The latest version of the Bentley GTC convertible, unveiled at the Frankfurt Motor Show in September, incorporates a number of developments including an extensively redeveloped structure and body elements that make strategic use of steel, aluminum, composites, and magnesium.
Dr. Ulrich Eichhorn, Board Member for Engineering, told AEI about some of the challenges involved executing the revised structure. “The big challenge on a large, extremely powerful, extremely fast convertible that is also supposed to be relatively compact is that you need a lot of stiffness to avoid shake, handling imprecision, etc.
“The load-carrying structure had to be steel to get the proper stiffness. We aided the stiffness with cross bracing under the car. We have two sets of cross-braces, one under the front that ties the front of the car into the sills and another triangulation that does the same in the rear.
“It’s a lot more difficult in the rear because you need to meet some structure there where there isn’t normally any,” Eichhorn shared. “So there is an added cross-brace that adds some stiffness in its own right, but mainly serves as an anchor point, so you basically have two triangles that meet at the tips and have broad bases where they attach to the body. That gives a lot of stiffness, both in bending and in torsion.
“The cross-car beam that also carries the instrument panel in any convertible is a very important source of stiffness, and we’ve developed some special attachments to the A-post that help the parallel movement of the header and reduce header shake dramatically.”
Bentley claims the GTC’s body is the stiffest convertible body in the world with a static measurement of 22,500 N·m/º (16,600 lb·ft/º). “We usually measure torsion,” said Eichhorn. “For a convertible it’s an incredible 29 Hz(for the BIW). We want that, do that, need that mainly to get the combination of refinement, NVH, ride, and sharp handling.”
Composite materials are used for the trunk lid and tonneau cover, which also features a cast magnesium inner, forming a stiff structure to support the roof at speeds above 300 km/h (186 mph). Like the Bentley Continental GT, the front wings are now made in aluminum using superforming to create the shape. Using a single aluminum sheet heated to around 550ºC (1022ºF), the wing shape is formed using air pressure. Welds and seams are also avoided by using this method. Eichhorn said that the result is very sharp lines: “A lot of people when they have seen it say, ‘This cannot be aluminum.’ Well, it cannot be aluminum in large volume, but if you’re only doing a dozen a day, or so, you can just about do that.”
Bentley has retained the fabric folding roof of the GTC, mainly, as Eichhorn pointed out, because that is what customers prefer. The design uses seven bows, with three layers of fabric. Eichhorn said that the roof will seal perfectly to the body at the claimed top speed of 340 km/h (211 mph).
“We use seven bows, where normally other people use five,” he explained. “That also helps the shape of the roof to stay smooth, to not give the ‘hungry cow’ effect that you sometimes get with five bows and with a thin roof.
“We use a special noise-insulating fabric that is about 50% heavier than normal and lasts the life of the car. Then comes a thermal and acoustic insulation layer, which in places is about 20 mm thick, plus we have inlays in specific strategic places as well. On the inside there is a cloth headliner, which looks very much like the slung headliner in a traditional coachbuilt car.”
The headlining incorporates a reading light and is made from a material developed by Bentley and Porsche, which Eichhorn described as a microfiber alcantara-type material that also has noise-attenuating properties.
“From a pure wind noise and outside noise insulation point of view, the coupe and the convertible are pretty much on the same level,” he reckoned. “For the first time on a Bentley convertible we have noise-insulating glass all-round.”
The double glazed screens incorporate an insulating layer between them. “This dramatically reduces the outside noise. Overall, it’s about 4 dB for the medium and high frequencies, especially the noise of oncoming traffic, etc.,” he added.
Another feature of the new model is application of a more powerful variant of the W12 6.0-L twin-turbocharged engine, raising power to 567 bhp (423 kW) at 6000 rpm and peak torque to 516 lb·ft (700 N·m) at 1700 rpm. The all-wheel-drive system has also been reconfigured to produce a front/rear torque bias of 40/60, compared with 50/50 for the original GTC. Next year, all Bentley Continentals will also get the option of the new Audi/Bentley 4.0-L direct-injection gasoline V8 with cylinder deactivation and engine start/stop.