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Body Beautiful

Audi’s Space Frame concept isn’t all about aluminum anymore. Now there’s a new version that combines steel with aluminum to create an intriguing hybrid structure.

Filip Brabec, general manager, Product Planning, Audi of America, says that while the new TT would “look very similar to a Golf to the equipment in the plant,” it is a much different car in that it is the first hybrid version of the Audi Space Frame (ASF) concept. It’s not a “hybrid” in the powertrain context. Rather, it’s a hybrid in terms of materials. The first ASF was launched with the first generation Audi A8 in the mid-1980s. It combined aluminum castings, extrusion, and sheet to create the vehicle’s load-bearing structure. And, until now, there was no structural use of steel in this design concept.

 

Steel and Aluminum Mix

The 2008 TT Coupe’s structure is 68% aluminum by weight, while the Roadster uses 58% of the non-ferrous material. This is supplemented by high-strength steel, which accounts for the remaining 42% of the Roadster’s structure, and 32% of the Coupe’s. On the scales, the Coupe bodyshell weighs 455 lb. of which 309 lb. is aluminum, while the Roadster’s shell weighs 553 lb. and has a steel bulkhead behind the seats that ties into the steel understructure that extends from the B-pillar back. Both cars have steel doors with extruded aluminum impact beams, and the Roadster features a steel trunk lid in place of the Coupe’s aluminum hatch. All of the remaining exterior panels are stamped aluminum sheet.

“The steel is used at the rear of the cars to balance the weight distribution, and improve handling,” says Brabec. “Yet the static torsional rigidity of both cars is up significantly.” Indeed. The Coupe’s number rose 66% while weight dropped 166 lb., and the Roadster’s jumped more than 100% while overall weight decreased by 188 lb. “The new Roadster is more torsionally rigid than the old Coupe,” says Brabec. Had Audi chosen to build the body totally from steel, the Coupe and Roadster—which are 5.4-in. longer, 3.1-in. wider, and sit on a 1.8-in. longer wheelbase for 2008—would be 48% and 45% heavier at the same rigidity level.

The list of aluminum components includes: 139 lb. of sheet, 99 lb. of castings, and 71 lb. of extruded sections. Castings are used primarily in high load areas where they perform more than one function. For example, the A-pillar node combines connectors for the sill and longitudinal front member, A-pillar, and windshield cross member. The side sills of the Coupe and Roadster are visually similar aluminum extrusions, though the Roadster’s have thicker internal ribbing for greater strength, and both “plug in” to the A-pillar node. Similarly, the front longitudinal members work in conjunction with the castings, front cross member and subframe, the latter of which is bolted to the front structure at six points. Transverse extrusions reinforce the passenger compartment floor, and the windshield frame has a high-strength steel reinforcing tube. Audi claims much of this technology was drawn from the spaceframe of its now-defunct A2 city car.

 

Fastening, etc.

A number of joining methods are used in the TT’s structure, which is assembled and painted at Audi’s Ingolstadt, Germany, plant and shipped to its Györ, Hungary, facility for final assembly. These include punch-riveting, clinching, self-tapping screws, laser welding, and adhesive bonding. The adhesive not only holds the sections together, it forms a protective layer that prevents a galvanic reaction between the steel and aluminum. These areas also are designed with different geometries so the materials’ expansion rates are matched when the body structure goes though hot-dip priming. In addition, Audi claims the robot-applied self-tapping screws melt the outer surface of the materials via friction to form a more complete bond. Finally, the Coupe utilizes what Audi calls a “zero joint” between the roof and side panel created by laser welding the two sections together. There is no gap between the panels, and the weld is lightly finished before painting. 

The steel/aluminum mix isn’t the end of the line for TT technology by any means. The steering column has a defined crumple zone of 3.9-in. and works in union with a mechanism that uses the relative movement between the front bulkhead and cross member to swing the pedals away from the driver’s feet in an accident. In addition, says Brabec, “The steering wheel frame is made of magnesium (the wheel weighs 6 lb. with the airbag) to reduce the effect of inertia on the steering, and the Coupe’s rear seat is made of plastic.” Its base locks into the structure (there are no upper fixing points), has a pass-through for two pair of skis, and saves about 2 lb.

 

Drivetrain

In comparison to the structure, the rest of the car is relatively ordinary. The entry-level (Coupe: $34,800, Roadster: $36,800) models use the VW Group’s turbocharged, direct-injected 2.0-liter inline four-cylinder (200 hp/207 lb-ft) driving the front wheels through the S-tronic dual-clutch gearbox. Laid out as two three-speed manual transmissions that are shifted automatically, via the gear lever or steering wheel-mounted paddles, the S-tronic matches one clutch to the odd gears and one clutch to the even gears, and has the solid input shaft for one gearset and clutch assembly running inside a hollow one for the other. This allows the transmission to pre-select the next gear and engage the second clutch as the first is disengaged for almost imperceptible shifts.

Each gear is assigned its own electro-hydraulic switch, which means it’s possible—if the powertrain computer agrees (it’s also tied into the suspension management controller)—to switch from sixth gear directly to second. For greater shift smoothness, the ECU automatically matches the engine speed by “blipping” the throttle between gears. With the narrow angle 3.2-liter V6 (Coupe: $41,500, Roadster: $44,500), and quattro all-wheel-drive is part of the package. This engine produces 250 hp and 236 lb-ft of torque, and comes with a choice of six-speed manual or six-speed S-tronic transmissions. In steady-state driving, 85% of the engine torque is sent to the front wheels, though either of the axles can receive 100% in extreme conditions.

 

Chassis

Audi engineers lowered the TT’s center of gravity from 12.6-in. to 12.2-in., increased the front track by 1.73 in. and the rear by 1.77 in., and reduced the unsprung weight of the front suspension’s transverse links by 5.5-lb. In addition, the front suspension’s pivot bearings, subframe and wishbones, as well as the rear suspension subframe, are now made from aluminum. The front suspension remains a McPherson strut/triangular lower wishbone design, while the rear is an independent four-link design that separates the longitudinal and transverse forces, and mounts the twin-tube dampers close to the wheels to maximize the width of the trunk. In addition, the instantaneous roll center was raised from 1.65-in. to 2.52-in. to reduce body roll, and Delphi-supplied magneto-rheological dampers are available as an option in all models.

The speed-sensitive power steering system now has electro-mechanical assistance in place of the previous car’s hydraulic power assist, in part to help improve fuel economy. Braking is by four-wheel discs—12.3-in. front/11.3-in. rear on the 2.0-liter and 13.4-in./12.2-in. on the 3.2-liter TT—with ABS, pad wiping, brake assist, and front pads that are 15% more aggressive. Electronic stability control is standard.