Q. What is the most ecologically sound choice for a bicycle frame — steel, aluminum, or carbon fiber?

David B.
Tucson, AZ

A. Dearest David,

I like your style. Not only are we talking bikes, the transportation darling of the green movement, but we’re talking the best bikes. Making a good thing even better is one of my favorite pastimes.

Everyone likes to talk about their two-wheeled chariots as if they’re emission-free — which they basically are, once you get them cruising. But you’ve put your finger on one inescapable fact: Bikes don’t just grow on trees. Most bikes are metal, and producing metal is hot, sweaty, energy-gobbling work. So what’s a well-meaning cyclist to do down at the bike shop?

As you note, major frame choices include steel, aluminum, and carbon fiber; let’s round out that list with one more: titanium. All four metals are considered energy-intensive to produce: Iron (the raw material for steel), steel, and aluminum alone account for 10 percent of our total manufacturing energy use. Exactly how energy-intensive each frame is isn’t totally clear, as I haven’t seen many comprehensive analyses from bike manufacturers (read on for the exception). But let’s roll on with a closer look at each.

Aluminum bikes tend to be cheaper than the others, and they’re very recyclable at the end of their lives. But in a Duke report commissioned by bike manufacturer Specialized, researchers found one aluminum frame required a lot more energy to produce, resulting in lots more carbon emissions, than a carbon fiber model (about 170 kg of carbon dioxide equivalents per kg of bike vs. about 60 kg). Using recycled aluminum would shave a lot off that total, but unfortunately, David, new bikes tend to use the virgin stuff for structural reasons.

Carbon fiber bikes, on the other hand, are prized by pros and weekend warriors alike because they’re strong but very light — with a high price tag to match that high performance. Made of carbon graphite threads coated with epoxy resins, carbon fiber frames came out ahead in the Duke report for energy use, but racked up the highest water use numbers: about 2,000 liters of water per kg of bike (the aluminum frame was about 1,200 liters). And because it’s a composite, carbon fiber is very tough to recycle. Specialized does have a take-back program in place, but recycling involves chopping the fibers into smaller pieces, so that reclaimed material won’t find another life as a new bike frame.

Then there are the steel bikes, which are durable workhorses but also kinda heavy. On the plus side, they’re also very recyclable, and old steel can be endlessly recycled into new bikes (though good luck tracing the source of the steel on any particular bike). And in general, producing steel emits less carbon than producing aluminum — 1.8 tons of CO2 per ton of steel to 2.2 tons of CO2 per ton of aluminum — though I couldn’t find a bike-to-bike analysis for those two.

Finally, there’s titanium, a rather niche option in the bike world. Enthusiasts like bikes made from element 22 for their favorable weight-to-strength ratio, durability, and “road feel,” but they’re quite pricey, and titanium itself is harder to extract and refine than aluminum.

But for all this carbon-counting, David, the point remains that all metal bikes are expensive, energy-wise. Knowing that, we’d do well to limit the number of new bikes we bring into the world — so let’s look at the longevity of these frames. Titanium is tough, doesn’t rust, and can be repaired if it gets dinged. Steel is even more bomber and easy to fix. Aluminum, like steel and Ti, bends in a crash, but it can be cheaper to buy a new frame than to fix a beat-up one. And carbon fiber? Well, when it fails, it tends to do so suddenly and spectacularly — as in, carbon fiber bikes can shatter underneath you. That’s not to say you can’t ride one safely for years, especially if you’re not aggressively racing, but it’s worth considering.

In the end, the best choice when bike shopping may not be steel, aluminum, carbon fiber, or titanium, but E) All of the above, if they’re used. As with so many other consumer choices, buying used is an excellent way to go because you’re avoiding new manufacturing emissions. Quality secondhand bike shops are usually fairly easy to find, and Craigslist et. al. can turn up some gems, too. If you’re a real bike guy, you might also look into a newly rebuilt ride based on a vintage frame for cool cred as well as planet points. Unless you absolutely need a brand-new set of wheels, that’s how I’d counsel you.

I should also mention one more choice while we’re at it: bamboo bike frames made from the fast-growing, sustainable crop. Word has it they’re durable, recyclable, energy-light in the production process, able to be made from U.S. bamboo, and possess a certain classy charm. Cheap they are not, however.

And finally, while it’s wonderful to choose our bikes wisely, let’s not lose sight of the fact that cycling instead of driving — no matter what the bike is made of — is one of the best everyday steps we can take to reduce our impact. Bottom line: Shop smart, then don’t sweat it.