A Look at GM's Kappa RWD Architecture

Twenty-seven months could be considered overnight when it comes to developing a new vehicle architecture, and that's how long it took a team of enthusiastic engineers to develop the rear-wheel-drive platform for the Pontiac Solstice - a concept-to-reality sports car coming in 2005.

The new Kappa architecture is the foundation for the Solstice production model as well as several exciting concept vehicles that demonstrate its adaptability. GM introduced the Solstice production model, as well as the Chevrolet Nomad and Saturn Curve concepts, at the 2004 North American International Auto Show. In addition to sharing the Kappa architecture, the concept vehicles reflect GM's global design and engineering resources. With cultural backgrounds from many corners of the world, designers came together at GM's Advanced Design Studio in Great Britain, as well as GM Europe's Advanced Design Studio in Sweden to collaborate on the concept vehicles' designs. Italian coachbuilder Pininfarina also helped assemble them.

"These vehicles have global appeal that translate well in any language," said Anne Asensio, executive director, Advanced Vehicles. "From the shape of the bodywork to the colors and interior materials, they each represent contemporary design."

Starting from Scratch

The impetus for the Kappa architecture's world-class platform was the Pontiac Solstice. A strong response to the 2002 Solstice concept car helped green-light the vehicle for production even though a compact rear-wheel-drive platform for it did not exist. A core team of engineers was assigned to develop one - and quickly. This efficient team collaborated globally to develop a flexible platform that would enable future production work for a distinct set of GM brands.

"We knew from the start that we would do it right or not do it at all; the support for that decision came from the very top," said Lori Queen, vehicle line executive for GM's small cars. "It has been an experience whereby the size and strength of GM was mobilized on a global scale to get the job done immediately."

To maintain the Solstice concept's aggressive stance, the chassis was developed to support the vehicle's short overhangs, long wheelbase and wide stance. This "wheels at the corners" design enhances handling and the overall feeling of stability. It also allows for a more comfortable interior, with increased legroom and hip room.

To ensure the lithe proportions of the Solstice were complemented with a firm foundation, engineers developed a lower-dominant tube structure for the chassis. Hydroformed frame rails, which run the length of the vehicle, are the basis of the chassis, while additional stampings form a rigid structure onto which the bodywork is attached. The hydroforming process uses pressurized fluid to form each frame rail from a single piece of steel, rather than several pieces of steel welded together. This creates a stronger frame rail and requires less time to form it. Additional components and stampings are added to the frame rail during vehicle assembly. A tunnel at the center of the chassis, which houses the transmission and driveshaft, is enclosed at the bottom to enhance stiffness. This built-in strength reduces chassis flex and cowl shake in a vehicle designed from the outset as a convertible.

"Convertibles typically are less stiff than vehicles with a fixed roof, so we set out to create the stiffest possible platform for a vehicle this size," said Queen.

The Kappa architecture's rigid structure also permits more precision when it comes to tuning the suspension. The Solstice features independent, SLA-type suspensions in the front and rear, along with coil-over springs wrapped around Bilstein monotube shock absorbers. The short-long arm suspension designs feature forged aluminum upper and lower control arms that are both strong and lightweight. The rear suspension also features a toe control link and the rear differential housing has an optimized three-point mounting design.

The Kappa's suspension design and geometry are not shared with any other GM vehicle architecture.

The Solstice also features four-wheel disc brakes, including 11.7-inch ventilated front rotors and 10.6-inch rear rotors, which combine with optional ABS with Dynamic Rear Proportioning to provide superior stopping stability. Eighteen-inch wheels are standard.

Dynamics Perfected

In addition to the robust chassis architecture and suspension components, Kappa engineers sweated dynamic details to produce a world-class platform, including:

  • Near 50/50 weight distribution for a balanced handling feel
  • Hydraulic engine mounts to provide a smoother engine feel
  • Optimized acoustic barrier and sound absorption package to provide world-class levels of interior quietness
  • Exterior components, such as mirrors, wind-tunnel tested to minimize wind noise
  • Tires selected for handling performance and low noise.

The Kappa chassis also was designed for the all-aluminum Ecotec four-cylinder engine. In the Solstice, the 2.4-liter version of the sophisticated DOHC powerplant features twin balance shafts to reduce engine-sourced noise and vibration. Technologies including intake- and exhaust-cam phasing and electronic throttle control are employed to maximize volumetric efficiency, boosting the smoothness and performance of the engine throughout the rpm range.

Clear Mission

Development time and cost were reduced by leveraging existing GM components where possible. The Solstice's rear differential, for example, is borrowed from the Cadillac CTS. Other proven component selections also aided Kappa architecture's development time. "It makes sense to leverage proven parts that fit the application," said Queen. "The parts are adapted to the Kappa architecture and do not compromise the vehicle's design or function."

Knowledge that the Solstice would be offered with a version of the Ecotec four-cylinder engine allowed the chassis team to design for the specific powertrain from the project's start. Because the Ecotec engine had not previously been used in a longitudinal layout, several accessory drive components required redesign to fit the new chassis.

"Knowing these challenges at the beginning helped us design it right the first time," said Queen. "We had a very clear mission on how to proceed with the architecture's design."

Although the primary focus of the Kappa architecture was the development of the Solstice, its platform is adaptable - as demonstrated with the Chevrolet Nomad sport wagon and Saturn Curve 2+2 coupe concept vehicles. The Nomad rides on a longer wheelbase than the Solstice for added rear passenger room, while the expressive Saturn offers a sophisticated, performance-oriented driving experience with a fixed roof.

"The Kappa architecture is a great platform for sporty, driver-oriented applications around the globe," said Queen. "Two years ago it didn't exist, but as the other concepts show, the additional possibilities are tantalizing."

Source: General Motors

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