FOCUS RS PERFORMANCE

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“Creating a 305PS, front-wheel-drive performance car is a significant engineering effort, but there is equal challenge in refining it to be a true, no-compromise driver’s car.” Dirk Densing, Chief Engineer, Focus RS

Pretoria, 25 August 2010 – Creating the power of a true Ford RS is one thing; managing that power into accessible yet exhilarating performance and assured traction is another.  This challenge was met by Ford’s renowned vehicle dynamics experts.

Under the guidance of Chief Programme Engineer, John Wheeler, the Dynamics team was led by Chief Engineer, Dirk Densing, whose relaxed demeanour belies his passion and pedigree in performance cars and motorsport.

This team has achieved a car capable of 0-100km/h in 5.9 seconds and a top speed of 263 km/h.

Wheeler and Densing were determined to create a dynamic mechanical set-up that would provide a pure driving experience, rather than manage power with constant electronic interference or truncation systems.

The most obvious change is the car’s wider track, but the team completed their solution with a suspension technology that was under development at Ford’s Research & Advanced Engineering centre (R&AE) in Aachen, Germany.  Called ‘RevoKnuckle’, this innovation was identified early on as offering significant benefit to a new front-wheel-drive RS.

The combination of wider track and RevoKnuckle front suspension is the reason Ford has been able to do what was thought impossible previously: develop a 224kW, front-wheel-drive car that is not only driveable every day, but faster in many situations than all-wheel-drive performance cars.

Exhaustive testing and refining in the hands of Team RS’ dedicated vehicle dynamics expert, David Put, has also created a set-up for Focus RS that forgives the bumps of every day roads, yet rewards on-track commitment.

“It sounds simple, but is a genuine challenge: make a potent performance machine as accessible and useable as any other Focus; able to transform instantly from everyday transport to track-day ‘supercar’,” explains Wheeler.   “It’s a combination of innovative technology and class-leading dynamics, finely honed through David’s expertise and commitment that made this possible.”

Torque Talk

The greatest engineering challenge was in managing the impact of the car’s 440Nm torque.  Torque steer – the impact of torque on steering in front-wheel-drive vehicles – is one of the key challenges in suspension development of front-wheel-drive cars.

In the real world, torque steer is most often experienced when hard acceleration, cornering or uneven surfaces are combined and its effects are apparent to the driver as a sudden or momentary disturbance on the steering wheel.  This is not ‘performance feel’, but a transient ‘error state’ which detracts from steering precision and performance.

 Wheeler explains the phenomenon:  “Torque steer is experienced when an unequal traction forces exists between the left and right front driven wheels. On a good flat surface, driving straight ahead, the left and right driven wheels have equal driving torque and everything is balanced.

“When left and right torque is unequal then the trouble starts.  In the design of front-wheel-drive suspension, two main aspects have to be considered to minimise torque steer: keeping the centre of the outer CV joint on the steering axis line, and minimising the offset from the wheel centre to the steering axis. This is most important in performance cars, where wider wheels create a greater offset to begin with,” he continues.

In the physics behind this phenomenon, traction forces at the tyre’s contact area with the road are translated in two ways:  one force ‘couple’ transfers harmlessly through the driveshafts, but a second force component acts at the wheel centre. This element creates a turning force about the steering axis.  The larger the offset between the wheel centre and the steering axis, the larger the turning torque.

In the design of the suspension geometry, therefore, the first priority is to keep the offset from the wheel centre to steering axis as small as possible.

Even though this is not an exact science, with wide tyres on uneven road, the tyre’s contact area with the road can move from the middle to the inside or outside of the tyre.

These unbalanced forces can be exacerbated further by a limited slip differential, as some have a very unforgiving characteristic of locking and applying transfer torque too rapidly.

Considering wide tyres, a wide track and wheel offset and limited slip differential are all present in the front-wheel-drive Focus RS, these areas were the first to be considered by the Dynamics team.

Tuned RevoKnuckle Technology

RevoKnuckle technology was originally conceived as the trend developed for high torque diesel engines to move from large, often all-wheel-drive vehicles into smaller, front-wheel-drive cars. The Ford RevoKnuckle approach is unique in its design and installation and has been developed and patented exclusively by Ford Research & Advanced Engineering (R&AE) and Team RS.

Ford RevoKnuckle is the product of six years’ work, beginning in 2001 and led throughout by project engineer, Marc Simon.  While Focus RS is R&AE’s first performance car application, Simon’s team was able to bring significant relevant experience to the project: in 2005, Simon and several other R&AE specialists were seconded to Ford’s World Rally team, developing the suspension layout and kinematics of the then-new Focus World Rally Car.

This gave them a sound understanding of the requirements for Ford RevoKnuckle in a high-performance road car, in both its functionality and durability.  “There’s a direct relationship between what we were doing in WRC and our work on Focus RS,” Simon states.

RevoKnuckle Structure

The performance demands of Focus RS mean it is the only model in the Focus range not to carry the traditional ‘MacPherson strut’ arrangement.

Regular suspension knuckles are a one-piece design; RevoKnuckle has instead two separate pieces – one part fixed to the strut; one part that rotates with the steering line of the car.

