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Gear Inches Question

Old 11-05-20, 04:30 PM
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Gear Inches Question

I've been bouncing this around in my head for a few days now. Did a bunch of research even and for the life of me, except for comparison purposes, I can't find a logical use for GEAR INCHES. According to the online calculator I found, my wheels have 101 gear inches. That tells me nothing. In reality, I have 86 inches of actual travel per revolution of the wheel, which if multiplied by the gear ratio that I now have of 52/13=4 times my wheel travel=28.66 feet per crank revolution. That's a number I can use and when I'm doing a cadence of 70 I am doing @23 MPH. Or 19.5MPH when I'm doing my normal cadence of 60. Both of these real world number jive with my GPS unit. So what the heck is the GEAR INCHES number for? Thanks
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Old 11-05-20, 04:48 PM
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Gear inches (GI) is a concept that goes back to the early says of bicycles, the high wheelers. A "gear inch" is how far a high wheeler would go with a 1" diameter front wheel. (Yeah, ridiculous but a kiddie on his tricycle has a 16" GI.) Racers (mankind will always race the next contraption that comes along) quickly figured out the bigger that front wheel, the faster the bike, limited only by the length of legs of the rider. (Their seats always looked to high, for very good reason.) When the chain bicycle came along and therefore gear ratios, they just naturally spoke of this new geared setup as being equivalent to the old front wheel, using the same (equivalent) diameter n inches.

So your 101" gear is the equivalent of a 101" high wheeler (to be ridden by the NBA's tallest).

Yes, an archaic way of expressing gear ratios. But it does describe the gears we use with simple 2 digits (3 for the really big ones). 20 to 120 covers most of what we ride. No decimals needed. Also has the nice feature that it takes wheel size into account, making comparisons between (say) a standard 27"/700c bike and a small wheeled folder easy.
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Old 11-05-20, 04:56 PM
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It's actually a pretty archaic/arcane term, honestly. It allows you to equate your combined gearing/wheel combo to an equivalent wheel of a particular size being driven directly by the pedals (with no gearing - like on the old penny-farthing bicycles).

So if your wheel is 27 inches in diameter, and your gearing is 52/13 (4:1), then your distance traveled per pedal revolution is the same as if you had a 108-inch diameter wheel being directly driven by the pedals with no gearing (not that you could actually straddle a wheel that big, which was one of the obvious limitations of the penny-farthings)

"distance traveled per crank revolution" is an equivalent measure to gear-inches, and probably more intuitive.
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Old 11-05-20, 05:14 PM
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Gear inches is how far the bike travels with every 360 degree revolution of the crank. It varies depending on what the combinations of front and rear gears are in use. In use it's just a method of comparing the gearing possibilities.

This is the most useful calculator I've found, even better then Sheldon's. It'll do gear inches, rollout, gain ration and speed. Easy to use.

https://mike-sherman.github.io/shift/
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Old 11-05-20, 05:16 PM
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The number by itself is fairly useless, but as a standard to compare bicycle gearing and wheel diameter combos between different set ups, it is useful.
If you can get everyone to agree on, and start listing a different standard that makes more sense, go ahead.
Sheldon Brown had some thoughts on the matter...I'll let you read his words rather than try and explain it.

Gain Ratios-- A New Way to Designate Bicycle Gears
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Old 11-05-20, 05:17 PM
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It's just a scale. Other scales are just as good and no better, if you use the measurement for comparison. I happen to think in gear inches, but you could argue that "rollout" is more meaningful.

I'm in favor of the US going to the metric system, and I can think in kilometers or miles. They're different scales.

But I would like it if we stuck to Fahrenheit. That's a sensible scale, because it encompasses human experience. The coldest we experience is about 0º, and the hottest we experience is about 100º.
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Old 11-05-20, 05:26 PM
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So you’ve been interested in getting one the of the new safety bikes, right? But what gear ratio should choose so that you can have a ride similar to your Ordinary? Why just measure the diameter of your front wheel on your Ordinary and then choose a gear-inch that matches and you’ll be able to keep up with your fellow wheelmen!

Some bicycle manufacturers in the late 1880s thought safeties would be ridden mostly by women or casual riders but racers would want Ordinaries if they where serious about racing and missed the bike boom in the following decade.
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Old 11-05-20, 05:30 PM
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Originally Posted by Steve B. View Post
Gear inches is how far the bike travels with every 360 degree revolution of the crank.
Not quite, although the values are related. Since "gear inches" gives you an equivalent wheel diameter, if you want to figure out how far the bike travels with one crank revolution, you have to multiply by pi to convert the diameter into a circumference. So with 100 gear-inches, the bike will travel (100 * pi = ~314 inches) with each revolution of the crank.
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Old 11-05-20, 05:45 PM
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Originally Posted by ScottRiqui View Post
It's actually a pretty archaic/arcane term, honestly. It allows you to equate your combined gearing/wheel combo to an equivalent wheel of a particular size being driven directly by the pedals (with no gearing - like on the old penny-farthing bicycles).

