Get in gear.

Have you ever shifted your bike into the wrong gear in the middle of climbing a hill? Impossible to pedal, right?

Now Ezra Gold and his fellow engineering students at the University of Rochester in New York have come up with a solution. "We created the `smart bike,' a bike that changes gears by itself," Gold says. The smart bike is designed to keep you in the right gear so your legs can ride on and on.

Bicycles, explains Gold, are compound machines, made up of many simple machines, such as levers and wheels and axles. Like all machines, says Gold, "the different gears on your bike increase or decrease the amount of force, or push, you need to apply" to do work - move an object (e.g., a bike) over a distance.

Usually bikers have to change gears by themselves - shifting gears for uphill climbs, and higher gears for flat terrain. But sometimes they forget to shift - or they wait too long. Result: That awful chain-clanking sound and tired leg muscles.

To make changing gears a snap, "we programmed a computer chip in the smart bike to measure a biker's pedaling and wheel speeds," says Gold. "The chip uses these measurements to calculate how taut your bike chain is." The tighter the chain, the more force you need to pedal.

To program the bike, you decide how fast you want to pedal. (It's best to cycle at a constant pedal speed of about 90 revolutions per minute, Gold says). "The chip remembers how fast you should be pedaling and shifts gears automatically to keep you pedaling at that rate - even when the terrain changes," Gold says.

The six-speed bike took about a year to make, says Gold, "and we're still making improvements." But the team of engineers says you may find their smart bike in a local cycle shop in the future.