Carbon vs Aluminum: Separating Environmental Fact From Fiction in the Frame Materials Debate

Is aluminum better for the planet? Does carbon fiber's performance live up to its high price? Will my frame end up as ocean fill? Is carbon fiber poisonous to work with?

Recently there has been a lot of finger-pointing within the sport, with aluminum’s supporters claiming that the abundant metal’s well established recycling channels make its products more earth friendly than carbon fiber. Some hold that somewhere in Asia, children are slaving away, waist deep in toxic chemicals to produce high-modulus pre-preg so dentists in more developed nations can shave two seconds from their Strava times. Entertaining stuff, for sure. Accurate? Not so much.

I've been a manufacturer, I've worked directly with factories in Asia, and as a journalist, I have visited many factories that produce frames and components with both materials. Like all hot topics, the truth can usually be found somewhere in the middle. This feature takes a step back from the hyperbole to compare the benefits and drawbacks of manufacturing mountain bike frames from either carbon fiber or aluminum.

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When you bought your last mountain bike, did you inadvertently finance its makers to rape and pillage the earth?


Once those materials are produced, an army of container ships continuously ply the globe, dropping off aluminum, carbon fiber, thermoplastic pellets, and steel to the places where frames and components are manufactured. Some of those same ships will then be loaded with containers of bicycles, destined primarily to European and North American populations who are hungry for high tech mountain bikes, but have lost their appetites for the dirty work that is required to create them.




ALUMINUM
Recyclable? Yes. Low impact? Not exactly.



Global statistics: worldwide, the cycling industry is barely a sliver in the pie-graph of aluminum users. World production of aluminum is estimated at 24.8 million tons annually, mostly consumed by 187 billion aluminum cans (100 billion in the USA alone). Architectural and common industrial applications are next in line, followed by automobiles (which on average use 300 pounds of the stuff), then aerospace. That is a lot of metal being added to the melting pot each year. And, there is a lot more of it laying around that could be recycled.

Aluminum Manufacturing

Melted down, all the aluminum contained in a five-pound frame would make a block about the size of half a sheet of printer paper and just one inch thick. When you consider how little material they have to work with, it’s a miracle that mountain bike designers have anything left after spanning the distance between the rear axle and the steerer tube to construct essentials like linkage rockers, shock mounts, suspension pivots, and the bottom bracket housing. That’s why, with one or two exceptions, aluminum frames are welded together from an assortment of pre-manufactured pieces.


Aluminum is easily formed and machined, so to avoid waste and to optimize the strength-to-weight ratio of a frame, manufacturers use a number of different processes to portion that five-pound block of metal into frame components. Highly stressed bits like dropouts, swingarm yokes, and suspension rockers are often forged. Threaded bits, like bottom bracket shells and places like shock mounts and pivot locations are CNC-machined to ensure precision. Some of that aluminum will have been pre-formed into tubing, which is then tapered, butted and profiled to maximize the strength and physical properties of the frame at each location. Welding those bits together creates a one-piece structure that could not be easily made using any other method. But, it's not a perfect process. Every welded frame must be heat treated and checked for alignment before it’s good to go.



Using these “best practices” to make key components and then welding the frame together produces the least amount of scrap, and is a key reason that aluminum is competitive in both price and performance. Presently, only a handful of bike makers have pushed welded aluminum construction to the point where it approaches the properties of the best carbon frames. One brand has recently made that claim, but they plan to use a very different construction method.

Is there a better way? Pole Bicycle Company proposes to CNC-machine an entire frame from plates of high-strength aluminum. To produce a lightweight, tubular structure, Pole will machine the frame components in halves, and then bond them together. Cannondale’s Hollowgram crankarms are a successful example of this technique. At present, Pole estimates the Machine frame to weigh 3.2kg (7.04 pounds) without a shock, so they have a ways to go to attain their goal.



