Is it better for the environment to buy a carbon bike or an aluminium one? It’s a question that’s done the rounds on the internet: carbon proponents point to the material’s clean production process; aluminium fans shout about just how recyclable the silver metal is. And in the end, there’s no wholly right or wrong answer.

However, the green credentials of a bike brand will increasingly come into play when people are deciding just which is the best mountain bike for them.

Trek’s 2021 Sustainability Report is the first of its kind from any big bike manufacturer, it looks into the environmental impact of the whole company, with the goal of reducing its carbon footprint. And guess what? The raw data shows carbon frames generate nearly three times as much CO2 as aluminium ones.

Three bikes get carbon rated

Chart shows the total emissions from the building of three bikes: a Trek Marlin aluminium hardtail, a Fuel Ex carbon full-suspension bike and a carbon Rail full-suspension e-bike

Take the Fuel Ex, used in the report as a comparison bike because it typifies Trek’s off-road range. As you probably know, the bike comes in multiple build options, and you can get it with either a carbon frame or an alloy frame – the former generates around 63kg of CO2e (carbon dioxide equivalent) to build, while the latter produces just 22kg. That puts a complete carbon bike at around 220kg and an alloy one at 120kg.

The story is the same for the Rail, Trek’s brilliant e-bike and former mbr E-Bike of the Year winner – the alloy framed bike produces around a third of the CO2e of the carbon model. To put that into some context and make us all feel good about riding bikes, to build a car you’re looking at anything from 5.5 tonnes up to 13 for an SUV.

Alloy beats carbon

The data doesn’t stop there either, the report is really interesting because it breaks down the bikes it analyses (Fuel Ex, Rail, Marlin and Madone) into their component parts and analyses the footprint of each, from headset to bottom bracket and everything else in between. The take-home message is that the top-end bikes generate more CO2e than their lower-tiered siblings, and bikes that come with premium components, like AXS or motors and batteries, also come with a significant CO2 punch.

Case closed then – alloy is king and we should be buying bikes with basic drivetrains and suspension. That’s not the full story, though – Trek’s report looks at the data from producing an estimated 1.6 million bikes a year, and from delivering them to its retailers for sale to you as customers. Aside from warranty claims (which the report does go into), that’s where the story ends.

Clearly that’s just the beginning for the riders who’ll swing a leg over the frame, because it matters just as much what you do with a bike as how it’s made. Carbon-fibre generates a greater carbon footprint, but it also lasts longer in use than aluminium. This means it could maintain its usefulness and value, pass from the original owner to the next, and offset that initial carbon outlay. The same could be said for higher-end products – buy the bike you actually want, keep it for longer and forestall the need to upgrade in a year or so’s time. Does carbon’s extended lifespan make up for this then? Nobody knows. It’s currently impossible to work out as there is no research data on the lifespan of a bike, so no real way of generating a CO2/km life cycle analysis.

Green miles

Trek takes a broader approach to this, carbon-fibre or aluminium doesn’t matter – ride 430 miles that you’d otherwise have driven in your car and you’ll be carbon neutral on the purchase of your bike anyway. It’s not specific to a model or frame material though, so you’ll need to add on a few extra miles if you’re riding a carbon-fibre e-bike.

Before we all fall over in a smug green delirium though, there is one slight problem – most miles on a mountain bike aren’t replacing road miles in the car. They’re optional. Trek has an answer to that though, namely you need to start using your mountain bike for day-to-day riding too.

“The good news is, any bike can be used that way,” Eric Bjorling from Trek told us. “The same bike we drive to the trails is almost 100% of the time appropriate to ride to a grocery store, school, work, etc. It doesn’t necessarily have to be all at once. 430 miles over the lifetime of a bike is very doable
for most.”

He’s right, most of us could clock up a few hundred miles running errands. The really bad news, though, is that many of us drive hundreds, maybe even thousands of miles a year specifically to ride our bikes, obliterating any of the gains we make riding our bikes to the shops. It’s really a systemic problem, where transport accounts for a sixth of our total emissions.

Future perfect

Trek should be lauded for its report, especially given that no other bike brand to date has done anything like this – exposing the carbon costs of an entire business to scrutiny. This needs to be just the beginning though, for the industry and our own personal riding habits.

We need a circular manufacturing process, where brands take back bikes to be recycled at the end of their life, argues Bernhard Isopp, a researcher and lecturer in sustainable mobility at the Technical University of Munich. “This would require efforts on behalf of manufacturers to use more recycled materials, and encourage people to recycle their bikes at the end of life,” he says. “Ideally, manufacturers would create a programme where they take back their old frames, or at least facilitate the re-entry of them into the recycling stream.”

Riders need to make greater efforts too. Covid has taught us that riding closer to home isn’t to be regarded as a poor alternative to a ‘proper ride’, and can sometimes be more rewarding than unthinkingly driving to the same trails we’ve been heading to for years. Eric Bjorling’s suggestion is to get more involved in trail advocacy as well, helping to maintain and preserve your local trails and the land on which you ride.“There is nothing in the world more effective at sequestering CO2 than a tree,” he says. “The more trails we have, the more natural land we preserve, the healthier we all will be.”