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I would like to buy a new road bike. However, I also want to make sure which material is the most sustainable. On the one hand in production and on the other hand in disposal.

I had already asked a question about carbon frames: "Is the carbon frame disposal problematic". It came out that carbon frame disposal is a problem. This is something I would actually like to avoid. You can choose between carbon, steel, aluminium and titanium.

I know there is bamboo and that would probably be the most sustainable material. However, I don't want a racing bike made of bamboo.

Peter Mortensen
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Maik Lowrey
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5 Answers5

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The process of manufacturing Carbon Fiber Reinforced Plastic (CFRP) parts—what we call "carbon" bikes—stands apart from the rest as extremely undesirable for the environment. Some bike companies have even decided they won't make carbon frames because of that.

The impact of carbon fiber itself is about 14 times more emissions than steel, but it's offset by its light weight - only 1/4 as much is needed. Almost all of the harm comes from the epoxy resin the fiber is bonded with. The process involves toxic chemicals in the resin, prolonged heating to set it, and leaves behind several times the frame's weight in plastic waste from single-use equipment. One unavoidable source of waste is the molds, made out of plastic, filling the entire inside of the frame while carbon is laid on top of them; these molds are destroyed to free the part and new ones are made for each frame. Avoidable waste includes tubing, wrapping, and excess resin. Unused material and used-up supplementary plastics end up as trash.

Recycling CFRP is somewhat possible, by burning off the epoxy (toxic) and chopping it into a powder, but the resulting material can't be used for products requiring strength. It's a growing problem in the aviation industry, and they're looking at environmentally friendlier thermoplastics to replace epoxy. Aviation's justification is that a plane burns over 1000 times its weight in fuel. Aerospace parts and processes also involve a lot less waste and toxic exposure.

As long as you're going with any metal rather than carbon, it doesn't really matter: being so light, bike frames have a very small environmental footprint. There's very little waste in making metal frames, and it's easily recycled. If any of the bike factory's workers commute by car, that's likely to have more impact than the metal used for the frame itself.

This is a big redeeming point for carbon bikes. The "worst" bike still has far less impact than even the "best", most-environmentally-friendly, even electric car, just due to the 100-fold difference in size. So if a carbon frame, fork, or full bike helps you enjoy the ride and skip the car, even sometimes, don't worry - it's well worth it! As an individual rider, any bike is better for the environment than any motorized transport it replaces. So any bike saves more than it costs.

But if I was in the business of making a million bikes, I'd check if I can meet each part's target specs with metal first. It's often possible (all-metal sub-6.8 bike), sometimes metal's better, it's also why planes aren't all-carbon. So if one needs more reasons to avoid cheap off-brand carbon parts, that's one - no-name factories also take more liberties with worker safety and waste disposal than high-end ones in Japan and Taiwan.

Bamboo is mostly an aesthetic choice. The highly stressed parts are still metal (or even carbon), only the tubes are replaced with bamboo. It's a cool look, but bamboo comprises about 10% of the bike's total weight, and the lacquer offsets the small amount of metal saved.

To go in depth about metals... Aluminium vs steel is an ongoing industry debate. Steel makers offer compelling reasons for steel, while impact studies slightly favor aluminum. When doing design analysis, steel starts out as more sustainable, but aluminum takes over when the proportions of non-fossil power and recycled metal in the mix increase. They're very close and are easily the most sustainable materials short of wood and concrete.

Titanium production involves more waste than steel or Al, but it's partially offset by its longevity. A bike-specific study found 3x more impact from titanium and stainless steel (which is up to 30% alloy) compared to carbon steel. In general, materials that cost more usually take more steps or resources to produce, and this holds here.

Summary:

