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Andy's Lesson On Gears and Changing Speeds
By Andy "The Mechanic" Wallen
The following is an excerpt from our Fall 2001 Ask the Mechanic column.
My name's Michelle and I'm doing a paper for school on Gears and Changing Speeds. Could you tell me a little about this subject? It would be a great help!
You can email back for more detailed information if
necessary, but here is an overview.
The standard mountain bike has three chainrings (also called chainwheels or sprockets) in the front and 8 or 9 cogs in the back. Road bikes usually have two chainrings and 8, 9, or 10 cogs, but some of them have 3 chainrings. Simply stated, you have what we call "big" gears and "little" gears. Big gears go fast, and little gears climb hills. On a standard road bike with 2 chainrings and 9 cogs, your biggest gear happens when you put the chain on the biggest chainring and the smallest cog. This gear is best referred to by the number of teeth on the chainring followed by the number of teeth on the cog, or 53/12. This is the fastest gear on the bike, because a single revolution of the chainring produces several (I'm not going into the physics of the thing here--to be exact, you'd need to count wheel rollouts, or perhaps your physics teacher could devise a formula for exactly how many) revolutions of the rear wheel. Conversely, the little gear happens when the chain is on the smallest chainring and biggest cog (usually 39/23 on road bikes), and this is the easiest gear to climb hills with, because a single revolution of the chainring produces something close to a single revolution of the rear wheel.
If you had a bike with only 3 chainrings (20-30-40) and 3 cogs (10-20-30), and no wheel size involved, you cold devise gear ratios, like 20/20=1:1; 20/30=2:3, etc. Bicycle people have devised a system for comparing gears called gear inches. To determine gear inches, you multiply the tire size in inches x the number of chainring teeth and divide by the number of cog teeth. While it is erroneous to use this number as a representation of a physical event, it is useful in that you can use the gear inch number, like 52 or 26 to compare gear changes on different sized wheels, or to better understand the difference in changing cog or chainring sizes.
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