Folding bicycles, wheel size, stability

One of the problems with public transportation in many American cities is the problem of how to get the last 2 miles from where your train / express bus / CalTrain stops to where you're actually trying to go (equivalently, how to get from your starting position to your train/bus stop). In some cities the solution is to take a different bus/train line and transfer : but in Los Angeles County and in Silicon Valley this is often untenable, and can turn a 20-minute ride on an express bus into a multi-hour transit saga.

A frequently proposed solution is to take one's bicycle on the train/bus. This works somewhat well for trains, but busses (I live close to an express bus line) in Los Angeles County tend to have only two slots on the front of the bus for carrying bicycles. If those are full when you catch the bus, you're out of luck. At certain times of day, even trains will restrict the number of full-sized bicycles that can be brought on board.

One way around this is to use a folding bicycle, such as the one pictured above. Conceptually it is great, and it makes a bike-and-bus commute feasible for me that would otherwise be difficult or unreliable. But it has issues.

On the one hand, it is difficult to fit under the seat. The Metro web page states Folding bikes with 20 inch or smaller wheels can be taken on board. Make sure your bike is folded and stored under a rear seat so as not to block aisles and doorways. . This is difficult and awkward. The size of the bicycle when folded (as can be seen from the pictures above) is dominated by the size of the wheels.

On the other hand, the stability of the bicycle (and its ability to handle rough terrain) affect its ability to go fast. Already riding my 20-inch-wheeled folding bike feels sluggish and unstable compared to my full-sized bicycle

So how might one try to go about making a folding bicycle more stable while reducing the size of its wheels? Here is a proposal.

For lateral stability it turns out there has been some interesting recent work on what makes bicycles stable. This video shows a small-wheeled bicycle that rides stably, without the help of trail or angular inertia. Further details on this work can be found. The authors even mention folding bicycles in their excellent TED talk.

For stability going over rough terrain, potholes and curbs : NASA has already faced the problem of making a lightweight vehicle that can be folded into a tight package and retain the ability to traverse large obstacles. Their solution for many of the Mars rovers was the rocker-bogie suspension system. It is not designed to go at high speeds, and may have neglected dynamic stability : but it may be a good start. Note that a "bicycle" designed with such a system might end up having far more than 2 wheels.

Neither of these, by itself, constitutes a "solution" to the problem of making a stable folding bicycle with small wheels capable of riding over rough terrain. But hopefully it provides a direction. And maybe it'll bring us one step closer to Richard Register's desire for cities built around transit and bicycles.

P.S. for graduate students, inventors and entrepreneurs in the United States it may not be completely implausible to get funding from DARPA to develop better folding bicycles