  • Regular carbon steel has the least environmental impact out of any frame material.
  • Aluminum has slightly more impact than carbon steel, but slightly less with mass recycling.
  • Titanium and stainless steel have ~3x the impact of aluminum or carbon steel. Their longevity might offset it however.
  • Carbon fiber has the most impact, at 14x, and additionally involves toxic chemicals and non-recyclable waste. Off-brand carbon is worse.
  • With bamboo, refer to the rest of the bike's material.
  • The impact of any bike is small compared to the fuel it saves.
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Therac
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    How do you know? So all the numbers etc. – Maik Lowrey Aug 06 '23 at 08:00
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    Engineering experience (though not with bikes), we have to do these analyses nowadays. I've linked articles with relevant research as well. Overall the impact of any bike is small, but steel and Al have it the smallest. – Therac Aug 06 '23 at 08:09
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    Steel has another benefit, probably out of the scope of this answer - it's more repairable. My aluminium hybrid commuter could have been welded back together but it would have cost more than the bike was worth. The steel of department store BSOs could be fixed by any fool with welding kit; CrMo is harder but not massively so. Good luck finding anyone to repair Ti, and it's not indestructible. – Chris H Aug 06 '23 at 19:47
  • @ChrisH worth mentioning the repairability of CFRP then. There’s a shop near me that specializes in diagnosing and repairing cracked (not shattered) carbon. Some owners decide the repair isn’t worth the price (to them) and leave the frame with the shop. The shop then repairs the bike anyway and sells it to someone else. – Paul H Aug 07 '23 at 00:00
  • (Note: I recognize that’s a small scale (verging on negligible) practice. But the shop does receive bikes from all over the US for repair) – Paul H Aug 07 '23 at 00:02
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    @ChrisH True, repairing titanium takes serious skill. On the other hand, any intact titanium frame, however old, is still an easy sell at a price worth the effort. There's no path to landfill for them, except by mistake. Accident losses can be accounted for by reducing expected lifetime, e.g. from 50 to 30 years, which is still longer than most. – Therac Aug 07 '23 at 02:48
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    @PaulH it's a growing area, but growing from a small base and labour-intensive. Now that carbon bikes are sold at mass market prices I doubt ever be cost-effective for more than a tiny minority of frames – Chris H Aug 07 '23 at 05:29
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Bamboo tends to use a lot of resin-based materials at the joints, and is usually lacquered or otherwise coated. So it's not actually very recyclable as commonly used, despite being sustainable in production. With it not being mainstream, you'd be breaking it up by hand. There's also quite a lot of aluminium in the joints (I've designed a set of dropouts for a club mate)

Carbon fibre recycling isn't really a thing, especially for bikes - they're going to landfill. The source material is at least in part a product of the petrochemical industry.

Aluminium is quite energy-intensive (electrically) to produce from ore, but is very recyclable.

Steel is widely recyclable, and takes less energy to extract from ore than aluminium. On the other hand, steel production is harder to decarbonise than aluminium, as coal is burnt directly in steel making.

Titanium can be recycled in theory, but tracking down somewhere to do it properly may noy be easy.

Overall, steel probably wins, and it can be surprisingly light, but high-performance steel bikes are quite specialised.

Realistically, aluminium gives perfectly respectable performance for most of us, with decent environmental credentials. If I was building up a (road) bike from scratch, I'd go for aluminum with steel forks. Most mass-market aluminium road bikes now come with carbon forks, and that what I got recently.

Chris H
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Carbon fiber has lots of problems, but the sustainability isn't one of them.

A carbon fiber bike is essentially a plastic bike. It's made of petrochemicals. The frame has weight of maybe two kilograms.

Carbon fiber has a carbon footprint of maybe 25 kg carbon dioxide per kilogram of product. So 50 kg of footprint for the frame.

Let's consider what will happen if you ride the bike 30 000 km. This will likely require around 200 kWh of secondary energy, or 720 MJ. At 25% efficiency, it's 2880 MJ of primary energy from food.

If you eat only cereal (unlikely), wheat has carbon footprint of 0.8 kg per kg of wheat. At 13 MJ / kg, you need 222 kg of it. That's already 178 kg of carbon dioxide emissions from the food you eat. And this is for someone who only eats wheat. Most people have more varied diet, creating even more emissions.

If you plan on actually riding the bike as opposed to storing it indoors as a trophy, the emissions from the food you have to eat to ride the bike far outweigh any emissions from the frame production.

Besides, carbon fiber can be recycled -- by burning it. Into carbon dioxide. This will create less carbon dioxide emissions than what was created when producing the frame. Also, although currently the carbon in the carbon fiber frame is from petrochemicals, tomorrow I'm sure we make it from trees, from waste or even from carbon dioxide captured e.g. from forestry industry factories (which makes its origin sustainable) or even directly from the air.

If you want to really consider sustainability, longevity is the key. For me, this is a chromium molybdenum steel frame and fork. However, I have to admit that for electric bikes, forming the chromoly steel tubes into shapes that can easily and tidily accommodate a battery is hard, so maybe for e-bikes aluminum is ok. The aluminum frame may even outlast the quickly obsoleted electrical parts of the e-bike.

