Opel Astra GTC: Exclusive High-Performance Chassis
07/20/2011
Opel
Opel's compact coupe challenger caused a stir when it became available to order in June. With dramatic styling created under the lead of Mark Adams, Vice President Opel/Vauxhall Design, the GTC is set to wow customers and rock the established front runners in its class.
But the GTC's appeal runs far deeper than its rakish lines. Unlike many three-door versions of existing compact hatches, the GTC - like its predecessor - will be a standalone model in Opel's range, conceived to resonate with a more driver-oriented customer base who value the way a car handles and performs as much as the way it looks.
To do this, engineers from both Opel in Rüsselsheim and Vauxhall Engineering Center in Millbrook, UK have developed a bespoke platform for the GTC. Key components - such as the HiPerStruts used on the front axle - have been derived from the 239 kW/325 hp Insignia OPC, while significant upgrades have been made to the GTC's unique Watt's link/compound crank rear end. So while the GTC line-up currently includes a broad range of engines producing from 88 kW/120 hp to 132 kW/180 hp, drivers can always be assured of class-leading levels of dynamic control.
High tech front suspension boosts GTC's appeal
From the start of the GTC's development program the target was to deliver a driver-oriented feel to the car, and one that also worked well on UK roads. "Our aim was to exceed the best in class with the GTC's dynamics," said Michael Harder, Opel engineering's supervisor for vehicle dynamics. "We also wanted to change the emphasis in steering and damper feel from the regular Astra 5-door to a set-up that was more focused, yet still comfortable enough for the most demanding roads in Europe and the UK."
Much of the development centered around the introduction of Opel's HiPerStrut to the GTC, a first for an Astra model. Based on the system currently seen on the 239 kW/325 hp Insignia OPC, the HiPerStrut uses the Astra's existing pick-up points, but reduces the kingpin inclination angle by 44 percent and shortens the spindle length (kingpin offset) by 46 percent versus the MacPherson strut-equipped models. This helps prevent torquesteer - a trait of many powerful front-wheel-drive cars - allowing drivers to make more use of the GTC's performance without the steering being corrupted.
Adding the HiPerStrut has also reduced the amount of camber change on the GTC's front wheels during cornering, improving grip. Steering feel is enhanced, too, helped by a reduction the steering system's friction levels. The geometry changes brought by the HiPerStruts also mean that the GTC can be fitted with wheels of up to 20-inches in diameter.
"The current Astra 5-door has always handled exceptionally well," said Michael Harder. "But with the GTC, we've raised the bar still further. Drivers will instantly appreciate the extra level of involvement - and grip - allowing them to exploit the potential of the basic Astra platform still further."
Rear suspension revisions complement GTC's ride-height and track changes
Compared with the Astra 5-door, the GTC's ride-height has been lowered by 15mm, while the wheelbase has grown by 10mm, from 2685mm to 2695mm. Both tracks are wider, too, at 1584mm (+40mm) front and 1585mm (+30mm) rear.
The Astra's GTC compound crank/Watt's link rear suspension system has also been revised, with bespoke roll-stiffness and roll-center height settings for this application. The system has many advantages over a modern multi-link design, including improved packaging, greater wheel camber stiffness and reduced suspension friction. The Watt's link also ensures that lateral stability is maintained at all times.
The Watt's link is carried on a small cross-member attached to the underside of the car, just behind the rear wheel center line. It comprises a short, pivoting center link with a ball joint at each end, to which the lateral links from the wheels are bolted.
In a straight line, the set up ensures excellent stability, but during cornering it minimizes lateral deflection in the same way a modern multi-link system would do. Opel's engineers estimate that the linkage absorbs around 80 percent of all lateral loadings on the rear suspension. In addition, the Watt's link allows for softer bushings, which no longer have to compensate for toe changes at the rear of the car, and thus provide greater compliance and ride comfort from the rear axle.
Unique steering program for GTC
Precision, Feedback & Confidence. These were the three target elements for the GTC's bespoke steering program. The challenge was to maintain the system's ease of use at lower speeds, but dial in a greater degree of involvement and slightly more effort at higher speeds.
The Astra GTC uses a rack and pinion steering system with speed sensitive assistance. But in order to provide drivers with more steering feedback, the system's electric motor is mounted directly on the steering rack, as opposed to the base of the steering column.
The key benefits of using an electric power steering (EPS) are well known. At low speeds, the level of power assistance is increased to minimize steering effort. At higher speeds, assistance is automatically reduced to ensure a high degree of steering feel for the driver. The second important benefit is that, because it does not require an energy consuming hydraulic pump and responds directly to the amount of power needed at any speed, fuel consumption is also reduced.
FlexRide enhances GTC's driver appeal still further
The Astra GTC chassis has been designed to perfectly integrate Opel's intelligent, fully-adaptive FlexRide chassis control system. It enhances driving stability with better cornering behavior and steering response by automatically adapting to road conditions, cornering speed, vehicle movements and individual driving style. Better road holding and vehicle balance also enhance driving safety in emergency situations. In addition, FlexRide offers three different settings, enabling a change to the car's character on demand by the push of a button: one can choose anytime between the balanced Standard mode, the comfort-oriented "Tour" mode or the more active "Sport" mode.
Bad roads: The ultimate challenge for chassis engineers
"The UK road system is like no other in Europe," said Gary Baker, Vauxhall Engineering Center´s chief dynamicist. "We obviously test cars in many different countries, but the UK's severely undulating and heavily cambered roads often reveal handling traits that would otherwise be hidden on smoother surfaced and more predictable continental roads. It's not just the surfaces, either. Corners with changing radii are commonplace in the UK, as are blind apexes and crests, which mean that extra demands are put upon our cars and drivers. A good example is a driver who has to make a mid-corner adjustment on an unfamiliar road where the bend tightens unexpectedly and he throttles-off or brakes suddenly. The car needs to work with him, and respond intuitively to steering inputs, but it still needs to be rewarding to drive under normal conditions."
Recognizing that drivers don't always have access to winding, smooth-surfaced roads, the GTC has also been tuned to deal with the worst rutted and broken surfaces, even when equipped with its optional 19- and 20-inch wheels (18-inch wheels are standard).