Cars with retractable hardtops that fold and stow themselves with the push of a button used to be little more than delightful oddities.

Now, thanks to lower-cost and more space-efficient designs, they will be appearing in a variety of segments and are predicted by some suppliers to reach 30% of all convertible programs by 2009.

The idea always has been appealing: hardtops give a car a sleeker appearance and provide a quieter, more secure interior. But until recently, folding-roof systems have been heavy, expensive and took up too much storage space.

The earliest designs, such as the ’34 Peugeot 301 Eclipse and the ’57 Ford Fairlane Skyliner, mechanically retracted the roof as one piece and stored it in the trunk. The Skyliner initially was a hit, but it was impractical and glitch-prone and was canceled after the ’59 model year.

Retractable hardtops always have been desirable. Now they’re becoming affordable.

The Mercedes-Benz SLK ushered in a new era of more practical designs in 1996 that partially fold the roof. Since then, numerous luxury vehicles featuring retractable hardtops have entered production, including the Lexus SC 430, Cadillac XLR, Peugeot 307 and the Chevy SSR.

Now, a new generation of lower-priced vehicles featuring retractable hardtop (RHT) roofs is entering production in Europe, and the trend is expected to spread rapidly to affordable cars as well as new luxury models.

General Motors Corp.’s Adam Opel AG recently introduced the sporty Opel Tigra TwinTop in Europe. It starts at the equivalent of $17,270 in Germany before taxes.

The Renault Megane Hardtop, another affordable car with a retractable roof, also is in production in Europe. And Ford Motor Co. showed off a hardtop convertible Focus Vignale concept similar to a model the company is expected to build in 2006.

The retractable hardtop trend will shift into a high-volume North American application next fall when GM offers a topless version of its Pontiac G6 midsize sedan. It will shock the segment by selling for less than $30,000, GM insiders say. Most retractable-hardtop vehicles in the U.S. have sticker prices of about $50,000.

ASC Inc., which was founded as a sunroof manufacturer in 1965, anticipates explosive growth for RHTs and currently is working on three such programs with auto maker customers, says Jeff Steiner, ASC’s executive vice president and chief marketing officer.

The convertible market has been relatively flat in the U.S., hovering around 350,000 vehicles annually, for about a decade.

By 2007, however, the number of convertibles in the U.S. should exceed 450,000 units, and most of that growth will come from the arrival of new RHT programs, says research firm CSM Worldwide.

However, price remains an issue. An RHT generally costs an auto maker twice as much as a conventional soft top. Steiner says a soft top may cost an auto maker about $1,200, compared with more than $2,000 for an RHT.

– with Brian Corbett