juhist
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    There's a excellent point here, that the bike is replacing or supplementing other forms of travel. "Sustainability" is a complex web, and if riding the worst bike (environmentally speaking) eliminates an even-worse form of transport then its a net-win overall. In other words, consider the whole system not a single cost. – Criggie Aug 05 '23 at 23:45
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    Carbon emissions is not the only measure of sustainability though. Just having waste matter laying around is another, or burning it to produce dioxins is another, and a carbon bike is more guilty of that than a metal bike. The carbon footprint you mention is also only for the carbon fibers themselves. It does not include the resin, curing energy, or the all the consumables used in some methods of composite production of each part. Resin infusion is particularly guilty of this with all the single-use disposable plastic tubing and films. – DKNguyen Aug 06 '23 at 00:21
  • @DKNguyen my understanding is that curing heat dominates the energy consumption of carbon fibre manufacturing. And only a few facilities could burn it both for energy and without a lot of pollution. – Chris H Aug 06 '23 at 08:46
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    @ChrisH When I say curing energy I am referring to curing the resin in an autoclave. But it sounds like you are using curing heat to refer to the the heat required to burn everything else in the organic precursor fiber to leave only a carbon fiber, done during the production of the fiber itself, not the composite. I imagine this would account for much of the energy required. – DKNguyen Aug 06 '23 at 17:07
  • @DKNguyen I was also referring to curing the resin (my experience of similar composites was in aviation and a long time ago, so I had a rather hazy idea but remembered that much). But por you mention it, fibre production probably is quite energy-intensive – Chris H Aug 06 '23 at 19:31
  • I was going to add a hyperbole to my own answer, comparing the whole Tour de France to one Formula 1 car, but turns out... Tour de France emits almost as much as Formula 1, season for season. Small numbers can add up. If e.g. the UCI replaced the 6.8kg rule with requiring metal except for forks and frames, I wonder how much it could amount to, not to count knock-on effects. – Therac Aug 06 '23 at 20:09
  • Isn't 2 kg for a carbon frame rather high? My Cannondale CAAD10 is only 956 g of aluminium. – Palle Due Aug 07 '23 at 10:54
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This is the wrong question to ask. The most sustainable bike is the one that you use to replace the most kilometers travelled with less sustainable vehicles during its lifetime. If you use it for 5'000 km before it breaks, no bike is going to be particularly sustainable. It still won't be bad, compared to a car, but not great either.

If you use it for 50'000 or 100'000 km at least, then you start seeing a benefit so big that it doesn't really matter what it's made of. It's going to trump most other modes of transportation that you could have used, with the exception of walking.

So: Look at the big picture. How are you going to use your bike? What other transport is it repacing? If you replace your sports car with a carbon fibre bike that you replace every year: Great decision. If you currently walk to work and are thinking about getting a repairable low maintenance bike made of recycled steel: It would be more sustainable to keep walking. If you currently commute 100km per week by unmotorized bicycle and are getting tired of it: Sure it's better to get an e-bike than to get a car.

Nobody
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    This is a very unscientific answer. The part about walking being more sustainable is not universally true because biking is more human energy efficient, requiring less food. Food is a lot of carbon emissions and land usage – cyanrarroll Aug 06 '23 at 20:20
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    This is an excellent example of "another viewpoint" and really helps see the question from another angle. – Criggie Aug 06 '23 at 22:11
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    On the flip side regarding walking, a pair of synthetic (essentially plastic) hiking shoes worn in the city lasts me about 1/3 the distance of a bike chain, and they go to landfill – Chris H Aug 07 '23 at 05:34
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    @Criggie Yes and no: the OP is about "race bike", which is very unlike to displace any other form of transport. The bike in that case is not an alternative to a race car, but to another sport. If the alternative is running, the footprint of the bike would probably superior (less equipment required, and if the OP participates to races, bikes impose serious constraints when traveling - larger car or additional energy losses because of the aerodynamic losses caused by the bike on the car, less public transportation options because of the bulk it causes,...). – Rеnаud Aug 07 '23 at 10:04
  • @Renaud Fair call - cycling does tend to be sport or transport and not both. A "road bike" could be either, and I assumed it was transport. Thinking through you're right, sports generally are not environmentally conscious and where they are tend to be afterthoughts. – Criggie Aug 07 '23 at 18:48
  • @Criggie It can be both, after all commuting by bike is a way to combine exercice and sport. But I would think that commuting on a race bike is the exception rather than the rule: after all, there are constraints with commuters that make race bikes lesser choices in most cases. – Rеnаud Aug 07 '23 at 19:37
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Anytime you go with carbon fibers or wood frames you're going to deal with resins and even though the frames themselves are compostable the resins are not, the most sustainable would probably be aluminum . It's very light and durable and outlasts steel frames due to it not rusting. And it helps the environment that they are being made from recycled material.

Mike
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    Aluminium doesn't rust, but unlike steel it fatigues cumulatively from even minor flexing. Rust can be prevented but fatigue can't – Chris H Aug 05 '23 at 17:38
  • Aluminium is also "infinitely recyclable" meaning that melts and recasting does not require new material to be added. Recycled glass and plastic generally needs some percentage of virgin material added each time it is remade. – Criggie Aug 05 '23 at 23:46
  • Also, welcome to the site - this is an excellent first post, please keep up the good work. You can also browse the [tour] to learn more how SE is organised. – Criggie Aug 05 '23 at 23:47
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    Carbon fiber is not compostable. I believe it's basically like graphite if you could take the epoxy out. – Weiwen Ng Aug 06 '23 at 14:07