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Path: utzoo!watmath!clyde!burl!ulysses!harpo!ihnp4!inuxc!pur-ee!ecn-ee!tk
From: tk@ecn-ee.UUCP
Newsgroups: net.bicycle
Subject: Re: Gear Ratios - (nf)
Message-ID: <1867@pur-ee.UUCP>
Date: Wed, 16-May-84 13:31:42 EDT
Article-I.D.: pur-ee.1867
Posted: Wed May 16 13:31:42 1984
Date-Received: Thu, 17-May-84 04:26:40 EDT
Sender: notes@pur-ee.UUCP
Organization: Electrical Engineering Department , Purdue University
Lines: 18

#R:sdccs6:-147400:ecn-ee:18200003:000:688
ecn-ee!tk    May 16 09:47:00 1984

-
The *size* of a gear is computed as:

  number of teeth on front sprocket               1
  --------------------------------- X  -----------------------
  number of teeth on rear sprocket     wheel diameter in inches

A little thought will reveal that this gives the equivalent diameter
of the front wheel of a "penny-farthing" type bicycle to get the same
mechanical advantage (you know, those things people were riding somewhere
around the turn of the century). It's a rather regrettable historical
convention; if you're in Europe, the ratio is given in meters, the 
distance the bicycle rolls in one revolution of the crank. But I guess
that's like arguing Celsius vs. Fahrenheit...