So if your wheel is 27 inches in diameter, and your gearing is 52/13 (4:1), then your distance traveled per pedal revolution is the same as if you had a 108-inch diameter wheel being directly driven by the pedals with no gearing (not that you could actually straddle a wheel that big, which was one of the obvious limitations of the penny-farthings)

"distance traveled per crank revolution" is an equivalent measure to gear-inches, and probably more intuitive.
Originally Posted by Steve B. View Post
Gear inches is how far the bike travels with every 360 degree revolution of the crank. It varies depending on what the combinations of front and rear gears are in use. In use it's just a method of comparing the gearing possibilities.

This is the most useful calculator I've found, even better then Sheldon's. It'll do gear inches, rollout, gain ration and speed. Easy to use.

https://mike-sherman.github.io/shift/
I think you are tying to say the same thing but you are wrong. Gear inches is the diameter of the wheel if the crank were attached to the axle like an ordinary (penny farthing, big wheel, etc.). To get how far the bike travels per revolution of the crank (aka “development”), you need to multiply that gear inch measurement by ∏ (to 2 places is enough). A 108” gear is going to travel 350” (29’) per one crank revolution.

An example of wheel that travels 108” (2.7m) per revolution would be a 22/18 or about 34 gear inches.
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Old 11-05-20, 05:55 PM
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In Europe the common method of expressing bike gearing is "development" which is the distance the bike travels with one full crank rotation. It is gear-inches X Pi but in Europe the distance is specified in meters.
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Old 11-05-20, 06:47 PM
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I’ve been riding a long time and never used gear inches. I’d sometimes use ratios on the fly to get a feel for what I was riding in what terrain. For me ratios are everything.

With the old 52/42 chainrings. If I was in a 52/21 I’d figure I’m about 2-1/2 so going to a 42, I have a range of 17-28. Didn’t mean much, but if it kept my brain occupied, especially if I were struggling I could do a quick, how much would this gear help.

Now at home I have an excel spreadsheet and I’ve kept nearly every gear change on each bike (decades) and the corresponding speed at crank rpm. I have what-if scenarios.

John

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Old 11-05-20, 07:45 PM
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Gear inches is merely a gear ratio like 52/13 = 4 times 27, which equals 108. It has nothing to do with how far the bike travels for each revolution of the tire. As others noted, you must know the true diameter of the tire and multiply by 3.14 to get the distance per revolution. It's really a rough equivalent of 100% being the top gear and the rest a percentage of the largest gear ratio.

A typical road tire has a circumference around 83 inches. With a gear ratio of 4/1 a bike will travel 83x4=332 inches per crank revolution.
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Old 11-05-20, 08:00 PM
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I use it as a simple comparison. I know what I have (although I forget), but it's easy to put into a calculator, and from there I can see if I want a bigger/smallerchainring (front sprocket in my case) or a bigger/smaller rear sprocket. It makes it easy to compare bikes with different wheel sizes instead of say, 32x38 gearing, calculating that ratio and then trying to make the calculations if one bike has 26" wheels, the other 700c or whatever.
Someone else said "it's just a scale". And it is. I can't use the number, say, "110 gear inches" for anything as a standalone number, but when compared to other bikes I also know the gear inches of, I can do just that.

It's like the Celcius scale: A number of, say "10" doesn't mean anything until you know that a) Water freezes at 0 degrees Celcius, boils at 100 (at sea level), and that below a certain threshold you will be cold, and above it, you will be hot. The number is just something to use to compare it to other things measured on the same scale.
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Old 11-05-20, 08:39 PM
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Ratios are useful only if you are comparing bikes whose wheel diameters are the same. My 52/14 is very different from my friends, because I have 700c wheels and he has 20" wheels.

Not only that, I can't keep track of which ratios are equivalent. Does everyone know that 52:20 is nearly the same as 42:16? I doubt it. Some reduce the ratio to 2.6 and that is accurate, but we still have the wheel size problem.
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Old 11-05-20, 09:39 PM
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I agree that ratios are wheel related. But for me it is relative to what I’m riding.

Even if I don’t figure things on the fly, it is nice for me to know where I am in comparison to using another chainring.

My mountain bikes have 34/24 chainrings and I really like to stay in a particular chainring as long as I can. I approach it like having (2) 1x setups.

I added a whopping 40t to the cassette range. If I can ride and occasionally need a 34/40 then I’ll stay in the 34 chainring. If I know I need a lower gear, or I’m always in the lower gears, I’ll go to the 24 chainring. The 24/28t cog is basically the same ratio as the 34/40.

I realize I could use gear inches, but it is just easier to use in relation to another chainring on the bike I’m riding.

John

Edit added: I will say that riding a 24/40 with 26” wheels is pretty hilarious. There are times when I’m praying the woman hiker pushing the jogger with a kid in it doesn’t beat me to the top.

