Vauxhall Mokka 1.7 CDTI - Café Culture

The UK is a key market for compact SUVs, with Nissan’s Qashqai and Juke models proving unprecedented successes. The Mokka is GM Europe’s first foray into this segment and its first car on the new global small vehicle platform, Gamma II. The car will be sold in a variety of guises, from Buick through Opel, but it is Vauxhall, GM’s UK subsidiary, that was instrumental in the UK and European chassis specifications.

The development of the Vauxhall model started right at the beginning of the Mokka’s gestation. “We didn’t start with one generic model and develop the European spec from it,” explains Gerry Baker, vehicle dynamics manager at Vauxhall. “We started right at the beginning of the program, with the very first mule vehicles, and developed a European car in isolation from there.”

Vauxhall Mokka 1.7 CDTI

The bulk of this work was not done at Opel’s International Technical Development Center (ITDC) in Rüsselsheim, but instead at the Millbrook proving ground in the UK. “We approached this by recognizing that different European markets have different preferences, and in some segments those preferences diverge more than in others,” explains Baker. “As we continued through the program and continued to review the requirements and differences, we recognized that this is one of the segments where there isn’t a great deal of difference.”

As a result of this, the European and UK specifications grew ever closer, a move accelerated by the fact that Mokka is an SUV. “When you’re off road,” explains Baker, “the things that we would normally tune differently – steering calibration, damper settings, bushes, and so on – none of that really matters, it’s all the same and the expectations are all the same.”

The extensive off-road testing program at Millbrook (see sidebar, opposite page) also resulted in a common standard for the Mokka’s electrical driving aids being defined, with the ABS, stability, and traction controls all sharing a pan-European state of tune. “There’s a great advantage to that,” explains Baker. “We can tune one system exceptionally well rather than trying to rush two.”

“We approached this by recognizing that different European markets have different preferences, and in some segments those preferences diverge more than in others,” explains Baker.

This common state of tune also extends to other subcomponents and systems within the car. Extensive testing in the UK was also backed up with programs in Germany and Spain, resulting in the European-spec cars all sharing damper settings, while the two steering setups ended up converging far more than expected.

“Quite late in the program,” says Baker, “we came to the conclusion that the typical European calibration, where the steering is light on center and the effort and steering wheel torque builds up gradually as you steer, didn’t actually benefit this car very much.

“The perception that this is the best thing to do on the Autobahn didn’t pan out and the use of the car in town said that we didn’t need a heavy steering setup. We don’t need that wide center window; instead we need the responsive feel you get from having a slightly steeper torque build-up curve.”

Meanwhile, the UK steering setup was criticized for its lack of feel. “The British settings came from a heavy, meaty, firm steering and we found that it made the car feel quite dead, dull, and unresponsive.”

The British settings came from a heavy, meaty, firm steering and we found that it made the car feel quite dead, dull, and unresponsive.”

The result for the UK car is a compromise between the two, with some of the ‘effort’ taken from the steering so as to lose any of the dampening, hysteresis, or ‘connected’ feel for the driver.

The Mokka’s column-mounted EPS has also been refined to reduce the ‘typical springy feel’ associated with that type of assistance. “Every time we develop a car,” acknowledges Baker, “there are more parameters to play with and it gets more complicated, but you can end up with a much finer finish. We ended up with two settings for the steering that were very, very similar. And even though the calibrations are different, they are so similar that we recognize that for this segment in future, we may not even try to do a separate UK steer, although it’s something we’d keep in mind.”

The front suspension uses MacPherson struts paired with offset springs. “The car has been designed for a wide range of intended uses,” explains Baker, “from an urban family carrier, to some capabilities for some fairly tough off-road expedition. So we wanted to reduce friction caused by the wheel position on the strut.” By allowing the strut to rotate through steering, friction is reduced and sensitivity is heightened in the steering, ultimately resulting in a “much smoother ride”.

“The car has been designed for a wide range of intended uses,”

The rear suspension is by way of a compound crank. It comprises a crushed tubular crossbeam, indented into a V section, the orientation of which dictates the roll-center height and position of the rear suspension, while also setting up the toe and camber combination for the axle.

“The length of the V-section indentation is critical to roll stiffness,” explains Baker. “With those parameters, we’ve got very fine control over roll-stiffness for handling balance and roll-center position for the front/rear roll force distribution.”

The dual nature of the car made the suspension settings critical. “We had to tune this vehicle for a much wider range of capabilities than a normal car, which was a challenge in itself,” says Baker. “First and foremost, it’s going to be carrying people around town, on motorways, and on relatively smooth roads. So a smooth ride was paramount.”

Vauxhall Mokka 1.7 CDTI Interior

The Mokka’s relatively wide track enabled Vauxhall engineers to give it good ground clearance without it becoming top-heavy and having a tendency to roll, or needing such a high roll-stiffness that the ride became uncomfortable. “The vehicle does exceptionally well off-road and we had a huge amount of fun defining those parameters,” states Baker. “But given that the basic architecture is moderate ground clearance and a wide track, none of the things we did for off-road capability impacted on the on-road refinement.”

As a result, the Mokka’s dampers have been tuned for on-road prowess, while the spring rates have been kept deliberately low to aid ride comfort. Because Vauxhall was able to tune the roll center heights, roll-stiffness has been kept to a minimum, reducing ‘head toss’ across moderately rough country roads.

“We were able to get the on-road comfort through damper tuning, springs, and ARBs. This means that the off-road capability comes from the ground clearance and the cleverness of the AWD system.”