A tandem bike I built with riders positioned side by side instead of front and back.
I found two matching generic 70’s era road bikes on Craigslist to build this project. Matching frames meant that there didn’t need to be any adjustments for differences in heights, wheelbases, etc., everything could be welded at 90° angles. And steel frames meant it was within my welding abilities (and my tools abilities), as well as being pretty durable and giving some extra strength margins.
I welded a piece of round tubing between the head tubes. I had to notch the tubing by eye with an angle grinder to get a nice fit on the curved head tube. I then welded another round tube between the rear wheel dropouts, and tested the strength by jumping on the newly welded tubes.
We gave the bike a test run without the steering joined, and it did not go well! It would take much more teamwork and coordination then we have to make any turns - the smallest misalignment of the two handlebars will cause one of the wheels to start skidding and shaking immediately.
I mounted some flat metal bar to each handlebar and ran it out far enough that we could turn sharply without it striking the handlebars. Then I mounted a strip of wood between each metal bar to test out the linked steering.
The linked steering worked fairly well, and I’ve since replaced the wood with a metal bar and cut off the long ends. But our test runs have shown the fatal flaw of this design - the bikes are rigidly linked, which means they can not tilt to stay vertical when the road has a camber. This makes it feel like you are about to tip over constantly, unless you are riding in the middle of the road! There is no easy fix to this sticky issue.
For the moment the tandem bike in storage has been overgrown by hops. So we’ll take it out again in the fall, but for now we have a new mobile trellis.