gear inchs formula
10-26-08 | 02:09 PM
#1
Thread Starter
Senior Member
Joined: Sep 2008
Posts: 542
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From: yreka, ca
Bikes: like 15. my favorite a 1951 schwinn spitfire cruiser. also have a 1959 amf roadmaster, 1962 jch deluxe cruiser among others.
gear inchs formula
it has been years since i figured out "gear inchs" for my bikes. isn't it like chainring x wheel divided by ? if anyone has the formula, id appreciate it. also, is their a different number i plug in for wheel size if i calculate bikes with 26x1.95/2.125 tires vs 26x1 3/8 and 700c wheels. and what does gear inchs actually measure? distance traveled on one rev? thanks
10-26-08 | 02:16 PM
#2
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Carbon: Fuji SL2.1 Di2.......Aluminum: Cannondale Synapse 105........Steel: Vintage Specialized Sirrus
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Carbon: Fuji SL2.1 Di2.......Aluminum: Cannondale Synapse 105........Steel: Vintage Specialized Sirrus
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10-26-08 | 04:08 PM
#3
Senior Member
Joined: Apr 2005
Posts: 17,196
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From: Ann Arbor, MI
Bikes: 1980 Masi, 1984 Mondonico, 1984 Trek 610, 1980 Woodrup Giro, 2005 Mondonico Futura Leggera ELOS, 1967 PX10E, 1971 Peugeot UO-8
Gear inch is the diameter of a hypothetical front wheel from high-wheeler, that will travel as far in one pedal revolution as your bike will. Gear used to be adjusted by buying a bike with a bigger front wheel.
The distance it will travel in one rev, the "development," is the gear inch number multiplied by the constant PI, equal to around 3.14, or 22/7.
The formula for gear inch is GI=wheel_diameter * chainring_teeth / cog_teeth, so yes, you had it real close. I don't think it needs to be extremely precise, so if you measure the wheel diameter with the tires inflated to 1/8 inch with a yardstick, I think you have good enough data for studying the spread and stepping of a gear system.
There are some standard ISO values for wheel diameter for all the standard tire types, but I'm not sure those are true for all commercial tires. I'd recommend just getting out a yardstick and measuring. Besides, the ISO documents cost significant $$$.
I know Sheldon recommended using "gain ratios" but I'm not sure that number helps. It doesn't help me, at least. It's just a different way of looking at measuring gear.
The distance it will travel in one rev, the "development," is the gear inch number multiplied by the constant PI, equal to around 3.14, or 22/7.
The formula for gear inch is GI=wheel_diameter * chainring_teeth / cog_teeth, so yes, you had it real close. I don't think it needs to be extremely precise, so if you measure the wheel diameter with the tires inflated to 1/8 inch with a yardstick, I think you have good enough data for studying the spread and stepping of a gear system.
There are some standard ISO values for wheel diameter for all the standard tire types, but I'm not sure those are true for all commercial tires. I'd recommend just getting out a yardstick and measuring. Besides, the ISO documents cost significant $$$.
I know Sheldon recommended using "gain ratios" but I'm not sure that number helps. It doesn't help me, at least. It's just a different way of looking at measuring gear.
10-26-08 | 04:46 PM
#4
El Duderino
Joined: Jun 2008
Posts: 492
Likes: 2
From: Madison, WI
Bikes: 84 Raleigh Portage, 83 Trek 620
Gear ratio is what I use. I like that it takes into consideration the crank length as that can make a big difference. I also like that it is a ratio and is not based on this or that measurement system.
10-27-08 | 05:53 AM
#5
Senior Member
Joined: Apr 2005
Posts: 17,196
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From: Ann Arbor, MI
Bikes: 1980 Masi, 1984 Mondonico, 1984 Trek 610, 1980 Woodrup Giro, 2005 Mondonico Futura Leggera ELOS, 1967 PX10E, 1971 Peugeot UO-8
Gear ratio is fine (really all these measures are fine!), but it depends on what you're trying to accomplish. If you're just trying to see if the distribution of gears is even and what the best gear shifting progression is, any measure should work as long as you have criteria for deciding what's good. If you're trying to estimate leg effort (pedal force, power, aerobic load, or ??), fitting crank length into the mix is worthwhile. But if you don't think an objective analysis of effort is meaningful, which I don't, then spread is what's important, and effort is judged subjectively. That's what I mean when I say gain ratios or gear ratios are not useful for me.
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