Responsive driving dynamics has been a Ford preoccupation and has won the company some acclaim since the Mondeo. ‘Dynamics and enjoyment are becoming one of the most important influences on design,’ says Ulrich Eichhorn, vehicle dynamics manager.
The multi-link rear-suspension set-up used on the Focus is a first for a car of this class. It is similar to the Mondeo estate system but has been optimised to reduce weight and cost of manufacture. The principle is make the suspension geometry stiff laterally and compliant longitudinally for ride comfort. This is achieved by a longitudinal ‘control blade’ whose compliant rear bush enables it to absorb bumps without affecting camber or toe-in. A toe link and camber link allow longer-than-average vertical wheel travel with good camber and toe control, impossible with the torsion beam more usual on cars in this segment.
With the rear wheels behaving themselves, Ford’s engineers aimed to make the steering as precise as possible and reduce friction in the front suspension. The front set-up looks like a standard MacPherson strut arrangement, but the struts have been angled to put them directly in the line of action of the resultant force from the wheel loads, eliminating bending and reducing friction.
The design was benchmarked against rival models from Peugeot, considered to be the leaders in this area. Again, the bushes have been designed to make the set-up stiff laterally and compliant longitudinally.
Starting with a good basic system has numerous advantages, says Eichhorn. First, it is inherently safer: ‘The car’s behaviour is the same at high speeds as at lower ones.’ It is stable in emergency lane-change manoeuvres at 100km/h, for example.
Second, although anti-lock braking and the ITT’s electronic stability programme to prevent excessive understeer and oversteer are available, they have been programmed to intervene less aggressively. ‘We wanted to have them as the icing on the cake, for use only in extremely difficult situations,’ says Eichhorn.
Third, it opens up more options for derivatives. ‘If you have roll steer, for example, you can compensate in some areas, but it could become a problem on derivatives where the loading is different.’