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As retractable hard-tops become more popular, buyers are becoming more discerning. OEMs must cut the weight and increase the speed of new developments. By James Griffiths

It used to be that having a hard-top convertible variant was a relatively straightforward business decision. If you can build it for a price that market research says your customers will pay, then you’ve got a live, and lucrative, project. A few years ago, the novelty of such systems meant less attention was paid to the aesthetics of the vehicle, how well it performed and what the added weight did to fuel economy.

Now, with more and more retractable hardtops on the market, buyers are starting to compare how fast they can get the top up and down, the luggage space and the fuel consumption penalty. The balance between weight, packaging, styling, material, price, crash requirements, luggage compartment space and stiffness is more important. And the more sections the retractable hard-top folds into, the greater the complexity and cost.

It is common practice to outsource the issue. Engineering firm Karmann is well known for its work on convertibles for a number of OEMs. “Each has its own philosophy and its own specifications for colour matching, handling, driver dynamics, sealing, visibility and legal requirements,” says marketing director Christian Eick. “Keeping down weight and cost is a particular challenge because the two contradict each other, but new processes and materials are helping.

Volvo worked with Webasto to develop the roof of its C70 convertible; Pininfarina integrated the unit into the vehicle. It discovered that simpler is not always better. The C70 needed a three-piece folding roof so it takes up less luggage space when folded down.

“A normal two-piece fits like a clam-shell with the hinge – but it’s not very space efficient,” says Olle Odsell, the vehicle’s technical director. “Our system worked by having the first section move between the second and third before it goes into the trunk.”

The weight a hard-top can add varies by design. The hard-top for the MX-5 Roadster coupe increases the overall weight by 37kg. Mazda admits this is heavy, but has taken steps to ensure that the extra weight is properly managed so as not to affect the ride quality. “The Roadster’s roof folds into the cabin within the wheel base, so any impact to the front and rear weight distribution and yaw inertia moment is minimal,” says chief programme manager Takao Kijima.

To keep the weight down and achieve the desired 200mm thickness, the outer panels were constructed with resin sheet moulding compound, which provides a stiff shell with high heat-resistance, allowing a much greater formability than would be possible with steel. “Using aluminium would have made forming more difficult and it would have required padding,” says Kijima.“We used glass-fibre-reinforced polypropylene for the inner panels, allowing a thinner cabin ceiling.”

Volkswagen’s EOS, has a much heavier, 90kg, roof. But the vehicle is designed more for comfort than sheer sporting appeal. Part of the reason for the extra weight is that the roof for the four-seater divides into five sections and includes a tilting and sliding sunroof.

“It’s made of steel and glass and the roof cross member was kept very thin so it does not intrude into the passenger compartment,” says Karl Heinz Bionner, the engineer responsible for the EOS system. The extra weight was necessary to maintain stiffness.

OEMs have looked at reducing weight in other areas of the car to compensate for a heavier roof, using aluminium for sections such as the trunk lid and bonnet and replacing conventional steel with high-strength alloys. High strength steels will become more important, says Magnus Sviberg, pre-development designer for Edscha, the firm that supplied the hard-top for BMW’s 3-Series. But it will not be the only interesting development for reducing weight in the future.

“The cost of aluminium has come down in recent years,” says Sviberg. “But there’s the possibility to go even further with plastics, which the Smart Roadster used, or interesting materials such as magnesium or carbon fibre.”

By replacing steel panels with aluminium, a weight saving of about 40 per cent can be achieved. This could be increased to 60–70 per cent if carbon fibre were used. “We have the technology to do it, but it’s difficult because of the cost,” says Sviberg. “The convertibles coming in the next three to fours years will not yet use magnesium or carbon fibre.”

The 3 Series Convertible Edscha worked on for BMW also emphasises quick opening and closing times for the hard-top. It takes around 22 seconds for BMW’s three-part unit to open – faster than the EOS and the C70, which do it in 25 and 30 seconds respectively, but a long way off Mazda’s 12 seconds.

“A three piece roof is much more complicated than a two piece,” says Sviberg. “Every time you add a panel, you need to be able to move it, latch it and then adjust it – there’s a longer tolerance chain.”

For luxury cars, hard-tops tend to be more of a challenge because of the extra acoustic, aerodynamic and styling demands. Sviberg says roof systems are becoming so complex that the next challenge is to start simplifying them. On the 3 Series, one such optimisation was the elimination of a moving parcel shelf. “It’s important to reduce the amount of movement,” says Sviberg. “You also always want to have more than one movement happening at a time.”

Next comes optimising the hydraulics. Technologies such as pumps with variable flow proportional valves will save time by starting slowly, speeding up and then slowing down again through the folding cycle.

But Sviberg is dismissive of a drive for faster folding roofs for its own sake. “It’s a sales argument to say ‘it opens in 20 seconds’,” he says. “In practical terms, those four seconds really mean nothing, but it’s easy for a customer to compare.”

With the increasing popularity of retractable hard-tops, is it conceivable that all convertibles will one day use them? Eick says not. “By 2010, in Europe we expect hard tops to reach 60 per cent. In the US, Canada and Mexico, they’ll account for just 10 per cent. There are completely different target groups for soft-tops and hard-tops.”

Sviberg agrees. “The soft-top has a very good image and is still going to be better to package for high-quality premium sports cars.”

But with hard tops getting progressively lighter and faster, it’s possible to envision a future where the choice to go with a soft or hard top simply comes down to a preference in style.