new Subaru Forester. But what is torque vectoring? How does it work to improve performance, and are there any drawbacks to it?
When roads are straight, performance is a fairly simple thing to achieve – add power, grip, and speed, and voila, performance is easy to come by. But when the road starts to twist, things get a little more interesting. In a straight line, the wheels on either side of a car rotate at the same speed. But when turning, the left and right sides of a vehicle form two parallel circles with varying radii. Because the outer wheels are tracking on a larger radius, they have to cover a greater distance in the same amount of time as the inner wheel covers the shorter distance.
To help with this, cars have been equipped with differentials – mechanical devices that allow the wheels to rotate at independent speeds. Differentials range from the simplistic to the extravagant in their designs and implementations – but it's these differentials that form the basis for torque vectoring. Torque vectoring, simply put, gives a vehicle the ability to vary the torque applied to each wheel. In doing so, a vehicle's handling characteristics can be altered to induce understeer and oversteer, or mitigate either entirely with the aim of neutrality, safety, or excitement in the form of smoky drifts.
Primarily though, torque vectoring aims to improve precision under cornering. But there are multiple ways in which torque vectoring can be managed and implemented. In the ranks of torque vectoring, four main types exist – each with their own pros and cons, and varying degrees of effectiveness.
This is the original torque vectoring, and the one most favored in high performance vehicles without budget constraints. Vehicles equipped with torque vectoring differentials originally made use of simple limited slip diffs – diffs that, upon slippage of an individual wheel, would send more torque to the opposite wheel – usually the outer wheel, thus increasing grip levels and minimizing torque-steer and understeer, particularly in front wheel drive applications.
In recent years, limited slip differentials have grown in prominence, but in rear- and all-wheel drive performance applications, these diffs have been developed further to include a range of electronic inputs such as yaw and pitch sensors and wheel speed sensors. These inputs and the computers that process them can then pre-emptively apportion torque to individual wheels – such as the torque vectoring differential found in Audi's RS-models and BMW's Active M Differential. By accelerating the outside wheels, a car will turn sharper and with less understeer.
This one has risen to prominence in recent years, particularly among front-wheel driven hot hatches whose wheels have to handle both driving and steering. Due to their 'flawed' base principles, a limited slip differential has limited effectiveness in such an application. To improve the torque vectoring capabilities, cars like the Mercedes-AMG CLA45, and even the Golf GTI, employ a brake-based system that pinches the brakes on the inner front wheel during cornering. By slowing the inner wheel, it performs the same function as a diff accelerating the outer wheel, sharpening turn in substantially.
But, brake-based systems have a couple of flaws. First of all, they're applying the brakes to make you go faster – it's a strange concept that seems oxymoronic, and sometimes downright moronic – but it's a cheaper means of adding torque vectoring to mass market vehicles, and as such we'll see it happening more frequently – we're already seeing it employed in the new Subaru Forester. However, the second, and biggest, fault with brake-based systems are that under duress and under repeated cornering situations, the continual applications of the brake to an individual wheel tends to over-use the brakes to the point that it cooks them.
The brakes heat up too quickly, and when it comes time to drop anchors hard ahead of a tight corner, they go soft and mushy and lose effectiveness. In this regard, brake-based torque vectoring systems are a short term solution; one that doesn't cater to the performance needs of those who look to eke out 100% of a car's potential.
Traditional differential vectoring and brake-based systems have existed for some time, but it wasn't until fairly recently that new clutch-based differentials came into existence. GKN Drivelines have been pioneers of these systems, supplying the all-wheel drive setups used in the Ford Focus RS and Buck Regal GS. These vehicles, and others sharing the same systems, feature a unique rear differential that features a clutch pack on either side of the axle that can engage and disengage the rear wheels individually to tailor the amount of drive sent to each wheel individually.
Unlike traditional differential-based systems, this effectively controls torque independently, allowing greater control and improved handling. Of course there are other added benefits – the Focus RS' 'Drift Mode' is chief amongst these. This mode uses the clutch-pack differential to allocate 70% of the torque to the outer rear wheel in a fixed split, driving the outer wheel harder and faster than the inner one to push beyond traditional torque vectoring and induce controlled oversteer.
In theory – at least until these systems have proven themselves – this system lends itself to advanced torque vectoring and the ability to tune cars in a multitude of ways that can lend them individual character traits based on each intended application. The versatility of such systems makes them highly impressive when it comes to high performance handling applications.
Then you have the new age version of torque vectoring – the one we'll see coming to the fore as the world drives towards an all-electric future. In its most simple operation, electric torque vectoring occurs when two electric motors are placed on 'one axle' – whereby one electric motor is affixed to each wheel and drives the wheels independently. This gives true torque vectoring capabilities as each wheel is individually controlled, driven, and adjusted, allowing individual wheels to be driven with up to 100% of the available torque.
More so than that by reversing the polarity to the electric motor, 'negative torque' can be applied, not just slowing a wheel, but effectively rotating it in the reverse direction of the opposing wheel. Not only can this ability be applied – as it is in the Acura NSX – to improve handling at speed, but it can be used in more pedestrian situations to improve low-speed handling an maneuverability. In theory, it can be used to effect on-the-spot turns. But, these systems require the complexity of an additional motor, the weight that brings with it, and a few potential drawbacks when it comes to power distribution and low-grip surfaces. Without locking functionality, electric drive with one wheel on a surface like ice becomes tricky.
There's also the limitation of an individual wheel only being able to use the maximum torque of the single motor assigned to it, rather than the total combined torque output. GKN drivelines have released a system that combines the principles of electric torque vectoring with those of the clutch based systems, that allows the use of a single, larger motor with higher outputs, to drive the wheels through a twin-clutch system. Torque can be apportioned left or right, with either wheel able to receive up to 100% of the total system's torque output.
When straight roads end and turns take over, torque vectoring can cut crucial split seconds off of lap times, and at times nip understeer in the bud before it blossoms into danger. It's the kind of technology previously reserved for high end sports cars – but now, it's mass-market technology that makes driving not only safer, but more enjoyable.