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As a low-carbon option for transport in urban areas, bicycles are synonymous with sustainability.
Now, bicycle manufacturers are turning their attention to sustainability, and the pioneers in this field have unearthed a few surprises. Here, we look at the four materials most widely used for bicycle frames – steel, aluminium, titanium and carbon fibre (CF).
Ninety years ago, engineers from Reynolds Technology in Birmingham, England, developed a revolutionary, light, strong, aerospace-grade, seamless steel-alloy tubing known as 531, reflecting the proportions of the elements manganese, carbon and molybdenum in the steel alloy.
Bicycle frames made from steel alloys dominated cycling until the 1990s, when large-scale manufacturers largely switched to aluminium frames because they were less expensive to produce than steel. This century, there has been a rapid growth in frames made from CF, which consists of woven threads of carbon material moulded and bonded using resins that harden. The characteristics of a bicycle frame depend on its shape and the materials used within it.
Engineers and frame-builders can create a frame that is light and strong using any of the four main materials, and each material has its advantages and disadvantages. Steel and titanium alloys, for example, are as versatile as they are robust and resilient, and can generate a frame for any purpose, whether for sport, commuting, recreation or hauling cargo.
Aluminium is almost as versatile. The lightest frames are made from aluminium and CF, with no material matching CF for its strength-to-weight ratio and mouldability. Therefore, elite-level and professional racing cyclists typically use CF frames.
Manufacturing bicycle frames has environmental impacts. So manufacturers such as Reynolds are looking at how to reduce impacts. Trek Bikes, based in the US, first looked at its carbon emissions for making bicycles in 2021, finding that CF frames had by far the highest footprint, and recycled steel the lowest. Starling Cycles in the UK, which makes bespoke steel bicycles using Reynolds tubing, found comparable results in its own investigations, with a CF frame having a carbon footprint up to 16 times that of a steel-alloy frame.
The biggest surprise for Trek Bikes was that it was using aluminium with a high carbon footprint, so it worked with its suppliers to change this. From this year, the company will use low-emission aluminium in all its products, which will reduce the embedded carbon in frames by 80% and make a huge contribution to lowering its scope 3 emissions.
So what encouraged Reynolds to look into its emissions? “One of our customers, Starling Cycles, asked us about it, so we decided to investigate,” explains Martin Shepherd, general manager at Reynolds Technology.
Reynolds worked with the Warwick Manufacturing Group at the University of Warwick. “They have an SME engagement team, and some of what they are doing is helping with environmental impact reports and greening the supply chain,” says Shepherd.
“Our investigation is cradle-to-factory gate, mainly because we felt that was all of the chain that was in our control. We choose the steel and titanium mills we work with, the way we heat our factory, what light bulbs we use and how we get to trade shows,” he says. “We felt it covered everything we could control.”
Reynolds makes alloy frame tubing from steel, titanium and aluminium, and the investigation looked at the first two metal alloys. The report confirmed that all the steel in their tubing is recycled, but like the few other manufacturers that have published emissions reports, noted how much freight and travel emissions contributed.
“The report has provided us with a conscience. We are mindful of emissions with every decision we make,” says Shepherd. “As travel and air freight were significant contributors to emissions, the company has changed the way it works and, as well as reducing travel, especially by air, is now working with customers to use sea and land transport.” Such emissions reports have also shown that even similar alloys, as Trek found, can have big variations in their carbon footprints, depending on where and how they are manufactured.
Metals such as steel, aluminium and titanium, thanks to their chemical properties, can be recycled indefinitely. According to the Bureau of International Recycling, at least 85% of scrap steel is recycled globally, with industrialised nations recycling far more. Aluminium is similar, with many countries having recycling rates near to 100%. Moreover, recycling metals has numerous environmental and economic benefits. Titanium is similar, although published investigations to date show that its carbon footprint is higher than aluminium and steel, but much lower than CF. So why are environmental impacts higher for CF frames?
CF appears to be a wonder material because of its strength, lightness and manufacturing flexibility. Its top three uses are in wind-turbine blades, aircraft and sports equipment. For example, using CF in aircraft wings can reduce fuel use by 5%. However, the process of making CF is energy-intensive.
CF bicycle frames have also yet to prove as versatile or as durable as metal-alloy frames, and CF itself has exceptionally low recycling rates – based on multiple sources, no more than 15%. Considering that uses of CF are projected to grow by at least 10% per year, recycling rates need to increase to meet demand, as well as reducing environmental impacts.
The international governing body for cycle sport, the Union Cycliste Internationale (UCI), is well aware of this paradox and is participating in the Carbon Fibre Circular Alliance, which aims to improve the environmental performance of CF.
The UCI has determined, for example, that 90% of CF frames end up in landfill. So, with the UCI, bicycle manufacturer SCOTT Sports SA has been participating in a demonstration project with the University of Bristol and UK company Lineat Composites to recycle CF frames. Lineat Composites has successfully converted used frames into unidirectional tapes of CF, which can be used in other products, such as skis. The recycled CF is not as strong as primary CF, but still sufficient for many products.
Meanwhile, another UK company, Hunt, which makes high-performance aluminium and CF bicycle wheels, has been working with a process developed for recycling the CF in windmill blades. This process uses an evolved resin that can be readily recycled when the product has reached the end of its life. The advantage of this process is that the original CF does not lose any of its strength.
While bicycles are typically used for local transport, most frames have already travelled thousands of kilometres before the first pedal stroke. If there is one thing Shepherd of Reynolds would like to see, it is much more localisation. “Here in Birmingham in the 1960s and 70s, you could source everything you needed to make a bicycle. All within a Birmingham postcode. And on our doorstep we had Dawes Cycles, Sun and Hercules, as volume bike builders. There’s only us left.”
Rick Gould MIEMA CEnv is an environmental scientist and writer
