Mercedes' 6th-generation SL is light in weight, heavy on technology

  • 03-Apr-2012 03:16 EDT
Merc3-12SL Aerodynamics.jpg

Mercedes instituted an intensive aerodynamics program for the new SL to pare the Cd down to 0.27 for the V6 version.

Creating new vehicle models is all about setting and achieving targets—defined by a combination of costs, legislation, environmental pressures, social expectations, and (sometimes overly) optimistic designers and ambitious engineers. Usually, targets have to be prioritized according to the role of the vehicle, and there will be clear delineation of needs to help keep everything in focus. Increasingly, as technology offers so much, all that is changing.

The new Mercedes-Benz SL, which is far more than just a “sports car,” epitomizes the challenge of integration and, inevitably, as with any design, some compromise—and the effort necessary to get that compromise right. The Director of Development for the new SL is Jürgen Weissinger, who faced the hugely challenging task of overseeing the integration of the wide raft of technologies that would make the car a multifaceted machine on a grand scale. These facets include premium luxury with more cabin space, combined open top and coupe (via its folding roof which comes in three versions), an enhanced safety focus, very high performance capability, and with ride/handling balance to meet a broad spectrum of demands (there are three suspension variants). The new, faster SL is larger than the previous model but far more frugal.

There was one defining engineering element of the SL (Super Light) that was fixed absolutely at conception: it would have an aluminum intensive (almost 90%) bodyshell. Previously described by AEI, this brought a combination of opportunities in terms of weight savings. The target was 120 kg (265 lb), which was bettered by 20 kg (44 lb) for the V6-engined version, which has a curb mass of 1685 kg (3715 lb) despite greater crash safety capability and more equipment, with associated improved dynamics, reduced fuel burn, and lower emissions. Additional challenges, though, involved its production.

“We had experience with the SLS, but many of that car’s parts are handmade; for the SL, we had to set up automated series production with particular focus on initial and total costs,” said Weissinger. Depending on applications, parts are produced via chill casting or vacuum die-casting worked into extruded aluminum sections or panels of various thicknesses.

“We also have used more magnesium in the SL than with previous models, with the upper and lower frames of the folding roof and the dividing wall between the passenger compartment and the rear of the car using the material. Altogether, magnesium is used for about 3% of the bodyshell.” High-strength steel tubing is integrated in the A-pillars.

Carbon fiber is obvious by its absence. But Weissinger explained: “We have to use the right material in the right place and at the right time; maybe in five years we will change some parts from aluminum or magnesium to carbon fiber. We can do that. We have very large cast aluminum parts, so we do not need a specially adapted line if we change material.”

The shape of the bodyshell, both as a BIW and fully dressed, was also critical. The previous SL versions had a drag coefficient (Cd) of 0.29 to 0.30, but the new car (V6 version) achieves a best 0.27 despite being larger, although its extra length helped contribute to the figure. “This is not just a best-in-class figure, but we believe it is the best of any series production sports car,” stated SL aerodynamicist Dr. Teddy Woll.

Because of increases in tire and body sizes to provide levels of comfort, handling, and safety required in recent years, sports car Cd figures have crept up. Porsche confirms that its 356 coupe (pre-A model) achieved a Cd of 0.296 in 1952, an early benchmark for series production sports cars. The latest 911 achieves a slightly better 0.29.

The new SL has received exceptional attention to aerodynamic detail, explained Woll. This includes precise shaping and grooving of the exterior mirrors to both smooth airflow and obviate the risk of rainwater and road dirt obscuring side window vision. Also, the rear deck is flat without the raised trailing edge typical of many aerodynamic solutions, but it nevertheless provides required downforce on the rear axle. Wind noise is very low with the hardtop closed, aerodynamics complemented by sealing precision.

Wheels are aerodynamically optimized, the underbody is flat, and a diffuser is fitted from the area of the rear axle to the back of the car. An active cooling air intake system contributes to aerodynamic efficiency via on-demand shuttering.

Woll believes Cd figures will continue to reduce, although it is a tough task: “A few years ago, we could never have imagined a wagon-type vehicle achieving a Cd of 0.24, but now we have it with our B-Class equipped with Eco package.”

Could this go lower without cars looking weird? Woll believes that vehicles looking very different from those of today might be acceptable if there was a major energy crisis with the associated need to make huge transport efficiency gains: “Then maybe we would have to have drag goals of 0.20 Cd or lower. But that is crystal ball gazing—and remember that a perfectly streamlined fish or bird’s Cd is around 0.05, but add wheels and that becomes 0.15!”

“Some compromise is a necessity for any vehicle, even the SL," Weissinger underlined. "We set drag targets to help achieve top speed and CO2, discuss overall development with engineering colleagues, and then go for those targets with the stylists; but compromise is a permanent game, a day-to-day process.”

Despite those inevitable compromises, fuel consumption for the 4.7-L V8 500SL has been improved by some 22% to 9.1 L/100 km in the combined cycle, power is up 12% to 320 kW (429 hp), and torque is up 32% to 700 N·m (516 lb·ft) from 1800 to 3500 rpm, while engine displacement is reduced by 0.8 L. 0-100 km/h (0-62 mph) acceleration comes in at 4.6 s. The 3.5-L 225-kW (302-hp) V6 is 30% more economical than the previous model.

At 4612 mm (181.6 in), the new SL is 50 mm (2.0 in) longer, and it is 57 mm (2.2 in) wider at 1877 mm (73.9 in).

Magic Sky Control (previously described by AEI) is an option for the folding hardtop, which takes 20 s to open or close. Although the word “magic” in this context may not conjure up an impression of traditional Mercedes’ design engineering and quality, the company is sticking with it and has now introduced Magic Vision Control for the SL. The system involves windshield washer fluid activated from minute channels (160 in total) in the wiper blade lip to avoid vision distorting water being pumped onto the glass. Operated with the roof lowered, the fluid does not enter the cabin. Heated wiper blades are an option.

Another interesting detail is the use of hands-free contactless opening and closing of the trunk lid. Sensors recognize a foot extended below the bumper. Ford will offer a similar solution for the new Kuga.

Free areas in the body structure in the front footwells are used as resonance spaces for the in-car entertainment’s bass loudspeakers.

An SL 65 AMG V12 twin-turbo developing 463 kW (621 hp) and maximum torque of 1000 N·m (738 lb·ft) will be launched in September. It is 170 kg (375 lb) lighter than the previous model. Performance figures include 0-200 km/h (0-124 mph) in 11.8 s.

Torsional rigidity of the SL is 19,700 N·m per degree, another target achieved but one likely to be raised still further on later generations of the car, believes Weissinger: “It is a question of weight, structure, and space—so it is that inevitable compromise again. Our task is to get the balance of compromises exactly right.”

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