Easy Riding

Burn calories, not fossil fuels. Biking for an hour at 10 miles per hour (which requires about the same physical effort as a brisk walk) can eat up 408 calories in a 160-pound person, and if that person commutes to work via bike at least four days a week (eight miles, round-trip), she can save herself 54 gallons of gas annually and approximately 1,140 pounds of carbon dioxide emissions.

Unfortunately, as clear as the benefits of biking are, choosing a "green" bike is less so. Each of the materials used in bike frames—such as steel, aluminum and titanium (raw materials that must be mined) or petroleum-derived carbon fibers—has harmful effects on the environment. But materials aren't the only important factor, says Scot Nichol, founder of Ibis Cycles. "Product longevity obviously comes into play here," he says. "If you don't replace the bike but every 20 years, then you're going to have a low [eco] impact and be the greenest guy on the block."

So the best advice for shoppers is to choose a bike that withstands the demands of your life, whether they include commuting around town or recreational off-roading. Aluminum frames, the most common and affordable, are less durable than steel frames, which are more expensive and harder to find. Carbon fiber and titanium bikes have a long lifespan but can carry a hefty price tag. As for those catchy folding bikes, they're great for space-challenged apartment dwellers, says Carlos Dall'Orso of the Manhattan Velo bike shop, but if space isn't an issue, buy full size. "Folding-bike hinges can go bad and require extra repairs," he says. "Full-size bikes are cheaper to maintain in the long run."

Product picks: Trek steel frame 520 touring bike for long trips ($1,239.99; www.trekbikes.com); Specialized aluminum frame Crossroads Sport for leisure riding ($330; www.specialized.com); Fuji aluminum frame Absolute 2.0 for daily commutes ($710; www.fujibikes.com, 215-824-3854); Dahon Speed D7 folding bike, rated a Consumer Reports "Best Buy" ($329.95; www.dahon.com); and Calfee's bamboo-frame bike for people willing to try anything (from $2,695; www.calfeedesign.com, 800-965-2171).