this weekWheel of Fortune

October 5, 2004

Automatic Transmissions for Bicycles

Reinventing the two-wheeler

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.

The first automatic bicycle transmission was designed by the Browning family, whose main claim to fame had been gun design. Now an independent company based near Seattle, Browning Components, Inc. focuses solely on bicycles and bike transmissions. Their most interesting innovation is a special gear with a hinged section (somewhat like a pizza slice) that swings in and out to guide the chain from one gear to the next. What’s great about the Browning mechanism is that the chain remains engaged in sprockets at all times, rather than simply dropping onto the next gear. This virtually eliminates the possibility of the chain slipping, and also makes it possible to shift smoothly and almost silently regardless of speed or load. Browning manufactures their own bikes (whose frames must be custom designed to accept the special transmission) and also supplies the transmission mechanism to other bike manufacturers.

Shifting More Than Gears
Shimano, the largest manufacturer of bicycle components such as brakes and shifters, has also begun selling automatic transmissions. One design uses a seven-speed, internally geared hub; another, which is not yet available in North America, uses a power-assisted derailleur system, but adds an automatic, powered suspension to adjust the comfort of the ride to fit current conditions. Bikes with the Shimano mechanism are significantly heavier and more expensive than their manual counterparts and are designed more for leisure riding than racing or mountain biking. The Browning mechanism, on the other hand, was first employed on bikes used for BMX racing, and adds somewhat less to the cost and weight of a bike.

Adding an automatic transmission to a bicycle seems—in the abstract at least—like a wonderful step forward in user interface. It replaces something awkward with something invisible, which is the way good technology should be. Whether or not the reality lives up to the hype (or will in the future), I don’t know. And something tells me it ought to be possible to create a purely mechanical automatic bike transmission. I’m not sure what happens to an electronic one when the batteries inevitably die; presumably it just stays in whatever gear it was last set to.

At this point, automatic bike transmissions are not taken very seriously among cycling enthusiasts. Some are put off by the extra weight; some feel it’s not worth the money just to avoid having to move a lever; and some just think automatic transmissions are for wimps. Having never used one of these bikes myself, I can’t say whether the performance would be improved enough to make me want to ride my bike more often, but at least I would no longer view gear shifting as the annoyance I do now. —Joe Kissell

More Information about Automatic Transmissions for Bicycles…

This article was featured in Carnival of Cars on Friday, July 7, 2006.

Here’s a sampling of articles written about automatic bike transmissions: Easy Rider by Kaspar Mossman in Scientific American (December, 2005); Bicycles with automatic gearshift are ready to roll from the Puget Sound Business Journal; Automatic for the Pedal from Wired News; and Let your bike do the shifting… from EXN.ca.

For more information on the Browning mechanism, visit the Browning Components, Inc. Web site or read The Browning Automatic Bicycle Transmission from Cycling Science. Browning’s own limited edition “Browning 12” bike is the least expensive 12-speed automatic transmission model, at US$995. Other models are available from Grisley Bicycles.

Information on Shimano’s mechanisms can be found on their Web sites. This page describes the Nexus AUTO 3, their internally geared hub.