I see that look in your eye. You want to see some sweet bicycle convertin’ design that makes falling off your bike and hitting a street sign look like fun again.

Well, here it is. It’s called the ZWEISTIL [two style], named by the designers, Stefan Wallmann and Daniel Knüpfer, for it’s unique variable frame construction that gives you the ability to drop it down cruiser style and turn the heads of the hunnies… or just save some gas.

Stefan fills us in on the greasy details. I think you’ll like the way he explains it.

Note from Josh: I hope you take just a few minutes to read through the account on the design of this bike. It touches on a lot of aspects that get overlooked in the design sites that throw up the pretty pictures but leave out the detail about how products actually get to the final stage. This project goes way beyond just creating some nice models to render. It shows proof of concept through full-scale working models. A task that isn’t quick and typically takes a few iterations to hash out all the issues.

The Idea
At first, our intention was, to discover new ways of riding with human power, thus inventing a new human-powered vehicle as answer for problems like air pollution and jammed cities, as well as the decreasing fit- and increasing fatness of mankind. The thought was simply to get people moving, without wasting valuable resources.

At first, different concepts popped up. Detailed investigation through some gyms made a concept evolve that was to be the perfect exercise-machine: to use as many muscles as possible and bring that force onto the road.

The First Concept
The first concept was created to be propelled in different ways: by arms, legs or both. Like a recumbent bike, a rowboat or a mix in between. The first thoughts about this bike were sketches and technical drawings, followed by small mechanical mock-ups created from LEGO parts. At this point we knew the principle would work and decided for a 1:1 mock-up. This three-wheeled monster we called “Eisenschwein” [Pig iron] utilized several freewheel hubs from scavenged bikes, as well as a “specialty part” for the tilt and propelling mechanism.

However, this was only the first small step to the ZWEISTIL. We decided the Eisenschwein’s footprint was too big for riding everyday and not very easy to handle, to put in a car, or drive on the sidewalk. So, we went away from the all-exercise-bike towards a more commuter-friendly vehicle that would provide more incentive to switch from a car to a bike.

A Recumbent That is Not a Recumbent
We went to Germersheim, a fair for specialty bikes (zillions of recumbents, velomobiles and the like) and from that, knew what to do: a recumbent that is not a recumbent.

Our research revealed that indeed, people had thought about such concepts before and created versions themselves with two seats, two bottom brackets and two handlebars or with some kinds of flip-over seats and handlebars, so they could stop to convert their bike back and forth, but our bike wouldn’t require you to stop.

The Final Process
We now knew what the bike had to do. We knew we needed a dampening spring to lower the frame SLOWLY. We skipped the backrest on the prototype for ease of building. The spoke and hubless front wheel were just for fun. For a bike that converts while you ride it, we also knew that the steering mechanism had to be “different”.

We printed two pictures, a normal bike and a recumbent, using them as an underlay to have the two final positions for the saddle, bottom bracket and handlebar. We then took some cardboard strips and needles to try out different frame geometries (using the needles as joints) to come to a frame that does what we wanted: translate from bike to recumbent.

It was time again for another LEGO prototype. This is the part were SolidWorks would have been a really great help. We needed to figure out a switchable frame with several joints and multiple dependencies. I know this could be done much easier in SolidWorks, but we had very little practice with the program and very little time to get the project done, so we relied upon our good ol’ model-building skills.

The LEGO prototype brought us far enough to build a first switchable bike prototype. Because of the lack of mechanical pre-planning, we welded the frame and cut it apart and welded again… and again, until we had a high-and-low-bike that lacked the desired degree of freedom, but proved that you could switch positions while riding without terribly injuring yourself.

Back in the metal workshop, it took us a rushing 2 days to set up the frame up. No time compared to the following tinkering and fiddling with the said mechanics (designers, not engineers remember…), but after a lot of work, we got it done. With countless aches and burns (freshly welded metal can be really hot even if it doesn’t look hot…) and punctures (Daniel put a spoke right THROUGH is index finger! the filthiest spoke I have ever seen!) we were finally riding a working prototype. You may notice the sense of pride in the video below.

The Finished Concept
Until recently, no computer programs were used in the design. We took a shortcut by using Photoshop for rendering of the bicycle, thanks to Scott Robertson’s great bicycle rendering tutorial. Additionally, we quickly made a foam model in 1:3.5 scale, painted it and presented the finished concept bike for our diploma thesis.

How 3D CAD Could Have Helped
For sure, the technical aspect of the vehicles innards could easily have been done in SolidWorks and the tilt mechanism (which had to be fixed later) would have profited from a thorough pre-calculation. Well, we´re designers, not engineers. (We kept that as mantra throughout the process). SolidWorks would have helped here in a way that we clearly could have skipped the cutting and re-welding by allowing proper planning of how the frame should behave. This slowed development of the final frame. It was a lot of trial and error, but also a lot of fun.

Admittedly, after a few months we realized the last steps to be quite a dead-end street for good industrial designers. We were asked many times if there was CAD-data that could be used for casting/molding processes. We wished many times we had the digital concept! Programs like SolidWorks could have helped immensely with the prototyping process (not every part of the process!) and at the same time make it a lot easier to prepare the product for manufacturing later.

Cost calculation could have been a lot easier as well. On the other hand, when only working digitally, many aspects of a product get lost, especially concerning usability and ergonomics. One can get great insights from a full-scale mock-up (we did several, also in wood using the exact dimensions) that would be impossible to have on the screen.

We were also aware of the possibility for rapid prototyping of a scale model, another point at which having the 3D data would have helped. We are good model builders and that kept the process fast, but it would surly would have speed up everything a little bit.

In the end, going almost completely analogue (except for final rendering) made the speed of the process possible for us. We had a mere 3.5 months to work on everything including pre-thinking, the Eisenschwein, the Highlowcycle (another variant) and finally the ZWEISTIL. With the additional research and design process, finding a way to do it in SolidWorks seemed inappropriate for us.

This would be totally different for a skilled user. I’m sure we would have really appreciated skipping some of the greasy hand moments, but (and here comes the nostalgia) we wouldn’t miss it! Time in that metal workshop was great – welding things and getting dirty, grinding, drilling, making stuff! Once there was a professor standing in the workshop telling us that he was teaching next to the courtyard (that was our testcourse) and every now and then he had to interrupt his lectures because all his students were staring out of the window…that felt just great.

Well, your now you know the process of making the Zweistil. I guess , as you may be quite keen with 3D CAD, you may have figured out some more way where the programs would have helped in the development.

If you would like to contact either about the bicycle or other projects you can reach them at [email protected] or [email protected]


Josh is founder and editor at SolidSmack.com, founder at Aimsift Inc., and co-founder of EvD Media. He is involved in engineering, design, visualization, the technology making it happen, and the content developed around it. He is a SolidWorks Certified Professional and excels at falling awkwardly.