I am lucky enough to have an incredibly creative and inspiring family. My sister builds exhibits for Children’s Museums in Minnesota, and I am always in awe on the ideas and installations she develops. So when she asked me if I wanted to design a bicycle-powered mobile exhibit at one of the museums, I immediately jumped on the opportunity.
The basic concept included:
- A children’s bicycle fixed to the floor
- A mobile in the ceiling with clouds, stuffed animals, or other items attached to it
- A device to translate the pedaling motion of the child to the rotation of the mobile
The drawing below shows the concept as envisioned by the museum curator.
To match the aesthetic of the rest of the exhibits, I used plywood and other wood products. To ensure serviceability of the system, I sourced all purchased parts as off-the-shelf bicycle components or from McMaster. Since the museum wanted the exhibit oriented in a specific way in the ceiling, I needed to change the axis of rotation of the chain by 90 degrees, thereby requiring the use of a transfer case. I created all of my parts with slots and tabs so simple tools and fasteners could accurately make assemblies. This allowed me to avoid the need to make precise fixtures. Additionally, slot and tabs provided me with the ability to use pocket screws to fasten plywood in the stronger, cross-lamination direction.
CNC machining provided an easy way to create the level of precision needed for a slot and tab design. I teamed up with my buddy who owns a CNC router to manufacture the component parts.
The original intent of the mobile was for it to span across the ceiling, but to ensure a safer installation the transfer case and idler case were mounted so the chain ran parallel to the wall. The design shown on the left is my original conceptualization of the installation and the design on the right represents the final installation, after adjusting for the updated mobile orientation.
Slots in the mobile mounts were needed to provide a method to tension the chain as it wears over time. I designed the mobile parts to be mounted to the chain with jeweler’s wire, allowing for minimal interference with the chain rings.
The transfer case required the most attention to design. It included multiple bevel gears and bearings and needed the most precision to function correctly. The main hurdle of the transfer case was creating an axle that would react torque through the bevel gears and bicycle chain rings. Using long 3/8” shoulder bolts provided a precise diameter for the bevel gears and bearings as well as the threaded section for a chain ring adapter to be mounted. The idler gear case was simply the same design of the transfer case with the bevel gears removed.
The last item to design was the stand and rear hub for the bicycle. A fixed-free rear hub (freewheel on one side, fixed gear on the other) was needed to avoid counter-rotation of the chain. The likelihood of the chain derailing unintentionally would sharply increase if the operator could pedal the mobile chain forwards and backwards. The stand was basic and provided support for the rear axle and a mount point for the tensioner on the chain running from the bike to the transfer case.
Given the opportunity to expand the installation in the future, I would add multiple idler gear cases to route the mobile chain into a visually compelling design on the wall. The installation went live on Earth Day 2017 in the Grand Rapids, MN Children’s Museum and was very well received.