Assuming Pole begins with a one-inch-thick plate, and it actually is possible to CNC-machine a five-pound frame that is safe to ride, it could take up to 100 pounds of aluminum to produce each frame (my calculations, not Pole’s). Some of the larger chunks of plate could be re-purposed, but the lion’s share would be waste - metal shavings, trucked off for recycling. Pole's designer, Leo Kokkonen was reluctant to elaborate on those numbers saying, "The machining process on our frame is a trade secret, so unfortunately I can't confirm any of your numbers on billet sizes. What I can say is that there are ways to save material on machining."

Is machining frames from billets sustainable? Perhaps for a boutique builder, but by my calculations, a production frame maker would have to import 50 tons of aluminum plate to make only one thousand frames – and then have to transport up to 95 thousand pounds of scrap for recycling. Even if you did manufacture your bikes in a country where pollution-free sustainable energy flowed out of unicorn butts, that would be an extraordinary waste of resources.


The down-side of manufacturing aluminum frames the traditional way is the sheer number of processes required to build them. There is at least one dedicated machine at each step and most of them require a trained operator, and presently, there is a worldwide shortage of skilled labor.



If you had to work a year at an aluminum frame factory or a year at a carbon factory, which would you choose?


Recycling Aluminum


Aluminum recycling began during the Second World War out of necessity, but the concept was supercharged by government incentives, enacted after beverage makers mastered the 12-ounce pop-top can and users littered the earth with them. Cans are pure aluminum, and therefore a premium source for foundries that specialize in blending high-strength alloys—recycled cans earn top dollar. After many governments levied tax-and-reward systems to end the aluminum littering plague, “recycling” businesses appeared on every street corner to cash in on the double bounty. In effect, the reason that aluminum recycling is so readily available in developed nations is a direct result of garbage-throwing human scum.



Recycled aluminum is separated into two basic groups: known sources that are not contaminated by paint or other non-aluminum substances. Most of that stream originates from machining or manufacturing businesses. “Contaminated” aluminum, is either an unknown alloy or any aluminum that has been painted, or is mixed with other metals. Items like engine blocks, step ladders, Airbus A320s, and bicycle frames fall into this category and generally are the least desirable aluminum recyclables, because they require much more labor and energy to reduce to a purified base metal, and also because the process produces greater quantities of toxic byproducts. For a baseline, aluminum cans average $2.00 a pound (including the $.05-per can subsidy); clean aluminum, about $1.55, and contaminated aluminum goes for around $.90 per pound in California.

After making phone calls to large and mid-sized brands, I was convinced that nearly every factory has an aluminum recycling program in place. Once those frames are sold, however, most tend to remain above ground and far away from the smelters. People are still reselling frames I made in the 1980s. Just for the record: if you decided your Commencal Supreme DH frame had served its useful life, you could choose not to sell it to some unsuspecting rider, hack saw it in half and recycle it – after which, you would have a clear conscience and $6.30 USD in your pocket.



CARBON COMPOSITES
Less recyclable, but also less waste.

How it’s made: Carbon fibers basically originate from crude oil that has been manufactured into acrylic fibers, or fibers created from pitch (I’m simplifying here). The ultra-fine fibers are heated in oxygen free furnaces until all of the compounds in the fibers other than carbon have off-gassed. The fibers are then post-treated to encourage them to bond to the resins which will be applied later to bind them into alignment when they are molded into their final shape. The machines that manufacture those tiny fibers are as long as football fields and there are only a handful of them worldwide.


On the plus side, carbon composite’s versatility and strength-to-weight ratio is unparalleled, but more about that later. Because “raw carbon” yarns are lightweight and the substance is inert, it can be rolled into skeins, stored indefinitely and easily transported. Manufacturing is labor intensive, but the technology is relatively simple. The assembly and molding processes are low key and don't require the heavy machinery required for metal fabrication, so factories can be located close to population centers anywhere in the world. On the negative side, once the resins are cured, high-strength carbon structures become more problematic to recycle than metal. Like cloth, wood, and paper products, each time carbon is recycled, the fibers become shorter and less useful for high-strength applications. It is possible, but highly unlikely that fibers recycled from bicycle frames could be returned to production to become frames again.

