median
don steward
mathematics teaching 10 ~ 16

Tuesday, 14 April 2009

bike gears


a 5-speed bike has the following gear ratios:

a)  3 : 7
b)  1 : 2
c)  3 : 10
d)  4 : 7
e)  6 : 17

put these ratios in order. smallest to largest

a 7 cog back wheel (rear sprockets, hub gears) and 2 cog pedal wheel (shifters, chain wheels) have the following numbers of teeth:

pedal wheel: 42 or 53 teeth
cog wheel: 13, 15, 17, 19, 21, 23 or 26 teeth

how many different ratios can you effectively get from the 14 combinations?
it isn't 14 because some ratios are too close together to be called different

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