Automatic Transmissions for Bicycles

When I first learned to drive, I learned on a car with a manual transmission. It never seemed especially difficult because that was what I got used to. In fact, the first time I had to drive an automatic, I remember being very confused. What was I supposed to do with my left foot? Do I not have to shift at all? And if it’s automatic, then what’s with all these different choices on the gearshift lever? I quickly got the idea, of course, but still preferred the increased control and responsiveness I got from making my own decisions about when to shift. It would therefore seem that I should have the same attitude about bicycles, which not only require manual shifting but typically have many more than four or five gears. But manual bicycle transmissions have always given me trouble, and I’ve frequently wished I could have the convenience of an automatic transmission on a multi-speed bike.

Yanking My Chain
For the record, I am not what you’d call an avid cyclist. I own a bike—actually a fairly nice one—that over the past few years I’ve ridden, on average, once or twice a year. Not that I feel I need to make excuses, but I live in a part of San Francisco that isn’t especially bike-friendly (on a hill, no less), and the vast majority of places I need to go are much easier to reach by train or by bus. Nevertheless, I can’t imagine not owning a bike, and I like the idea of bike ownership very much—good exercise, good for the environment, and so on. But even when my bike was my sole form of transportation a number of years ago, I never fully grasped the way bicycle gears worked. That is to say, I understood the mechanics, but actually using them was another story—the logic of how one must manipulate those levers to reach the desired balance between torque and speed always seemed a bit like a black art. It was not a simple linear progression of lower to higher as on a car, but a function of the ratio of the front gear size to the rear gear size, both of which are variable. My usual practice was just to fiddle with the controls until pedaling felt about right, then leave them where they were until I couldn’t stand it any longer.

Another problem with shifting gears on bicycles is that the derailleur—the mechanism that moves the chain between gears of different sizes—is by nature imprecise. Although some designs are better than others, over- or undershooting your desired gear is common, and if you’re pedaling too fast or under too heavy a load, the chain can easily slip off the gears entirely, requiring a greasy manual adjustment. Wouldn’t it be nice if bikes could figure out how to change their own gears as painlessly and accurately as cars with automatic transmissions?

Gearing Up for a Change
Sure enough, automatic bicycle transmissions have been in development for almost 30 years, though only recently have they become commercially available. Mechanically, the main thing needed for an automatic bike transmission is a motor or piston that moves the chain between gears in place of the standard lever-operated cable. This is a relatively straightforward engineering problem, but the difficult thing is working out how and when to tell the gears to shift. That computation currently requires the use of a tiny, battery-operated computer along with sensors that determine the current gear and the speeds at which wheels, pedals, and sprockets are moving. The computer constantly recalculates the optimal combination of front and rear gears to keep the rider at a consistent pedaling cadence, automatically signaling the gears to shift lower when going uphill or higher when going downhill. Using a controller on the handlebars, riders can, if they wish, adjust the gearing to provide a more intense workout or a gentler ride; they can also override the automatic shifting entirely and use it as a power-assisted manual transmission.