Global statistics: Global production of carbon fiber is pegged at 135,000 tons (compare that to 24,800,000 tons of aluminum). The largest producers for 2017 were in North America, with the US and Mexico churning out 48,700 tons. Japan is next largest at 27,100 tons, and then China at 13,300 tons. Aerospace uses about 80-percent of the world’s carbon fiber production, with another 15-percent gobbled up by sporting goods manufacturers. Of those, golf and snow sports are by far the largest carbon consumers, with cycling trailing somewhere off the back. The automobile industry is anticipated to become a larger player as it struggles to meet stringent fuel and emission targets looming ahead.


Manufacturing Carbon Fiber

To convert the yarn into high-strength (high modulus) carbon composites that cycling manufacturers use, the yarns are either woven or arranged parallel (unidirectional) and then squeezed between rollers that saturate the fibers with catalyzed epoxy-type resins. Non-stick paper or plastic film is applied to the material, which is then stored in rolls. At that point, the clock starts ticking for the pre-impregnated material, because once the resin is mixed with the catalyzer, it slowly begins to cure, so it must be used immediately or placed in refrigerated storage to retard that process. Carbon material made in this way remains sticky (like adhesive tape) for a specific time, which is critical to layering and shaping the fibers during the molding process.

If you had to work a year on the assembly line, would you choose Giant’s carbon or its aluminum factory?


Pre-impregnated carbon material is formulated so it will not cure completely until it is heated to a specific temperature, at which point, the resin becomes viscous to ensure proper bonding between layers and to accelerate the catalytic process. When complete, the epoxy materials are transformed into a plastic compound that is virtually inert and that cannot be re-melted into its original state, like more common thermoplastics.


There are a number of ways that carbon is molded, but most of the top manufacturers today have adopted similar methods. Molds are machined from large steel plates that separate into halves. Front triangles are usually made in one piece. Swingarms are more complicated to mold and are generally made in two pieces, which are bonded together in a second operation. Each frame size requires a different mold, although most designers try to use one swingarm for the whole size run. Most bike makers say that molds run from $40,000 to $80,000 per model, depending upon how complicated the frame design is. Molds last from one to three years, depending upon how much force it takes to separate them after the parts are cured.


To ensure the strongest and lightest product, the pre-impregnated carbon is cut into a large number of shapes which are mapped and numbered, so workers can place them in the correct location and order. To assist this process, the frame maker molds a mandrel (usually EPS foam), slightly smaller, but identical to the finished frame. A slender, inflatable nylon bag is taped around the mandrel, which the lay-up workers then apply the carbon strips to. When all of the carbon is applied to the mandrel, it is carefully laid into the mold. The halves are closed and it is transported to a heated press. The nylon bladder is pressurized to force the layers of carbon together against the inside of the mold while the press runs through a heating cycle that can take well over an hour. When the mold has cooled sufficiently, workers pry and hammer it apart – and if all goes well, the frame emerges with minimal sanding and clean-up required.



Carbon’s strength-to-weight advantage over any other frame building material is undeniable. European supplier Dexcraft Composites states in its carbon vs. aluminum white paper that a component made from standard carbon fiber of the same thickness as an aluminum one will offer 31-percent more rigidity than the aluminum one, and at the same time, weigh 50-percent less and have 60 percent more strength. High modulus carbon can boost those numbers significantly. Carbon road bikes and some XC racing machines approach those numbers, but the reality is that carbon mountain bike makers err on the conservative side, which results in lower weight savings - about one pound between comparable carbon and aluminum frames.


