Hey Gear Heads! The Physics of Bicycle Gear Ratios *
AbstractIf you have a multi-speed bike, you know that you can make it easier or harder to pedal just by shifting gears. Ever wonder how that works? You can investigate this a number of ways. A basic approach is to use a selection of spools of thread (with different diameters), a board with two nails, and a rubber band. Place a spool over each nail, and put the rubber band over them. Mark the 12:00 position on each spool so that you can count revolutions. Turn one spool through a full circle and note how far the second spool turns. Try with different combinations of spool sizes. Explain how your results relate to bicycle gears. You can also do this with a multi-speed bike: turn the bike over, and mark a position on the rear wheel with tape so you can count revolutions. Or, maybe your bike has a speedometer and cadence monitor (this uses magnets on the crank and wheel, and fixed sensors mounted on the frame to count). Have a helper hold the rear wheel up while you move the pedal at a fixed cadence (make sure there is no slack in chain). Record the resulting speeds for each gear combination. Count the teeth on the front sprockets and rear gears. Divide the number of teeth in front by the number in back for each gear combination. Knowing the wheel circumference, you can calculate the wheel's angular speed (revolutions per minute, or rpm's) from the recorded speed. Graph your results. Is there a relationship between the ratio of the gear teeth and wheel rpm's? (Idea from Wiese, 2002, 62-67.)
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Last edit date: 2017-07-28
BibliographyWiese, J., 2002. Sports Science: 40 Goal-Scoring, High-Flying, Medal-Winning Experiments for Kids. New York, NY: John Wiley and Sons.
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