A Hybrid Bike

A Hybrid Bike

Semester Bicycle is not a piece of furniture, nor a sculpture, nor a building.  It’s not any of the things we usually talk about here.  It’s a very smart hybrid design, however, with intriguing implications for construction.

Begun as a Kickstarter project, Semester Bicycle is a collaboration of two American companies, makelab and HERObike, an attempt to make a sustainable product using locally sourced materials and local labor.  Local in this case is Greensboro, AL, the self-proclaimed Catfish Capital of Alabama.  Apparently, Greensboro not only has an abundance of catfish, but also of bamboo.

The bike is advertised as a bamboo bicycle, but it is, more accurately, a hybrid of steel, bamboo, and carbon fiber framing.  The front fork and rear triangle are steel.  The three main frame elements appear to be hexagonal bamboo beams, but are, in fact, bamboo skins on carbon fiber cores, a bamboo/carbon fiber/polymer composite.

The method of casting the carbon fiber cores is ingenious, and in keeping with a project whose goal was to invest as much as possible of its Kickstarter capital in the product, while minimizing the expense of tooling.

The bamboo is cut and shaved to form the six sides of a hexagonal tube.  (Each segment, seen in cross-section, is a trapezoid.)

A carbon-fiber sleeve is slipped over a straightened length of recycled bicycle inner-tube.

The strips of bamboo are laid out flat, side-by-side, so they could be wrapped together into a tube.  The top surfaces of the bamboo strips are painted with epoxy resin, and then wrapped around the carbon fiber.

The inner tube is inflated to press the carbon fiber against the inside of the bamboo tube. The carbon fiber becomes saturated with liquid resin.

When the resin cures, the carbon fiber is cast in place to the inside of the bamboo hexagon.  The inner tube can be deflated and removed.  The result is a strong, lightweight bike frame with excellent vibration damping.

The bike is made in three sizes.  The largest has a frame weight of just under 2kg (4.4 lbs).  It is available with handwoven bamboo fenders.

The choice of bamboo has aesthetic as well as structural value.  It also has a unique property of communication: bamboo has become a symbol, a signature material for the concept of rapidly renewable sustainable materials.  Using it for a bicycle is a statement about environmental consciousness.

However, the fabrication concept is not limited to bamboo, nor for that matter, to carbon fiber.  Why couldn’t this type of internal inflatable formwork be applied on a larger scale to make hybrid structural members for self-supporting structures, including buildings?


Images via designboom.com and the Semester Bicycle Kickstarter web page.