The molded layering process used to make the frames, while time consuming, provides options to strengthen or lighten the structure as needed that are either impractical or impossible with aluminum. Unlike metals, which must first be formed into useful structures and accurately fitted before being assembled into a final product, one roll of carbon fiber can be molded into any number of shapes and used to make any size frame. A good carbon frame emerges dimensionally correct, without need to re-size its bearing locations seat tube, or threads - which lends itself well to building dual-suspension frames, where even minor misalignments can wreak havoc. Carbon’s additional strength and the repeatability of the molding process has reduced warranty returns for most bike makers. Workers must be careful and attentive, but not necessarily skilled. Typically, lay-up takes place in air conditioned rooms, and the manufacturing process is safe from beginning to end.

The downside of manufacturing carbon frames is the material costs begin around $20 USD per pound and the start-up cost for molds, engineering, and proof-testing is very expensive. The lengthy lay-up process is tedious, and it has to be done correctly. Layup is an entry-level job with a high turn-around, which makes it hard for factories to retain experienced workers.

Rumors abound about exposure to toxic chemicals related to carbon production, but in my experience the majority of workers are at low risk. Some epoxy-type resins that are used for high-strength carbon composites react with human skin. Once mixed and embedded into the carbon those effects are mild, but they are accumulative. Prolonged exposure to skin can eventually cause hyper-sensitivity and allergic reactions. Plastic gloves are enough to protect workers, and help to prevent moisture, oils or grime from affecting the layup. Smaller manufacturers purchase their carbon pre-impregnated, which is quite safe to handle. Larger frame makers, like Giant Bicycles, however, buy their carbon dry and, in order to have a fresh batch on hand for each production run, they pre-impregnate their own carbon as needed. Understandably, the mixing-room staff who prepare those chemicals and operate the machines face an elevated risk of exposure.

Recycling Carbon Fiber

Today, the cycling industry’s carbon frame and component makers do not generate enough waste to attract the attention of recyclers. Toray, one of the world’s largest carbon fiber producers, says that an estimated 50 million pounds of carbon fiber scrap are produced annually, and that 2 million pounds of that scrap is generated in Washington State, where Toray services Boeing’s airliner production and aerospace ventures.



Aerospace and military suppliers must track their materials from the ground to finished product, so they discard any material that could be questionable. Toray pegs the scrap rate for aerospace at 20-percent, which is astronomical compared to bike makers, who typically clip remnant carbon pieces into small squares and use them to reinforce hard-to-reach sections of their frames. The high cost of carbon, and the fact that the lay-up process affords many opportunities to use small, odd-shaped pieces is the constant that drives carbon makers to enforce frugality - from the world’s largest frame producers, like Giant in Taiwan, down to a small wheel builder like Dustin Adams, founder of We Are One Composites in Kamloops BC.

“We make 125 rims a week,” says Adams. “And our total scrap rate is one plastic trash bag. Most of that is the paper backing we pull off the carbon.” Adams commented about an image that was being tossed around the interweb of a stack of discarded carbon frames behind an Asian frame factory. “That probably came from the frames used to qualify their customer’s designs,” he said. “You have to destroy a number of frames from each size run for every customer, and some of those factories take on a lot of clients.”


Adams says he saves up his cured scrap until he has enough of it to send to a recycler, who converts carbon waste to chopped fiber. Brands like Trek, Specialized, and Ibis also send their carbon fiber scrap, which consists mostly of warranty returns, to become chopped fiber as well. Most of the world’s carbon recyclers convert cured carbon into short “chopped fibers” by heating the composite material to burn off the epoxy matrix, which leaves raw carbon fiber. The resins assist the burning process. The fibers are sized and then sold to be used to reinforce molded plastic (like you’d find in a pedal) or made into fibrous mats, which are used to manufacture structural panels and under-the hood bits for automakers. Chopped fiber is also mixed with asphalt and used to reinforce concrete.



facility and its sister in Japan are beginning to produce continuous-fiber products recycled from large-scale aerospace sources. To augment that waste stream, the first airliners and military aircraft to use composite wing and fuselage parts are due to be scrapped this year.


