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Lightweight Construction

Detroit Sets Benchmark

| Editor: Janina Seit

Of all things, the road cruisers from Detroit are setting standards in lightweight construction. We take a view across the Atlantic Ocean to Cadillac.

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( Source: Cadillac/Jim Fets )

The design of the CT6 presents production with considerable challenges. Cadillac uses an unusually high number of joining techniques. Orange: 38% steel. Grey: 62% Aluminium.
The design of the CT6 presents production with considerable challenges. Cadillac uses an unusually high number of joining techniques. Orange: 38% steel. Grey: 62% Aluminium.
( Source: Cadillac )

When the German manufacturers launch a luxury saloon of just under 5.2 m in length, a six, eight or twelve-cylinder engine — or a hybridized four-cylinder engine — is usually fitted under the bonnet. The Cadillac is different: in the top model of the US luxury brand, the entry-level engine is a conventional 2.0-litre four-cylinder. The 198 kW/269 hp unit is sufficient — because the CT6 weighs only 1,639 kg.

The American is by far the lightest vehicle in the luxury class. It beats both the Audi A8 and the Jaguar XJ, both with a solid aluminium body, and is even lighter than the BMW 5 Series — one class below. The XT5 represents a similar achievement for Cadillac: Despite improved space and a plus in equipment, the XT5 is 132 kg lighter than its predecessor with a weight of 1,808 kg — although the Cadillac is equipped with a V6 engine in most markets. The XT5 is not only the lightest vehicle in its segment: it also beats the Audi Q5 and BMW X3, which are one class more compact.

A Light Crossover

on a mix of steel grades, some of which are laser-welded. For A and B columns, Cadillac uses pressure-hardened steels of the highest strength. Multiphase steels are also used for the columns and some structural elements in the base group.

The roof and large parts of the floor assembly are made of steel with medium yield strength; Cadillac uses, for instance low carbon steel for the outer parts. Leone attaches great importance to the use of modern simulation and analysis methods. He says: "The requirements for crossover models are specific. The weight reduction not only improves driving dynamics, but also fuel consumption and packaging.” The XT5 is built at the Spring Hill/Tennessee plant. The mixed aluminium construction of the CT6 is even more demanding: The "Omega" architecture was developed in 50 million computer hours and more than 200,000 simulations; the architecture was calculated seven times and evaluated in its entirety. For each of these cycles, the engineers evaluated more than 2,200 load cases. In addition, there were three separate interdisciplinary projects that analyzed weight savings, material selection and joining techniques.

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The Lightest Luxury Sedan of the Upper Class

With an aluminium share of 62 %, the CT6's structure is aluminium-intensive. All in all, however, the Detroit company has decided to use a mix of 13 materials. Cadillac uses high-strength steel in the A and B columns as well as in the safety cell, partly in conjunction with aluminium.

The rear is fitted with an aluminium crash element, while the front and side flanks combine aluminium and steel. “If we were to use aluminium for these parts, we would have to use more material for the sound insulation, which would ultimately result in a higher weight," explains CT6 chief developer Travis Hester.

Nevertheless, the use of aluminium is still worthwhile: Cadillac has calculated that the absence of light metal and substitution with high-strength steel would have increased the weight by almost 100 kg. Bending frequency and torsional stiffness even exceed ATS. The design of the CT6 presents production with considerable challenges, because the manufacturer uses an unusually high number of joining techniques. These include aluminium spot welding, aluminium laser welding, aluminium arc welding, spot welding for high-strength steels, flow-hole-forming screws and self-pierce rivets. Cadillac places a total of 3,073 spot welds per car and uses 268 m of structural adhesive. “This mix of joining techniques is not applied by any other manufacturer," says Hester, "It promises more precision than ever before”. He proudly reports that the process is "choreographed like an orchestra. Incidentally, a robot arm lifts the finished body-in-white onto a conveyor belt on the first floor that drives the next station. "An absolute novelty for such a large vehicle," says the developer.

Cadillac invested more than $ 300 million in a new body-in-white plant in the Detroit suburb of Hamtramck which is equipped with 205 robots. In addition to the CT6, small quantities of ELR electric sports cars have recently left the production line, and further models based on the Omega platform could be added in the future. A larger Cadillac called CT8 and a top saloon of the sister brand Buick are being considered too.

The result is impressive: the Cadillac CT6 moves with a light-footedness that is not expected to be the case with a vehicle of this size. The precise driving characteristics, the very good performance and the high level of acoustic comfort are directly attributable to the unique lightweight structure. And because common sense is not everything, the CT6 is also available with a 301 kW/410 hp V6 Biturbo. As a result, the mileage is now at V8 level.

This article was first published by Automobil Industrie.

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