Turning away from MacPherson struts

It began with a query concerning the electrically-assisted steering system in the new Ford Territory, but ended with a dissection of MacPherson struts -- and why they are compromised.

During the launch of Ford's SZ Territory, the company's Vehicle Dynamics Manager, Alex de Vlugt provided an in-depth run-down of the way Ford Australia's product development team has tailored the EPAS (Electric Power Assisted Steering) to suit the upgraded SUV -- and the local environment.

"In terms of the power steering, it's unique in that it doesn't have any hard component in common with anything else in the world, but in essence it's a mirror to the Mustang steering gear," de Vlugt began.

"The Mustang steering gear is very similar. It's got different mounting points because of the packaging of the vehicle, but the control boards -- all the controlling electronics -- [and] the driving motor are identical...

"The tuning is completely unique. We started off with the Mustang tuning and we didn't leave anything there in common at all."

If it seems like a stretch for the power steering system from a relatively small coupe to be adapted to suit a larger SUV, de Vlugt concurs, but it's not so much the respective vehicles' packaging or buyer type that distinguishes the two -- it's their differing front suspension systems.

"It's a conceptually different vehicle," de Vlugt said of the Territory -- contrasting it with the Mustang -- "but what has the most direct influence is different front suspension geometry that really leads to a need for a different boost curve.

"Even if you want to get -- on a total vehicle level -- a similar feel, you're already on the track of a different boost tune. Because of the different geometry layout we've got in our front suspension, the boost curve, in their most basic shape, already look different from the Mustang's.

"There's also less visible parameters in the tuning, which have to do with damping and with inertia feel compensation, which are completely unique, simply because of the different hardware layout.

"There's nothing common at all...

"If you were to take our tuning, simply as it is, and download the software into a Mustang control unit, it wouldn't feel good at all -- because you've got a mismatch between your boost... against the fundamental geometry layout..."

"That really leads to a couple of very distinct characteristics."

The Mustang is underpinned by MacPherson struts and the Territory features what Ford Australia calls a 'Virtual pivot control link' double-wishbone system. Ford developed the MacPherson strut system for models dating back to the 1950s, including the original Mk 1 Zephyr/Consul, but the Territory's system owes more to work done by German prestige manufacturers in the 1990s.

"The key reason that we are using the double-wishbone [configuration] is that it gives you a couple of fundamental geometry benefits," de Vlugt continued. "The first one probably is that you have got a better opportunity to make kingpin axis more upright than in a MacPherson strut.

"On the MacPherson [strut] you've always got it a little more tilted and the kingpin inclination is more inboard, which really means that you've got different behavior off-camber, over steering wheel angle. And on the double-wishbone it's far more linear than on the MacPherson strut."

"So if you go to extreme lean angles [with MacPherson struts], then you can end up with almost positive camber on both wheels -- on the inside and the outside corner... whereas double wishbones... you tend to go towards more negative camber, evenly distributed.

"If you turn to the left, then you get more negative camber towards that direction -- which will basically optimise your tyre properties better."

More complex and heavier, the double-wishbone system does offer other virtues, including better ride quality than MacPherson struts can typically provide.

"With a more upright kingpin inclination the disturbance angle from unbalanced effects or small disturbances from the road, your lever arm is smaller than [in] a MacPherson strut typically," de Vlugt explained.

The "lever arm" in double-wishbone systems equates to the lower control arm of MacPherson struts and needn't be as long to provide the same level of wheel articulation, but in addition to conventional notions of ride quality, the Ford engineer also points out that double-wishbone systems cope better with conflicting design parameters such as anti-dive under braking and compression of the system over bumps.

"By and large, you can say that a double-wishbone [system] gives you a better opportunity to balance different kinematic properties," said de Vlugt.

"Over road obstacles, where you get vertical movement forced on the suspension, then a MacPherson strut has got a very strongly defined relationship between how much you can let the wheel move rearward over those bumps and how much anti-dive you can get out of it.

"Typically, MacPherson struts are limited to building in too much anti-dive, because the front suspension will get very harsh if you do that. Double-wishbone has got more design freedom to create more anti-dive -- and at the same time, not have harshness issues."