Like tandems and recumbents, folding bicycles provide a unique alternative for cyclists concerned about space, portability and convenience.

The concept is simple: With a few adjustments, a smaller-wheeled or full-sized bike becomes compact and more easily transportable – often in less than 30 seconds.

Until in recent years, however, the folding bicycle industry suffered from a hard-to-overcome dilemma. Folding bikes not only looked different, a prevailing thought was that they folded at less-than-ideal moments.

As such, folding bikes were popular among boat owners, private pilots and recreational vehicle enthusiasts. They packed their folding bikes in tight spaces and used them emergencies or for short excursions to complement their main recreations.

Mainstream cyclists, however, largely remained unimpressed and were among the non-buying majority.

That's an image, of course, folding bike manufacturers like Dahon, Brompton and Bike Friday, among others, believe is changing.

With improved technology, ease of use and increased restrictions for full-sized bikes on some metropolitan public transportation systems, folding bikes are increasingly appealing.

Yet, the question remains: Are folding bikes reliable?

"We still get that kind of reaction," said Joshua Hon, director of sales marketing for Dahon, the world's largest folding bike manufacturer. "We get it everywhere. But in Japan now, the product has become mainstream. And in Europe, it's on the way to becoming a mainstream product."

Frenchman A.J. Marcelin is often credited with inventing the folding bicycle in 1939. His version was a 16-inch-wheeled bike called "Le Petit Bi."

But it was Hon's father, David Hon, a physicist and laser technology expert for Hughes Aircraft Corporation, who brought the folding bike back en vogue.

Like many commuters in 1975, Hon grew tired of waiting in gas station lines during the oil and gas crisis. He decided to again rely on cycling as transportation - like he did in college. But he also realized transporting bicycles long distance wasn't practical.      

Seven years later, the first "Dahon Folder" was introduced. Like other folding bikes, the Dahon Folder folded and unfolded
with a series of easy-access, adjustable brackets. And commuters who wanted to transport bikes long distances no longer had to worry about problematic roof or trunk racks.

Despite vast media attention, however, Hon couldn't persuade any manufacturers to license his fledgling product. Instead, Hon quit his job, acquired venture funding, relocated to Taiwan and built a factory.

More than 20 years and 1.5 million sales later, Dahon remains the industry leader. It offers dozens of models and styles, featuring 16 to 26-inch wheels, aluminum to steel frames, weighing 23 to 31 pounds and priced from $179 to $1,800.

Dahon also has a new racing folding bike, the Allegro. It doesn't fold, but separates into two pieces, fits into a suitcase, weighs 18.7 pounds and is priced beginning at $1,800.

Many other folding bike manufacturers offering smaller-tired options to full-sized racing bicycles, are also now in the mix.

Brompton, the British manufacturer, has had its bikes sporadically available in the United States for several years. But the importer named after the London neighborhood is now expanding to a hopeful nationwide U.S. retail distribution with three styles, five models and three and six-gear options.

Touting its offering as "the only bike in the world to combine a first-class ride with all the convenience or ready portability," Brompton handmade steel three and six-speed models, weigh 24 to 27 pounds and cost approximately $600 to $1,110.

Brompton is also hoping its offerings will help cycling commuters battle the increasing restrictions against full-sized, non-folding bicycles on public transportation systems. One of Brompton's brochures features a commuting businessman, dressed in suit and tie, carrying his folding bike in one hand while reading a newspaper as he walks along a train platform.

Bike Friday, a folding bike manufacturer since 1991, offers single, tandem and triple, custom-make folding bikes, beginning at $998. Bike Friday's motto is "Performance That Packs." Its bikes, like other manufacturers', fold into suitcases not subject to additional airline luggage charges.

Montague, promoted as the largest manufacturer of full-sized folding bikes, has gained particular notoriety with its "Paratrooper" model. The 24-speed offering, priced at $649.95, has been used internationally as a military bicycle.

"Having a folding bike gives you a lot more freedom of mobility," Hon said. "You can say, 'Hey, honey, I'm going to ride the bike out to the shopping mall. Why don't you meet me there and we'll go back by car?' "

"Or you can ride your bike to the train station and then take the train. You can't do that as easily or at all in some areas on the bike, especially in New York or Chicago or Los Angeles. You just can't always combine a bike with public transportation very easily."