Tuesday, April 14, 2009
SAN FRANCISCO engineer Keith Chilcote invented a bike with a computer-controlled automatic transmission. Superficially, Chilcote's 11-speed bicycle doesn't look all that different from most other bikes - except that it has a small computer display on the handlebars. The computer itself, weighing a few ounces, hides beneath the seat. On the hub of the rear wheel is a collection of 64 tiny magnets that are arranged in a ring as big round as a 45-rpm record.
As the wheel turns, the magnets turn with it, passing over a sensor; the rate at which they pass lets the computer determine how fast the wheel is spinning. With so many magnets zipping past, the computer can calculate speed about 120 times a second, allowing the tiniest change to be detected. Comparing this rate with the gear the rider is using, it computes how many revolutions the pedals are making a minute. Human legs pedal most efficiently at 75-rpm; when the computer notices the speed going above or below that, it shifts gears, it slides all the teeth out a little or in a little, forming a new circle with a new circumference. Each tooth can stop at 11 different points along its track.
There are times, however, when a rider wants to pedal faster than 75-rpm - in the final leg of a race, for example. To keep the computer from interfering at such critical moments, Chilcote is developing a pressure sensor for the rear axle. The sensor monitors the force applied to the wear wheel with each pedal push. If the computer detects that the feet are spinning faster than 75-rpm but that pedaling force is nevertheless increasing, it's smart enough to know that the rider probably wants it that way, and it switches to a sprint program. This program allows pedal speed to reach a dervishlike 82 rpm before finally shifting gears.