Industrial Engineering

In the Frame: the 3D Printed Bike

The Engineer Januari 2020
By Stuart Nathan

As the world’s first 3D printed bike frame hits the market Stuart Nathan investigates the advantages of additive manufacturing for the bicycle industry 

Since additive manufacturing (AM) has become more mainstream, the hallmarks of its use are increasingly familiar: radical curves, branching structures, shapes reminiscent of bone and coral. A cursory glance at the frame of the Emery e-bike reveals none of this: it’s quite a different shape from a traditional bike, but the geometry seems quite simple and it’s based around the familiar triangular structure that has formed the basis of frames for over a century. The viewer might think that perhaps the frame has some exotically-shaped reinforcing structure inside its tubes. But no, like any other bike frame they are hollow. Why then, you might ask, bother 3D-printing it at all? It looks like it could be made perfectly well using conventional manufacturing methods.

The frame came about because of a desire by Silicon Valley additive manufacturing specialist Arevo to find a commercial product to demonstrate the advantages of the process it has pioneered for printing carbon fibre-reinforced composites, which it has until now employed for aerospace components.

Deciding that a bicycle frame would be attention-grabbing, the company spoke to the last American winner of the Tour de France, Greg LeMond, who had been impressed by its technology during a visit. LeMond suggested they contact Bill Stephens of Studiowest, a Colorado-based sporting goods design consultancy. Stephens is a veteran bicycle designer, with experience in working in carbon fibre for high-performance frames.

Arevo’s printing process works by placing continuous strands of carbon fibre using a printhead mounted on a robot arm. As with most 3D-printing systems, it’s as much about the design software as the printing hardware. Arevo’s systems work by performing finite element analysis of different orientations of fibres to determine how the forces and stresses of usage will affect the final printed article. This allows the composition of the piece and the internal structure of the composite to be optimised in a similar way to the geometry optimisation of a more familiar powder bed-printed article with its distinctive bird-bone shapes.

Left: Bike frames have to cope with dynamic forces at several points. Centre: Clearances between bike components are often very tight. Right: Overall weight is low for printed carbon fibre

Stephens admits to having doubts when first approached. “I’m pretty sure Arevo thought ‘we make aerospace components. How difficult can a bicycle be?’ so I gave them my perspective. Bicycle frames are very challenging. There are many points where they have to withstand multiple dynamic forces from different directions at once. The consequence of failure can be severe injury to the rider. The tolerances are extremely tight because of all the moving components in a very small area and the need for the human rider to be able to move unimpeded. And bicycle designers are obsessed with reducing weight. I wouldn’t have been surprised if they’d decided to go for something like a tennis racket instead. But to their credit, they decided to embrace the difficulties and stick with the bicycle idea.”

“There was definitely an education both ways, where they taught me about materials and I taught them about all that’s going on dynamically and from an engineering standpoint with a bike frame.”

As to why 3D print was used, Stephens explained that the big advantage is the ability to make engineering items without tooling. “To design a carbon frame, we currently have to sit down – designers, engineers, marketing people – and forecast years in advance what we think is going to sell. We then design and engineer like crazy. At a certain point, we have to put the pencils down and write a big cheque to a toolmaker, who then spends weeks or months cutting steel. Those steel moulds we now own, and you’re locked into that design for the life of the product.”

“And then there’s the production process. Your production team lays in the carbon fibres by hand. There are inevitably inconsistencies from one frame to another, because even the most skilled person can’t produce identical work every time. It’s a lengthy process – tens of hours per frame – and that limits the number of units you can produce and consequently keeps the price high.”

“The Arevo process just kind of turns all that upside down. There’s no need to forecast that far out ahead; and we’re not locked into any single design. In the bicycle world, there’s a lot of trends in components. These are all changes that are prohibitively expensive, when you’ve got to cut into those steel moulds. With the print on demand process, any change that we choose to make to the design can be a software change. The next print can reflect a different physical geometry. That’s a kind of design freedom and flexibility that doesn’t exist today.”

“Moreover, you can print a run of 1000 frames faster than conventional carbon builds and be confident that everyone will be identical.”

As the project progressed, Arevo developed its process to accommodate a much larger print volume, which made it possible to consider printing the entire bike frame as a single part. “Initially, I thought, this is big enough to at least print one side,” Stephens said. “And then, I threw out to the team what I thought was just a radical idea, to print the whole frame as one piece, but I didn’t think they were going to be able to do it.”

 “The idea of being able to design a bike frame as one continuous dynamic unit is just fantastic, from a design standpoint. There’s so many different dynamic loads happening at the same time, that to lay out a fibre orientation network that gives us different dynamic qualities at different parts of the frame on different axes, and consider that as one engineered unit, that’s a whole new way of thinking in terms of bike frame design.”

This would simply not be possible with traditional ways of building carbon fibre bikes, because it is just too expensive to produce moulds to make a prototype and check how it would work. “You just wouldn’t take that risk,” Stephens said.

Unexpectedly, Stephens has also found that 3D printing is a very good way to make wheels. “One of the other major benefits of the Arevo carbon fibre process is that this is a thermoplastic process, versus traditional carbon fibre which is a thermoset process. What that means practically is the material is much tougher and more impact resistant than traditional carbon fibre bikes. That would be a very risky wheel to make out of traditional carbon fibre.”

Stephens believes that this technology could make carbon fibre frames much more affordable. The fact that the Arevo frame forms the basis of the Emery E-bike is more a reflection of the way the bicycle market is currently developing: it was originally made as a conventional bike and is perfectly suited to that type of construction. “It’s complete freedom to set aside preconceived notions of how we’ve just always done things, and consider new forms, new directions, new mechanical concepts, that we otherwise wouldn’t be able to explore,” he said. “I foresee an opening up of ideas. That’s very exciting for me as a designer.”

Reference: https://www.theengineer.co.uk/frame-3d-printed-bike/