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There are about a billion bicycles in the world, twice as many as motorcars. Almost 400 million bicycles are in China

The world speed record on a bicycle is held by John Howard of the US. In 1985 he reached 245,08 km/h (154 mph), cycling in the slipstream of a specially designed car

Most bike crashes occur in parks and on bike paths or driveways. Most bike crashes occur in the summertime between 6 and 9 p.m. A helmet is 85 percent effective in reducing the risk of head injury.

Randy Draper on his electrically assisted mountain bike climbing Haleakala volcano, Maui. The road is billed as the world's steepest. It climbs nonstop from sea level to the peak of the 10,005 feet high volcano Haleakala on Maui, over a distance of 37 miles. The race up it, called Cycle to the Sun, was conceived of as the world's toughest bicycle hill climb. But in this year's race, held on August 17th, a rider on an electric power assisted bicycle, using advanced batteries and a motor he made himself, has completed this endurance feat for the first time, near the head of the pack. Randy Draper, the constructor and pilot of the modified bike, was drawn to the race not as a competition against human power, but rather as an extreme test of his unique design approach to light EV's. Using an unusually efficient 750 watt brushless motor he designed himself, a modified three-speed hub shifter from bicycle component maker SRAM, and a 48-volt nickel-zinc Evercel battery pack, his full-suspension mountain bike is a test bed where total over-the-road efficiency is the guiding rule. I think it's a world record.. I now have a 100+range bike at 20-30 mph 1600 watt peak but rarely use over 750 watts. Got to 10,005 feet from sea level in 3 hours 3 min and 58 sec.

On a penny-farthing bicycle, one pedal gave the wheel one turn. A lot of pedaling was needed to get around! With the advent of gears, bicycles can move as fast as cars.

Bicycle Gears

You have probably seen a picture of the funny-looking "penny-farthing" or "high-wheeler" bicycles -- the ones with a huge front wheel and a tiny rear wheel. You might even have seen someone riding one in a parade or in a movie. These bicycles became popular starting in 1870, but by the turn of the century were replaced by the "safety bicycle." A bicycle from 1900 or 1910 looks almost exactly like any bicycle you see today. Today's bicycles have two wheels of the same reasonable size, a pair of pedals in the middle of the bike and then a chain that connects the pedals to the rear wheel.

So why did penny-farthing bicycles ever exist? In a penny-farthing bicycle, the pedals and the front wheel are directly connected just like they are on a kid's tricycle. That means that when you turn the pedals one time, the wheel goes around one time. That's an inexpensive way to build a bicycle, but it has a problem. Think about a kid's tricycle. The front wheel might be 16 inches (40 cm) in diameter, or 16 * 3.14 = 50 inches (127 cm) in circumference. That means that each time a kid on a tricycle pedals through one revolution of the front wheel, the tricycle moves forward 50 inches (127 cm). Let's say that the kid is turning the front wheel at 60 rpm, or one revolution per second. That means that the tricycle is moving forward 50 inches per second. That is only 2.8 miles per hour (4.5 kph). If the kid pedals twice as fast, at 120 rpm, the trike is moving at just over 5 miles per hour (9 kph), and the kid looks like his legs are about to spin off because 120 rpm is a lot of pedaling!

If an adult wants to ride a tricycle at a reasonable speed, maybe 15 mph (24 kph), and if the adult does not want his or her legs to fly off, then the tricycle's front wheel has to be pretty big. If the adult wants to pedal at 60 rpm, the front wheel needs to be 84 inches in diameter -- that's 7 feet (more than 2 meters) in diameter!

The first thing that causes bicycles to have gears is the fact that gears can reduce the wheel size from 7 feet in diameter to something reasonable. As described in the article How Gears Work, gears and gear ratios are a good solution to this problem. For example, if you put a gear with 42 teeth on the front chain wheel and a smaller gear with 14 teeth on the rear wheel, you have a 3-to-1 gear ratio. Now the back wheel can be 84 inches / 3 = 28 inches (71 cm) in diameter -- about the size of a normal bicycle wheel. This is a much safer approach.

Bicycle Bearings

Bicycles use ball bearings to reduce friction. You can find ball bearings in:

The front and rear hubs for the wheels
The bottom bracket, where an axle connects the two pedal cranks together
The fork tube, where the handlebars are allowed to turn
The pedals
The freewheel, where they do double-duty (In the freewheel, they also help provide the one-directional feature.)

The bearings in the fork tube are typical and are shown in the following figure:

The ball bearings (yellow) ride in a cup (red). The cones (dark blue) screw onto the light blue tube that is attached to the fork. The cones are adjusted to be tight enough that there is no play in the fork, but not so tight that they squeeze the ball bearings and cause them to bind. The wheel hubs and pedals work exactly the same way, with the cones providing the adjustment. In the crank axle, one of the cups provides the adjustment instead of the cones. A little bit of grease in the bearings makes them even smoother.

Periodically, you have to disassemble the bearings on a bicycle to clean out the dirt and put in fresh grease. Some more expensive bicycles have sealed bearing cartridges that never need adjustment or lubrication.