Almost ready for recycling: The bottom line for carbon frame makers is that, as a whole, they are a small producer of carbon waste and much of that goes into landfill sites. Carbon composite is considered inert, so its detrimental impact in landfills is its bulk, but that resource may come to a swift end. Concerns about auto makers and aviation ramping up their carbon use propelled the European Union to ban composite waste from landfills and many countries are following suit.

Some bike makers do recycle. Telephone calls and emails to a cross-section of the industry indicate that almost every carbon manufacturer is under pressure to come up with a pilot recycling program and would happily do so if they could find someone who would take it. Hans Heim, CEO of Ibis put it bluntly: “The small amount of returns we get and our pre-production products had been accumulating for years. We didn't throw away our carbon. We wanted to initiate our own recycling program, but when we tried, we couldn’t get them to return our calls.” Ibis paired up with another prominent brand in California that sends a truck every few months to pick it up. As it stands, carbon manufacturers will most likely have to pay recyclers to take their waste until customer demand for their products comes up to speed.


And, what about you? Owners of carbon fiber bikes are even less likely to recycle their frames than their aluminum-riding friends. Carbon bikes are more resistant to fatigue and corrosion, generally out-last aluminum, and fetch a better price on the resale market. In addition, broken or cracked carbon frames can often be repaired to full strength, while most aluminum frames cannot be welded back into service without stripping them down to bare metal and sending them off to heat treat. Aluminum may be more easily recycled, but on the other hand, you may be able to pass your carbon bike down to your great grandchildren.




So, Which is Better: Carbon or Aluminum?

If I personally was going to launch a new mountain bike factory, I would build with carbon. My reasoning is that aluminum construction has evolved to its pinnacle and offers little room for improvement. Perhaps a breakthrough in additive manufacturing (3D printing) could breathe new life into aluminum construction, but as it stands, it's a way to make a very good bike frame that's going nowhere fast.

My litmus test is simple: if I gave a million dollars to an aluminum factory to improve a frame, and did the same to a carbon factory, I doubt the aluminum version would be significantly better than the best aluminum bikes are today. Carbon, however is relatively new to bike makers and has a long way to go before it could be considered a perfected process. One example: automation is a fact of life for metal fabrication, but has not taken root as a viable option for labor-intensive carbon frame production. Recycling will hopefully soon be a fact of life for the composite industry as a response to public pressure and new regulations. The potential for improvement and to remain competitive is much greater with carbon.

If I were teaching sustainability and best-use practices at a middle school, I'd own an aluminum bike, because I would not have to argue my decision with students beyond the black and white fact that aluminum is the most recyclable material that lends itself to mountain bike production.

If I was a bike brand, concerned about the human cost of those who made my bikes, or was worried about "ocean fill," I'd first choose the best material for my bike design and then I'd research a factory that has established safeguards in place for its workforce and documented environmental protocols. In my experience, bicycle manufacturing jobs are sought after in the countries where they exist, and most of the cycling industry adheres to environmental standards that often exceed those in their countries of origin.

If all I rode were downhill bikes or 33-pound enduro sleds armed with tire inserts and 1100-gram tires, I don't think the one or two pounds that a carbon frame and wheels might save me would mean all that much in the pedaling department, so either carbon or aluminum would be fine.

For professional riders and those who want the pinnacle of performance, carbon is king. Carbon offers a lighter weight, longer-lasting, corrosion-free frame, a more lively feeling chassis, wheels that remain straight almost forever, and a more attractive return on the resale market.

Everything can be broken. Regardless of the material or the amplitude of their riding, some people just break stuff more often than the rest of us. If you are producing your own waste stream, then you should probably choose aluminum and recycle your broken frames and components. Recycling is earth-friendly, but so is conserving resources. If you don't break things that often, consider a long-term investment. Carbon may be less recyclable, but well-made carbon frames and components have the potential to maintain their performance and appearance for many years.


The Takeaway