ix1037@sdccs6.UUCP (Christopher Latham) (05/15/84)
In some of the articles in this group people talk about 33 inch gears or 80 inch gears. Just what exactly is meant by this, and how is it computed. Christopher Latham U.C.San Diego Dept. of Applied Mechanics and Engineering Sciences ..sdcsvax!sdccs6!ix1037
fish@ihu1g.UUCP (Bob Fishell) (05/16/84)
(oo)
The references to "33-inch gears," etc. goes back to the old velocipedes
of bygone days. A 33-inch gear is the equivalent of having the crank
connected directly to a wheel of 33 inches in diameter. The bigger the
gear, the bigger the virtual wheel, and the faster you go per rpm.
Gear ratios are easily computed. Just count the teeth on your chain
wheels and the freewheel. If you have a 27" wheel diameter, the
virtual wheel size is 27 * #teeth in chain wheel / #teeth in freewheel.
For example, a 52-tooth chain wheel coupled with a 14-tooth freewheel
gear on a 27" bike gives a 27*52/14=100.28" virtual wheel. This is
commonly called a "100-inch gear."
--
Bob Fishell
ihnp4!ihu1g!fishtk@ecn-ee.UUCP (05/16/84)
#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...
kwd@cygnet.UUCP (05/16/84)
[] >> In some of the articles in this group people talk about 33 inch gears or >> 80 inch gears. Just what exactly is meant by this, and how is it >> computed. >> >> Christopher Latham >> U.C.San Diego >> Dept. of Applied Mechanics >> and Engineering Sciences >> ..sdcsvax!sdccs6!ix1037 >> ------------------ You mean you really don't see how these folks are getting around on 80 inch gears!! Actually the term is "gear inches" and it is basically a measure of "how hard you have to pedal". Formula is Gear Inches = ( F / R ) * W F = Number of teeth on front sprocket R = Number of teeth on rear sprocket W = diameter of real wheel in inches (typically 27) Thus if you shifted "to a gear" (I never know what my friend means when she says she's in 4th gear) such that you were using a front sprocket with 50 teeth and a rear sprocket with 25 teeth, you would be riding with (better word?) 54 gear inches. Typical range available on a ten-speed bike is 40 through 90 gear inches. Roughly speaking: 90 - moving at a good clip, maybe downhill 68 - crusing around, flat and level 40 - uphill, lots of pedalling 27 - climbing mountains (I have a 28 in front and back!) I found that graphing out the combinations on my bike helped me use my gears more effectively... know where the overlaps are, etc. Karl Danz Cygnet Systems, Inc. P.S. people who insist on calling front sprockets "chain rings", etc. can send all flames to /dev/null.
ron@brl-vgr.ARPA (Ron Natalie <ron>) (05/17/84)
Something is wrong here if I use 1/wheel diameter I keep getting values less than 1". Makes a very small wheel for a penny farthing bicycle. =Ron
joels@tektronix.UUCP (05/23/84)
Gear ratios on bicycles are measured in inches of Effective Wheel Size
(EWS). A gear of 33 inches is equivalent to having the pedals drive directly
a wheel of 33 inches diameter. The gear ratio is calculeted for each
combination of chainwheel and rear sprocket as follows.
EWS= TF/TR*RD
WHERE: TF= number of teeth in front sprocket
TR= number of teeth in rear sprocket
RD= diameter of drive wheel in inches
This works for any bicycle.
Joel Swank
Tektronix, Beaverton OR