In place of the regular, inverted ‘L’-shaped lower suspension mount, RevoKnuckle features a ‘C’-shaped mount, connected to both the wheel hub, via two arms and to the strut, which is attached at its top.

This approach afforded engineers far greater flexibility to set the car’s suspension geometry to minimise torque steer, particularly by reducing the critical distance – known as the ‘king-pin offset’ – between the wheel centre and the steering axis line.

The structure of RevoKnuckle provides a layout where the familiar spring/ damper strut and lower wishbone dictate the basic wheel control and geometry, but provides a separated king-pin axis, more often associated with a double wishbone design. This provides more freedom in suspension set-up – such as application of camber, castor and trail – without the need for an expensive suspension re-design.  Most significantly, it also creates a king-pin offset less than half that of a conventional McPherson with wide track.

“RevoKnuckle allows the strut and lower wishbone to dictate the basic wheel control and geometry, but provides a separate king-pin axis – in effect, moving the turning line of the wheel closer the wheel centre,” explains Densing.

Ford RevoKnuckle Patented

Ford succeeded in combining the new RevoKnuckle with the proven Focus front suspension layout that is normally equipped with McPherson struts. The work to create this unique combination is now protected by a Ford patent.

RevoKnuckle has been patented by Ford for a key aspect of its design: the additional connection between RevoKnuckle and the front anti-roll bar, critical for its high resistance to rotation.

“RevoKnuckle is a straightforward piece of suspension design, in an innovative application that makes it absolutely right for a high-performance car.  The explanation may be a little scientific – the result is genuinely dynamic,” says Simon.

Innovative Mechanical Approach

In Focus RS, RevoKnuckle has been developed to work alongside the differential in enhancing traction and drive in a range of situations.  Focus RS’ Quaife epicyclical design relies heavily on friction of the gear surfaces which builds with differential speed, and has been tuned finely to avoid snatch and rapid torque changes.

Such a mechanical approach was very deliberately chosen over other available solutions, such as electronic torque truncation, or Electric Power Assist Steering algorithms to ‘mask’ torque steer effect by adding ‘counter-steering’ force into the steering system.

“We weren’t interested in masking torque steer or reducing performance to the point where it wasn’t an issue.  We have managed to reduce torque steer to a minimum, yet keep all the performance an RS should have,” concludes Simon.

According to John Wheeler, the importance of RevoKnuckle to Focus RS cannot be underestimated:

“All-wheel-drive or rear-wheel-drive have long been default options for managing torque in a high-performance vehicle.  We used AWD on RS models in the 80s and 90s, but it creates significant compromises in weight, fuel economy and the cost/ performance balance.  Through combined efforts of our Research and Dynamics teams, we have created a solution that simply redefines what is possible with front-wheel-drive,” he says.

Summarising the benefits of RevoKnuckle, Densing says: “RevoKnuckle gives the adjustability of a race-car style, double wishbone suspension, in a cost-effective McPherson architecture.”

Stopping Power

With such performance capability on tap, it’s no surprise to find Focus RS offers a vice-like braking performance.  Large 336mm x 30mm, ventilated front discs are gripped by stiffer, single 60mm piston calipers, supported by 302mm x 11mm rear discs.  Focus RS also boasts unique high-friction brake material and a tandem brake booster, all combining to generate up to 1.2 g under braking and give Focus RS a stopping distance of just 34.8 metres from 100km/h.

Anti-lock braking (ABS) with Electronic Brake Force Distribution (EBD) is standard, as is Ford’s Electronic Stability Programme (ESP), here tuned specifically for the car, to allow a high level of sporty driving before intervening.

“In some cars, ESP limits fun for the enthusiast, because it can be used to disguise chassis weaknesses,” explains Densing.  “In Focus RS, the ESP system is tuned for later and shorter interventions, so you don’t feel engine intervention and only occasional brake intervention, even on race circuits, or in rain.”

The ESP system can be deactivated for circuit driving.  However, this may not always be necessary, as Densing highlights.

“Because it’s so precisely tuned, our engineers have lapped the Nuerburgring just as quickly with ESP on, as with it off,” he states.

Stable High-Performance Set-Up

By employing RevoKnuckle, the Focus RS Dynamics team was able to retain Focus’ suspension layout, including ‘Control Blade’ independent rear suspension set-up for stability and controllability, albeit tuned and lowered by 20 mm for a performance driving character.

“Ford Focus’ fundamental dynamic character makes an excellent base for a high performance car – it’s agile, responsive and stable.  It required only subtle changes for Focus RS,” explains Densing.

At each corner, Focus RS is set up with stiffer springs and higher spring rates, with rear spring rates increased 40%, versus Focus ST.  These are matched to thicker and longer, 24mm anti-roll bars, aiding stability and mechanical grip.

Front and rear track width has been increased by 40mm versus Focus ST, with the resulting wider wheel offset and increased overall vehicle footprint sharpening handling and improving stability.

At the same time, the front subframe has been lowered 20mm, giving a low roll centre height.

