I built a detailed road bicycle entirely from Three.js geometry — no imported model files.
The main lesson was that directly positioning individual meshes quickly becomes fragile. I ended up defining the bicycle through shared geometric constraints first — wheelbase, chainstay length, head angle, rake, etc. — calculating anchor points from those parameters, and only then generating the frame, wheels, drivetrain, cables and other parts.
The article walks through the approach, including some of the mistakes, the geometry relationships, instancing, and how the 62 parts fit together.
Article: https://www.wormhole404.com/writing/threejs-bicycle/