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In the Colorado desert in 1992, a speeder was clocked going 110 kilometers (68 miles) per hour. No ticket was issued to the driver. A lapse in law enforcement? Not at all. The Dexter Hysol Cheetah, an experimental bicycle, had just broken the world record for human-powered speed. The Cheetah incorporated many new innovations in bicycling-a recumbent seating angle, aerodynamic airing, and the latest lightweight materials. The result was a milestone in the annals of two-wheel transportation. Two principles of physics explain how a bike works. First, angular momentum, the same force at work in a gyroscope, makes wheels want to keep turning in the same direction and position as they have been. So as your bike wheels spin underneath you, they're actually helping you stay upright as their angular momentum resists changes in the bike's upright position. Second, because of the way bicycles are constructed, inertia swings the upper part of a bicycle away from the center of a turn, even as the front wheel dips into the turn, keeping the bike in an upright position. Bicycles have undergone few design changes since they were first invented. The earliest known bicycle design dates back to the 1490s, when a student of Leonardo da Vinci sketched a vehicle which looks remarkably like today's bicycle. The first functioning velocipedes of the 19th century also strongly resembled today's bikes, with two wheels of the same size. The old-fashioned bicycles most people think of with enormous front wheels and tiny back wheels were actually invented later. Called ordinaries, these bikes were fun and fast, but quite unstable and dangerous. Most innovations in biking happened as a result of the energy crisis of the '70s. Many of these innovations have already been incorporated into competition-class bicycles, such as aerodynamic carbon-fiber frames. Other improvements include wheels that attach on only one side, two-wheel drive for more traction, and tension wires that offer extra stability for less weight. Recumbent bicycles are also becoming popular, in part because the rider's lower center of gravity makes the bike more stable. Not only are you less likely to fall off - it's a shorter distance to the ground if you do!