Automotive Engineer

Hot hatches are designed to perform on narrow, twisting country roads, rather than on the seemingly endless straight sections on motorways. Shifting down gears and turning hard into corners is where drivers gain the most enjoyment. But increasingly powerful engines mean that the chassis must become more capable too, keeping the vehicle on course if the driver goes into a bend a little too fast.

OEMs have different approaches, especially when trying to counteract torque steer. Some vehicles, including the Renault Sport Megane and Clio, use dual-axis MacPherson struts to compensate for the power being sent to the front wheels, while others, such as the Volkswagen Scirocco, just use standard struts but with optimised geometry.

Now, the Astra GTC coupe uses a dual-axis design too. Gerry Baker, vehicle dynamics manager for the GTC, says: “We couldn’t have made such a driver-focused car without it. The obvious choice would’ve been the classic MacPherson strut but with higher-powered engines you can get torque-steer and variability in the feedback that you can’t tune out, however clever your electronics are.”

The Astra GTC chassis has been developed to work with six engine variants, the most powerful a 1.6-litre turbocharged gasoline unit that produces 132kW of power at 5,500rpm and 230Nm of torque from 2,200rpm. It enables the GTC to reach 100km/h in 7.8 seconds and gives the car a maximum speed of more than 220km/h. The most powerful diesel engine is a 2-litre unit developing 121kW/350Nm.

Development work was conducted at Opel’s facilities in Rüsselsheim, its nearby proving ground in Hunsrück, and at Millbrook in the UK. “The project started two years ago. Ride and handling engineers were involved right from the beginning when we were target-setting to understand what the vehicle’s dynamics were going to be,” says Baker.

The programme’s aim was to change the emphasis in steering and suspension feel from the regular Astra to a more performance focus yet retain the standard vehicle’s comfort.

The set-up for the front suspension was taken from the larger OPC Insignia sedan. Although the GTC uses the same pick-up points as the standard Astra, the kingpin inclination has been reduced by 44% and the spindle length has been shortened by 46%. While the front of the car poses engineering challenges, the rear axle also needed to be adapted. Baker and his team had to revise the Astra’s torsion beam and watts-linkage set-up to maximise vehicle performance.

“The crossbeam is a pressed steel tube with a U-profile pressed into it. By adjusting the length of the U-profile and making the radius smaller or larger you can define how stiff it is which affects roll stiffness of the suspension. And as well as being a structural beam it also performs a role in maintaining camber stiffness,” says Baker.

By rotating the crossbeam so that the apex of the U-section is either vertical or tilted fore and aft, Baker could move the roll centre of the rear suspension to adjust the roll movement and the roll stiffness, giving him the ride and handling characteristics he wanted. It also changes the toe and camber characteristics as the wheels move up and down.