Bike Walker (Walking Bicycle)

I’ve had an image of this contraption in my mind for years and years: How cool would it be to have a gadget that you sit on and peddle like a bicycle, but instead of wheels it has 4 legs like a horse?  So a couple years ago I started designing and analyzing the geometry, and am ready (technically) to start building it.

At first I considered just modifying an existing bike frame, but as the design matured I decided that was unworkable, so I’ll build the entire thing from scratch.  The angled axis of the fork on a regular bicycle provides stability for balance because of the wheels, but in my case it provides no such benefit, and would only add complexity to the mechanism to compensate for it.  So the steering axis is vertical, and the front and rear leg attachments are identical.  The seat and peddles are similar to a bicycle, but to facilitate powering the front legs I’ve opted for a timing-belt drive instead of a chain drive.  So with the turning mechanism and the belt-tightening mechanism, the frame consists of several moving parts (not counting bearings). I’m still considering an electric motor to provide some assistance to power the machine, which will add a few more moving parts.

Perhaps in the future I’ll optimize the design for weight, but I’m going to build the first version out of steel.  The stock material will be mostly 1/8-inch wall 1-inch square-tubing and 1 3/8-inch solid rods.  The rod will be cut and bored to hold the joint bearings, and square tubing will be used for all of the structure.

The front and rear parts of the frame are each made of about 40 parts, each of which must be cut, formed, and welded (or bolted) together.

All four legs are identical (except right/left mirrored).  Each leg consists several moving parts, which consist of a whole bucket full of parts which must be cut, formed, and welded.  Not to mentions bolts and nuts and bearings.

Altogether, the whole contraption consists of over a thousand cut and formed parts, welded into around 60 moving parts, and assembled with dozens of additional parts and dozens of fastener parts into the final assembly.  Whew!

At first I envisioned myself cutting and forming all those parts by brute force, but after further consideration, I’ve concluded that without the assistance of CNC, it will be impossible to hold the tolerances and reproducibility necessary to assure the designed performance.  I will also make some fixtures to hold the different part types for milling to increase reproducibility save set-up time, and I’ll make some fixtures for welding the sub-assemblies because the position tolerances of the end-joint on the leg parts are pretty high.

This image doesn’t show the seat or handlebars, but you get an idea of the complexity.  The feet consist of one-direction-rotation wheels, because although the feet lift off the ground with the step they do not move at a constant velocity while on the ground.  So although the feet bear weight three at a time, not all of them are propelling the thing forward at any time.