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    Biking

    Bicycling Down Through the Gears

    By Vince Hancock
    Special to The Washington Post
    Wednesday, May 14, 1997; Page H08

    The first bicycles with gears and chains had only one "speed." Even competitors in the rigorous Tour de France climbed mountains and sped into valleys without benefit of gear changes -- a form of bicycling almost unthinkable today.

    Modern cyclists are accustomed to using various gear combinations that allow pedaling at the most comfortable rate.

    The first step toward multiple gears was the two-speed bike that had different gears on either side of the rear wheel. To shift, one stopped, removed the rear wheel, flipped it around, refitted the chain and reattached the wheel.

    Today, cyclists can shift effortlessly through as many as 24 gears while continuing to pedal, thanks to an invention known as the derailleur -- a French word meaning just what it sounds like. Derailleurs allow cyclists to match the optimum pedaling ability of human legs to the differing demands of bicycling uphill, downhill or on level ground.

    When the chain is on the largest gear at the pedals and the smallest at the rear wheel, each turn of the pedals makes the rear wheel go a greater distance than in any other gear combination. But this is hard work on level or uphill terrain.

    If, say, the largest gear in the front has 52 teeth and the smallest in the rear has 13, one complete turn of the pedals will rotate the rear wheel four times. With the same front gear and a 28-tooth gear in the rear, one turn will move the rear wheel less than two times, but pedaling will be easier.

    Rudimentary derailleurs were first used in the 1890s, according to Raymond Henry, an author of a forthcoming book, The Dancing Chain: The History of the Derailleur Bicycle.

    The first, easy-to-use derailleur was invented in France in 1910 by Paul de Vivie and shifted among four gears at the pedals. The first modern rear derailleur was patented two years later by a Frenchman named Joanny Panel, according to David Herlihy, a bicycle historian.

    HOW DERAILLEURS WORK
    When you click your bicycle's shifter, what happens?

    The shifter pulls a cable that moves the derailleur, forcing the chain from one gear to the next. Or, if shifting in the other direction, a spring in the device pulls the cable back.

    You might think that gear teeth would stop the chain from going sideways, but the teeth have a special shape that helps the chain to slip off. In the smoothest-shifting systems, some teeth are shaped like a steeply sloping roof, a few are shorter than others and some are slightly twisted. This allows the chain to slide up and off that gear and onto the next even while you're pedaling hard.

    FRONT DERAILLEUR
    Start by looking at the front derailleur. Gear changes occur on top, making everything easier to see.

    The front derailleur has two cage plates, one piece of metal on each side of the chain. Their job is to push the chain sideways. On most front derailleurs, when shifting from a smaller gear, or chainring, to a larger one, for example, the cable pulls the cage plate causing it to shove the chain toward the larger chainring.

    The plate presses the chain against a ramp on the hidden side of the chainring, forcing it to ride up the ramp. The cage plate also helps to lift the chain high enough so that, as the plate continues pushing, the chain slips sideways to the larger chainring, completing a shift.

    Shifting in the other direction, from a larger to a smaller chainring, the cage plate again pushes the chain. But, instead of ramps, the chain uses part of the chainring that has one or more shorter teeth.

    As these shorter teeth pass through the cage plates, one chain link comes off of the gear. The plate keeps pushing the chain, and the chainring keeps turning. The chain then comes off all the teeth and falls to the smaller chainring.

    REAR DERAILLEUR
    The rear derailleur works much as the front does but upside down.

    The chain runs in an "S" shape under the rear derailleur on the two little wheels, variously called jockey wheels, jockey pulleys, pulley wheels or guide wheels.

    The top jockey wheel, called a guide pulley, forces the chain under the gear, or cog, you want. The bottom jockey wheel, called a tension pulley, keeps the chain taut even though the length of the chain's path over various gear combinations changes with each shift.

    The tension pulley moves toward the back of the bike to take up slack to provide, in effect, a shorter chain and toward the front for a longer chain, depending on the different combinations.

    When you shift from a smaller cog to a larger one, the cable pulls the derailleur toward the bigger cog, moving the jockey wheels as a unit directly under the larger cog onto which it's going to shift. This causes the chain to press against that cog.

    The chain rubs against the cog's side and catches in a "notch," also an upside-down ramp. As the cog turns, the notch comes right side up, becomes a ramp and helps to lift the chain up and over, finishing a shift.

    Many cogs have slightly twisted teeth -- in this case to the right when you look from behind the bike -- that help to grab the chain and complete a shift.

    During a shift, a rear derailleur also moves diagonally toward the back of the bike, helping the chain to move toward a larger cog. The derailleur's center part or body, where the manufacturer's name appears, makes this happen since it's shaped like a parallelogram, a tilted rectangle aimed toward the big cog.

    When shifting from a larger cog to a smaller one, both jockey wheels move directly under the cog you picked. The chain comes to a tooth twisted to the left and begins to move off of the larger cog. As you pedal, the chain moves to the smaller cog, completing a shift.

    GOING UP?
    1. Front derailleur presses the chain against the larger chainring.
    2. A ramp on the larger chainring catches the chain, lifting it off the smaller chainring as it turns.

    GOING DOWN?
    1. Front derailleur aligns the chain over/under the chosen gear.

    2. When the chain reaches a shorter tooth (or a tooth that is twisted to the left on the rear cogs), it leaves the larger gear and moves to the smaller one.

    DERAILLEUR MECHANICS
    A spring in the derailleur pulls the chain sideways, usually toward the smallest gear. The cable from the gear shift pulls the derailleur in the opposite direction.

    Vince Hancock is a bike mechanic in Arlington.

    © Copyright 1997 The Washington Post Company

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