As we’ve learned, the Coupé and Roadster concepts were created in parallel, giving the engineers three years to pay particular attention to bodyshell rigidity, driving dynamics, the soft top and NVH. Around the Greater Stuttgart area this spring, the new SLS AMG Roadsters cover their test routes under mild disguises. Easily identified by those in the know, there are no Mercedes stars or model designations to be seen, just black adhesive foil at the front, rear and along the sides.
With several months to go before the world premiere at the International Automobile Show in Frankfurt/Main in September 2011, the team at AMG has been finishing up the new SLS AMG Roadster. The latest simulation software and extensive test drives on all continents make the 571hp Roadster fit for its global launch in autumn 2011. Tobias Moers, head of overall development at Mercedes-AMG, explained: "Owing to the omission of a fixed roof and gullwing doors, it was necessary to design the side sills more robustly. But not primarily to meet the legal requirements in the event of a crash – in principle, the Roadster would also manage this with the side sills of the Coupé. However, studies and test drives showed that the driving dynamics could be improved further by doing this. Therefore, thicker side sills and chambers were developed for the open-top SLS AMG. In order to achieve handling dynamics identical to those of the Coupé, the Roadster has two further features designed to increase the rigidity of the bodyshell: firstly, the crossmember carrying the dashboard has additional support struts at the windscreen frame and the center tunnel. Secondly, a strut between the soft top and fuel tank makes the rear axle more rigid. These features prevent unwanted vibrations and make use of vibration dampers unnecessary.
Such measures also mean the soft top can be safely opened and closed electro-hydraulically while on the move at up to 31mph. The bodyshell of the Roadster tips the scales at only 535 lb – which is comparable to the 531 lb Coupé bodyshell. The Roadster also has a reinforcing crossmember behind the seats to support the fixed rollover protection system. The 250-watt 6.5” Bang&Olufsen BeoSound subwoofers are actually accommodated in the crossmember itself, since the Coupé’s parcel shelf is not available.
Another challenge was acoustic insulation of the crossmember. Numerous improvements became necessary, including a special bulkhead within the aluminium crossmember in order to satisfy the audiophiles.
The three-layered fabric soft top of the SLS AMG Roadster, which is deposited behind the seats in a Z-formation to save space, is also relevant to handling dynamics. Constructed from magnesium, steel and aluminum to optimize weight, the combination ensures a low centre of gravity, yet is able to withstand speeds up to a maximum of 197mph (electronically limited).
Whether open or closed, even at top speed, there must be no intrusive noises or resonance. Likewise the push-on glass draught-stop and the paneling in the interior must be vibration-free. The testing for this was carried out in extreme conditionsby test drivers on high-speed tracks in Papenburg, Nardo (Italy) and Idiada (Spain).
The interior acoustics also benefit from a seamless, bonded rear window comprising a single-layer of safety glass. A special production process not only ensures a smooth transition between the outer skin of the soft top and the glass, but the design also leads to low wind noise at any speed when the roof is closed.
Ensuring that the soft top is wind- and waterproof is a complex undertaking, and Mercedes-Benz used the experience gained during 125 years of experience. The five challenges for the AMG developers were water, sand, dust, heat and cold. One special feature of the soft top is the continuous water pocket, which is attached below the soft-top to catch rainwater and direct it down through two apertures on each side.
The rain test at the Mercedes Technology Center (MTC) in Sindelfingen is particularly demanding, and every new vehicle bearing the Mercedes star is required to pass it – whether it has a fixed roof, a soft top or a vario-roof.
Extreme quantities of water are used to ensure that the result of the development work is watertight, which is a particular challenge in the case of roadsters or cabriolets. Sixteen tests must be successfully absolved before approval is granted. Whether during the hose test, when all soft top, door and flap seals are sprayed with a water-hose, the continuous, overnight rain test, the fording test, the icing, swirl and high-pressure tests, or the final session in an automatic car wash – the rain test simulates every conceivable situation that can occur on any continent.
In addition to various test facilities such as the water chamber and the climate/ wind tunnel, the AMG specialists have recourse to test drives in all climatic regions of the world, where problems can be identified and solutions sought. In Laredo, Texas, for example, there is a particularly fine dust, which finds its way into practically any gap – and tests the seals to the absolute limit. The fabric soft-top, which can be conveniently opened and closed in just 11 seconds at the touch of a button, at speeds up to 30mph, must also submit to various tortures. One of these is the standardized soft-top endurance test used for all new Mercedes roadsters or cabriolets: 20,000 closing cycles on a stationary test rig must present no problem for the hydraulic cylinders, electric motors and joints. There are also 2500 closing cycles while on the move, whether in heat, icy cold, high humidity or dry desert winds.
Testing of the entire vehicle, focusing on the powertrain, chassis and suspension include 10,000km (6200 miles) on the Nürburgring's North Loop and 10,000km in city traffic. This is an extensive program that will only end at the world premiere on 13 September 2011, when the new SLS AMG Roadster is presented at the International Motor Show in Frankfurt/Main.