Front-wheel drive and high power and torque outputs are not always easy technology associates, despite advances in electronic traction control systems. The new 2.5-L, five-cylinder, turbocharged Ford Focus RS returns exceptional figures for a front-wheel-drive car, including 224 kW (300 hp) and 440 N·m (325 lb·ft), a combination that calls for special control solutions.
To achieve them, Ford elected to adopt the motorsport-derived Quaife automatic torque biasing (ATB) differential. Working together with Ford’s RevoKnuckle suspension, it has been developed to keep torque steer to an almost zero level under the most challenging conditions, while retaining required steering quality.
Ford wanted to use front-wheel drive for the RS mainly to save weight against a four-wheel drive system. "The problem was how to put so much power through the front wheels without tremendous issues with traction and torque steer," said Quaife Technical Director, Michael Quaife.
The RevoKnuckle uses the existing MacPherson strut architecture but with a two-piece knuckle that incorporates a "C" shape lower suspension mount. This creates a kingpin offset less than half that of a conventional MacPherson system with wide track. In effect, this moves the turning line of each wheel closer to the wheel center and provides significantly greater freedom to optimize suspension parameters including caster, camber, and trail.
As well as substantially reducing torque steer, the new architecture can operate effectively with a high-performance limited-slip differential (LSD) as it allows the benefits of the ATB to be maximized. The system has been designed to avoid compromising steering feel and smoothness.
The Quaife ATB is a new development of proven motorsport technology. Michael Quaife explained that, when one wheel starts to slip, the patented system automatically biases torque delivery to the wheel with the most grip. In cornering, it maximizes the amount of power that can be used by biasing torque distribution to the outside wheel. This combination of functions allows the driver to apply more power through corners and on loose surfaces and makes the vehicle potentially safer and more stable in difficult conditions.
Power is delivered to a sun gear on each driveshaft by a set of six helical planet gears (12 in total) running within the input drive hub. As one wheel starts to slip, the torque differential across the two sets of planet gears increases, causing the set transferring the most torque to progressively lock. This reduces their rotational speed and, also progressively, transfers more torque to the wheel with the most grip.
"We believe that only a helical LSD can provide an appropriately high level of functionality without compromising other aspects of dynamics, such as steering feel and refinement," said Quaife. "Our system uses natural forces within a helical gearset so does not require the electronics or friction materials used in most other LSDs. It is a simple, highly robust solution that is also exceptionally effective."
The new version of Quaife’s ATB has been engineered to maximize the synergistic benefits of its pairing with the RevoKnuckle suspension. Quaife worked with Ford’s vehicle dynamics team throughout an 18-month development program using computer-aided engineering techniques to lighten and strengthen the existing design and adding a sixth planet gear to help handle the torque from the turbocharged engine.
Particular attention was paid to calibration of the torque-biasing function to provide an appropriate blend of capabilities in both dry and wet for both road and track driving and to ensure subtle intervention that feels natural to the driver.
The new differential is produced in a recently completed 2136-m² (23,000-ft²) facility designed specifically for high-performance road-car programs. Stated Quaife: "Our latest investments in manufacturing systems allow us to produce gearboxes and differentials in volumes from 5000 to 50,000 per year as well as a wide range of other specialist steering and transmission components."
The Quaife company was founded in 1965, initially supplying five-speed gearsets for motorcycle manufacturers, Norton and Triumph, and later developing a wide-ranging motorsport business, leading to a Formula One victory in 1986 when Gerhard Berger won the Mexican Grand Prix in a Benetton using a Quaife ATB differential.
In 2000, the company achieved success in the European Truck Racing Cup with Caterpillar Truck Racing running a Quaife differential. Since then, the technology has helped secure podium finishes across a wide range of motorsport series. Road-car programs have included the original Ford Focus RS (2002), differentials for Chrysler (including for the Dodge Viper) and General Motors, and various driveline technologies for other vehicle manufacturers.
The Quaife LSD is also increasingly popular with specialist vehicle builders who are converting standard road cars for users such as the police and rescue services to improve off-road capability. Selected Quaife technologies are also available for aftermarket applications.