Arachnid: Folding Belt Drive Bike

Building a Belt Drive

Arachnid was created as my own ideal for a commuting bike. My main goals were creating a bike for going to school and one able to be packed into any car. This made it possible to use my bike after driving to work as well as getting picked up by friends with my bike.

Drawing inspiration from past Cal Poly Bike Builder designs I created a combination of builds to refine a folding mechanism by integrating it with a traditional triple triangle design. This took the function of previously built folding bikes and improved the aesthetics and frame rigidity of those designs.

Folding

A second headset was used on the seat tube to provide a second pivot point on the bike. This design was inspired by swing bike design. A locking mechanism was integrated into the triple triangle shape to keep it rigid.

Belt Drive

Needing to tension a belt meant that my dropout options were significantly reduced. Because of this I chose to make my own customized dropouts to fit with the spider theme of my bike

A Bike of My Own

Building my own bike let me add all my own details that made it personal to me. This included a spider head tube badge, webbed dropouts, and my initials.

2D Design

The first drafts of the bike were built using BikeCAD to rapidly make changes during ideation while accounting for bike industry standards.

3D Design Verification

After the design was finalized, the bike was recreated in 3D CAD. Solidworks was used to check clearances of the folding mechanism as well as to design custom dropouts.

Manufacturing

Making this bike required a large amount of customization and additional setup. With so many breaks in the frame, keeping everything rigid and consistent during welding became my biggest hurdle.

Couplers

Couplers provided three breaks in the front triangle for the bike to fold and an additional break in the drive side chainstay for the belt drive.

Dropouts

Custom dropouts were first laser cut to achieve sharp corners in the webs. Post machining on a fixture plate was used to provide lightening, stepped profiles, and improved finishes on the sliding surfaces.

Frame

Custom spacers were made to keep the secondary headset in place while welding. A combination of TIG and nickel silver brazing was used to make the frame of the bike.

Brazing

In addition to brazed parts of the frame, bosses were silver brazed on

Wheel Building

Building the exact bike I wanted meant building my own wheels.

Powdercoating

A hammer finish powder coat was chosen for its web like texture, durability, and ability to hide scratches. The powder combines a black base coat with a crackled silver/purple top layer that shimmers in the light.

MADE 2025

Presenting my bike at one of the largest custom framebuilding conventions was an absolute honor. As one of three bikes presented by Cal Poly bike builders I represented the boundaries that our club is pushing. Having professionals compliment and appreciate our work to make experimental designs was extremely rewarding. Additionally, being the only student group at the convention gave us the opportunity to represent what is possible at other schools.

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