Convertibles are a diversified niche business with 60 different models capturing just 900,000 units of annual global production.
That's only about 15,000 units per model and a combined 1.5 percent of passenger-car output.
But convertibles sales are booming. And because the open-air feature adds high-margin incremental volume for automakers, making roof modules, or sometimes even the entire car, is attractive to suppliers.
That's especially true for retractable hardtops, a relatively new development. Last year, about 250,000 convertibles worldwide were retractable hardtops, according to Polk Marketing Systems.
In Europe, folding hardtop sales grew to 188,000 last year from 99,000 units in 2003. Polk forecasts 220,000 European units this year and 340,000 in 2010.
Polk does not expect softtop convertible volume to decline. But it does expect folding hardtops to take all the growth in the convertible segment over the next decade because of folding hardtops' advantages -- attractiveness, security and ease of operation.
"By 2015, we expect the hardtop share to increase to 70 percent of the overall convertible market, without cannibalizing existing softtop presence," said Thomas Mawick, marketing analyst at Polk Marketing Systems.
He believes the world convertible market will be 1.2 million to 1.4 million by 2010.
The industry is unsure how big folding hardtops will become.
Germany's Edscha, which will enter the folding hardtop market in 2006 or 2007, sees softtops remaining popular. It expects 2010 global volume of 830,000 softtops and 520,000 folding hardtops.
"We may have a too-conservative forecast," said Walter Pecho, managing director of Edscha roof systems. "We still must see whether customers will appreciate the new models."
Paul Queveau, CEO of France's Heuliez, said softtops will remain popular for sports cars.
But he added: "I do not expect Peugeot and Renault to return to softtop convertibles."