Ford has overcome one of the most challenging aspects of high performance front wheel drive cars – giving its new Focus RS 300bhp (221kw) and 324lbft (440Nm) only a whiff of torque steer. The trick is a combination of clever new suspension and a motorsport-derived Automatic Torque Biasing (ATB) differential from UK drivetrain specialist Quaife.
For its highest performance European model, Ford wanted to retain front wheel drive because it is lighter and more efficient than four wheel drive and for many it gives a more tactile driving experience. The new suspension system, called RevoKnuckle and based on a traditional McPherson strut architecture, delivers a king-pin offset less than half that of a conventional McPherson system with wide track, in effect moving the turning line of each wheel closer to the wheel centre. This provides significantly greater freedom to optimise suspension parameters such as caster, camber and trail.
This tuning flexibility has allowed Quaife and Ford, working closely together throughout the 18 month development programme, to maximise the synergistic benefits of the ATB differential’s pairing with RevoKnuckle. Particular attention was paid to calibration of the torque biasing function to provide an appropriate blend of capabilities in the dry and wet for both road and track driving, and to ensure subtle intervention that feels natural to the driver.
The ATB used on the RS is a development of a Quaife technology proven over several decades of motorsport. Power is delivered to a sun gear on each drive shaft by a set of six helical planet gears (twelve 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 progressively transfers more torque to the wheel with most grip. In cornering, the ATB maximises 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 more stable in difficult conditions,” says technical director Michael Quaife.
“Our system uses natural forces within a helical gear set 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 differential is manufactured in a recently constructed 23,000 sq ft (2,136m2) facility designed specifically for high performance Original Equipment programmes. “Our latest investments in manufacturing systems allow us to produce gearboxes and differentials in volumes from 5,000 per year to 50,000 per year, as well as a wide range of other specialist steering and transmission components,” says Quaife.
“Selling vehicles in this challenging market increasingly requires strong branding and exciting halo models. Motorsport technologies can help provide this quickly and affordably with excellent reliability,” explains Quaife, who’s company has worked with competition teams from series as diverse as truck racing and Formula 1. “Over the last 12 months we have invested in an all-new flexible manufacturing facility specifically to support vehicle manufacturers with this type of programme, providing volumes optimised for niche applications at prices the big suppliers cannot match.”
The company’s OE capability now includes design, prototyping and manufacture of a wide range of driveline components for light and heavy duty applications including off-road. Quality standards are very high. According to Quaife, there were no warranty claims on the previous generation ATB built by his company for the first Focus RS and much of the work on the new LSD has been to ensure a similar level of reliability. Other roadcar programmes have included differentials for Chrysler (including the Dodge Viper) and General Motors.
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