Empire Cycles is a British bike designing and manufacturing company in the North-West of England. The company offers innovative designs to the world’s mountain bikers and downhillers. Recently, the engineers at Empire collaborated with Renishaw, a manufacturer of metal-based additive manufacturing machines, to create a 3D-printed metal bike frame. The engineers designed the mountain bike to take advantage of Renishaw’s additive manufacturing technology, allowing them to create a titanium frame that would be both strong and light. Through the use of topological optimization software, the new frame is some 33% lighter than the original. The frame was additively manufactured in titanium alloy in sections and bonded together.

The engineers used topological optimization software, programs that help determine the “logical place” for material using iterative steps and finite element analysis. The program suggests where you can remove material from areas of low stress, helping to enhance a design for load bearing. The resulting model is both light (due to the low volume) and strong. By working together, Renishaw and Empire Cycles developed a bicycle design for additive manufacture that eliminated many of the downward facing surfaces that would otherwise have needed support structures.

The additive manufacturing machine used Titanium alloys as the build material. These alloys have a high Ultimate Tensile Strength (UTS) of more than 900 MPa when processed using additive manufacturing and near perfect densities of greater than 99.7%; this is better than casting and, as any porosity is both small and spherical, it has little effect on strength.

Titanium alloys are more dense than aluminum alloys, with relative densities of around 4 g/cm3 and 3 g/cm3 respectively. Therefore, the only way to make a titanium alloy version of a part lighter than its aluminum alloy counterpart is to significantly alter the design to remove any material not contributing to the overall strength of the part.

The original aluminum alloy seat post bracket is 360 g and the hollow titanium version is 200 g, a weight saving of 44%. The engineers think that with further analysis and testing, weight could be reduced further. The original bike frame weighs in at 2100 g. Redesigned to make use of additive manufacturing, the weight drops to 1400 g, a 33% weight saving.

The project’s aim was to produce a fully functioning bicycle, so the seat post bracket was tested using the mountain bike standard EN 14766; it withstood 50 000 cycles of 1 200 N. Testing continued to 6 times the standard without failure.

Testing of the completed bicycle frame will continue, both in the laboratory using Bureau Veritas UK, and on the mountainside using portable sensors in partnership with Swansea University.

There are lighter carbon fiber bikes available, but Chris Williams, Managing Director at Empire Cycles, has researched this already and said, “The durability of carbon fiber can’t compare to a metal bike, they are great for road bikes, but when you start chucking yourself down a mountain you risk damaging the frame. I over-engineer my bikes to ensure there are no warranty claims.”

Chris had already produced a full size 3D printed replica of his current bike before he approached Renishaw, so he had a good idea of what he wanted to achieve.

Renishaw originally agreed to optimize and manufacture the seat post bracket only, but after this proved successful, decided the whole frame was a practical goal. Chris updated his design with guidance from Renishaw’s applications team on what would build well, and the frame was sectioned so that it would fully use the AM250’s 300 mm build height.

The key benefit for Empire Cycles were the advantages that this construction method bestowed. The design has all of the advantages of a pressed steel ‘monocoque’ construction used in motorbikes and cars without the investment in tooling that would be prohibitive for a small manufacturer.

The potential performance has not been completely explored yet, but Chris and Renishaw hope to continue to develop the project. As no tooling is required, continual design improvements can be made easily; and as the component cost is based on volume and not complexity, some very light parts will be possible at minimal costs.

Research into bonding methods resulted in Mouldlife providing the adhesive, and technical specialists 3M providing test facilities. The next goal is to look at iterative improvements in bonding methods, such as specific surface finishes.

The wheels, drive train and components required to finish the bike were provided by Hope Technology Ltd.

In summary, this additive manufacturing approach delivered a number of advantages:

Design freedom

–Rapid iterations; flexibility to make design improvements right up to production
–Ability to make shapes derived by topological optimization
–Ultimate customization and tailoring – make one-offs as easily as production batches

Construction
—Complex shape with internal strengthening features
–Hollow structures
–Built in features, such as the rider’s name

Performance, titanium alloy
—Seat post bracket 44% lighter than aluminum alloy version
–Extremely strong – tested to EN 14766
–Corrosion resistant and long lasting

Renishaw
www.renishaw.com