The highly acclaimed Focus independent rear suspension has been refined to reflect the RS’ high performance intent, particularly in managing higher lateral acceleration. This is achieved through a rigid, cast suspension knuckle and larger rear wheel bearings, the combination of which improves stiffness significantly, creating the low rear suspension camber compliance critical to a responsive high-performance car.

The rear anti-roll bar has also been modified to control immediate roll, heightening vehicle stability and maintaining a fun-to-drive character.

The Dynamics team studied and developed further the ‘elastokinematics’ of the Focus rear suspension, generating a more direct build-up of cornering forces through reduced lateral compliance, for a more immediate and linear steering response.

Finally, David Put himself worked closely with tyre manufacturer, Continental, to develop a specific compound tyre for Focus RS.  The car’s 19-inch alloy wheels are thus wrapped in 235/35/R19 bespoke Continental tyres, giving superior road contact and huge lateral grip.

“Behind the wheel, all this translates into a direct response and a high level of cornering controllability, without sudden surprises for the driver.  On a circuit, the driver can feel exactly what the Focus RS is doing and predict it safely,” says Densing.

Dynamics Tuned for Precision

Ford’s renowned driving dynamics DNA comprises four pillars: stability, precision, comfort and agility and each Ford car carries a different balance of these attributes.

In outright racing cars, comfort is the least important.  Yet, while Focus RS drivers will accept a little less comfort for greater high-speed stability, agility and directional precision, engineers needed to maintain the sophisticated Focus ride and handling as far as possible, as the car must be equally competent as everyday transport.

“Focus RS has a different character from Focus ST, with a more overt sporting nature.  RS is intended to meet the needs of the serious performance driver when this is demanded, yet still provide a fully acceptable and rewarding vehicle for daily driving.  In dynamics terms, that’s a fine balance between raw, race circuit performance and the controlled precision required for on-road driving,” says Wheeler.

With stability and controllability in check thanks to the suspension set-up and RevoKnuckle, dynamics engineers moved on to concentrate on agility and precision.

In the Focus, this is characterised by a fundamental turn-in tendency that tucks in the car’s nose on throttle lift.  This is more pronounced in the Focus RS, by a more acute castor angle to the front wheels, for a keener dynamic response to throttle adjustments.

Focus’ Electro-Hydraulic Power Assist Steering (EHPAS) has also been tuned with a lower ratio (13.2:1, versus 14.7 for Focus ST), providing sharper responses and requiring less steering effort in cornering, with just 2.32 turns, lock-to-lock.

Ensuring the car’s weight did not rise too far was critical to handling as well as ensuring good performance feel.  Thanks to RevoKnuckle, which negated the need for all-wheel-drive, and the close attention paid to detail throughout its development, Focus RS weighs in at 1,467 kilograms. This is 75 kilograms heavier than a three-door Focus ST, an increase spoken for by the larger wheels, tyres and brakes, limited slip differential and aerodynamic and cooling aids.

“It doesn’t just feel faster; it’s also sharper and more responsive,” explains Put.  “In cornering, you will notice how the improved braking performance, higher lateral mechanical grip and sharper steering responses allow you to carry more speed into and through the corner apex. The car also carries a flatter attitude in a corner and has a more rapid load transfer.  It reacts faster to directional changes on twisty roads or race tracks and you can adjust its line on the throttle, so you can get back on the power quickly out of the turn.”

Aerodynamics

Aerodynamics also played a part in the Dynamics team’s work.  Says Densing: “Aero is very important to any car, but usually concentrated on not creating lift front to the rear.  With its sophisticated aero package, Focus RS generates genuine downforce at circuit speeds and we’ve covered thousands of kilometres refining computer simulations to find the right balance of downforce without too much drag.”

The target for high speed stability of a performance car is to position the aerodynamic centre of pressure at a controlled point behind the centre of gravity.

In Focus RS, the starting point was the standard Focus shape, crafted to generate moderate front end and rear end lift at higher speeds. The challenge was to transform this lift into downforce and bring the aerodynamic centre of pressure forward, without undue sacrifice to the drag coefficient and top speed.

Extensive wind-tunnel testing was conducted to refine aerodynamic performance, with a target of delivering about 40 points of downforce at the front and 10 points at the rear.

The resulting aerodynamic elements for Focus RS achieve these targets, creating 26% more downforce than Focus ST, yet with a drag co-efficient (Cd) of 0.38.

The result of all this effort is a car that is not just fast in a straight line. It is the fastest ever car around the infamous handling circuit at Ford of Europe’s Lommel Proving Ground in Belgium – known to the team as ‘Route 7’ – beating even the Ford GT and cementing its dynamic credentials.

In addition, Focus RS has completed over 400 laps (over 8,000km) of Germany’s fearsome Nuerburgring Nordschleife during its circuit performance development.

The purpose of all this work was to ensure Focus RS continues and enhances Focus’ reputation as a driver’s car and stays true to its core philosophy of generating pure driving enjoyment by flattering the novice driver, while rewarding the expert.

The Ford RS comes with a 5 year / 90 000km Service Plan and a comprehensive 4 year/120 000km warranty with service intervals of 15 000kms. Customers will also benefit from 3 years of Roadside Assistance and a 5 year corrosion warranty.