hen Ford were developing the Mk2 Focus RS they had a problem to overcome. Big power (300bhp), and more importantly, huge torque (324lb/ft) running through a front-wheel-drive, MacPherson strut front suspension set-up would cause significant torque-steer.
With modern cars having LSDs and/or layshaft systems to overcome most of the torque-steer associated with uneven-length driveshafts, the main culprit for causing the phenomenon is now the angle of the kingpin offset – a line between the bottom balljoint and the top pivot point, ie the strut top mounted on the inner wing. This angle is quite large using traditional MacPherson strut design. Other problems are not being able to get the CV joint centre near enough to the wheel centre (especially on wide rims as the tyre would then rub on the strut body), and something known as the scrub radius – the patch of tyre contact on the ground between the angle of the kingpin and the tyre centre line. When a car is accelerating hard through a corner with a rough/damp surface, these factors combine to make torque-steer even worse.
For the powerful new Focus RS, an expensive and weighty all-wheel-drive system could have been designed, or Ford could have gone for a complex and costly double wishbone suspension. But the company didn’t want to move away from FWD and MacPherson struts, so some sideways thinking was needed.
Engineers in the Advanced Research & Advanced Engineering (R&AE) Department in Germany therefore developed an innovation they dubbed the RevoKnuckle – from a concept first thought about in 2001. This is a C-shaped addition to the lower part of MacPherson strut that allows for an independent kingpin assembly to be used. And with this being independent to the angle of the main strut, it allows a better kingpin offset angle, and can also be positioned directly in the centre of the wheel – eliminating both of the major torque-steer contributors, and reducing the scrub radius too.
Although that all sounds very complex, the easiest way to grasp what’s going on with the RevoKnuckle is to look at the set-up on a car. Traditionally, when a MacPherson strut-equipped car is jacked up and the steering wheel turned, the whole of the strut rotates with the hub components. With the RevoKnuckle in place when the wheel is turned only the hub, brakes, wheel and tyre swivels.
Having that separate lower section, is the key to why the new RevoKnuckle system can comfortably deal with huge amounts or power and torque without suffering from so much torque-steer. If you want any further proof, just take a new Focus RS for a drive!