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Old 11-05-20, 09:49 PM
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Originally Posted by HillRider View Post
In Europe the common method of expressing bike gearing is "development" which is the distance the bike travels with one full crank rotation. It is gear-inches X Pi but in Europe the distance is specified in meters.
Multiply meters by 3.3 to get feet development.
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Old 11-05-20, 11:29 PM
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For my personal use gear inches hasn't been that significant. But my kids ride track, road and BMX and knowing gear inches has proven invaluable. I'm building a road bike for next year for my daughter and was trying to decide on the crank options 50/34 or 48/36. A quick look at gear inches and the 48t with a 12t small cog on the cassette with a 26" wheel with 28c tire will be only a couple inches under the max rollout allowed by USA cycling for junior riders. Saved me buying the wrong gearing. With track and BMX having a nice big gear inch chart printed out means I can pick the next closest gearing without having big changes to fine tune what's best for them.
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Old 11-06-20, 08:04 AM
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Originally Posted by Bigbus View Post
I've been bouncing this around in my head for a few days now. Did a bunch of research even and for the life of me, except for comparison purposes, I can't find a logical use for GEAR INCHES. According to the online calculator I found, my wheels have 101 gear inches. That tells me nothing. In reality, I have 86 inches of actual travel per revolution of the wheel, which if multiplied by the gear ratio that I now have of 52/13=4 times my wheel travel=28.66 feet per crank revolution. That's a number I can use and when I'm doing a cadence of 70 I am doing @23 MPH. Or 19.5MPH when I'm doing my normal cadence of 60. Both of these real world number jive with my GPS unit. So what the heck is the GEAR INCHES number for? Thanks
With all due respect, other than gear inches and development (28.66 feet in your example), it would be pretty difficult to compare gearing between two bikes, or between alternative gears, with anything else you've listed.

Think about it. The travel per wheel revolution number is embedded into both gear inches and development. Cadence is highly individual -- I think a cadence of 70 is borderline low, while you think it's high. But with variation between riders, much less variation within a ride, speed is hardly useful to compare bike hardware. You want to calculate speed anyway? If you can do the numbers in your head, you can do it for gear inches, too -- just multiply gear inches by pi times cadence times 60 minutes per hour times the conversion from inches to miles, and boom! you're there. You could also set up a spreadsheet to do the calculation for you, put in either gear ratio or gear inches, plus the cadence, and out comes your answer.

If you prefer to use development, that's fine. Gear inches has more history (and therefore more pre-calculated examples) behind it.
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Old 11-06-20, 09:25 AM
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Originally Posted by noglider View Post
It's just a scale. Other scales are just as good and no better, if you use the measurement for comparison. I happen to think in gear inches, but you could argue that "rollout" is more meaningful.

I'm in favor of the US going to the metric system, and I can think in kilometers or miles. They're different scales.

But I would like it if we stuck to Fahrenheit. That's a sensible scale, because it encompasses human experience. The coldest we experience is about 0º, and the hottest we experience is about 100º.
You're absolutely right Noglider, though I disagree on the sensibility of the Fahrenheit scale. Besides, the important bit is that all good tea temperatures are based on the boiling temperatures of water at 100º Celcius.


Kelvin should be pretty easy to convert to if we want to make the whole world change though.


I might be a metric based person but gear inches is useful as a random scale to see how the gearing on a bike compares.

For example:
  1. Koga-Miyata SilverAce with Sturmey Archer XL-RD5(w) 5-speed hub 33x16T (Gear-Calulator)
    • I sometimes run out of gears on the high end on a gentle downhill or with a tailwind and I only use the lowest gear when riding up one of the tall bridges here.
  2. Batavus Randonneur GL with 52/42/26 x 11-34 (Gear-Calculator)
    • I barely, if ever use the 52x11 setup but I am very glad when riding this bike in the mountains for the low gear. Before this I maxed out with a 32 x 32 low gear at 27.4 gear inches.
  3. Future project bike plans: 43/26 x 10-speed 11-36 (Gear-Calculator)
    • The specific crankset I have in mind would have a very similar useful spread with a 10-speed 11-36 cassette and would offer gears closer together in the high end than I have now.

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Old 11-06-20, 09:44 AM
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Thanks to everyone for your explanations and the history lesson. It's pretty obvious to me now that the "gear inches" number is strictly a comparison tool and is useful in that respect. Using it for actual real world math is not a good idea.
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Old 11-06-20, 09:55 AM
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Originally Posted by noglider View Post
But I would like it if we stuck to Fahrenheit. That's a sensible scale, because it encompasses human experience. The coldest we experience is about 0º, and the hottest we experience is about 100º.
I’ve got to disagree. The Fahrenheit system is a goofy system based on nothing. 0°F is the freezing point of a water brine of water, ice and ammonium chloride. Why use ammonium chloride in the mixture? There is no place on Earth where you are going to run across a ammonium chloride brine outside of a laboratory. And why set the boiling point of water at 212°F? I’m amazed that we don’t have fractional temperatures on the Fahrenheit scale analogous to our goofy ‘Merican measurement system. The system Fahrenheit based his scale on...the Rømer scale...did use fractions. The Fahrenheit system gets even goofier when you look at how he developed it. From Wikipedia

According to a letter Fahrenheit wrote to his friend Herman Boerhaave,[14] his scale was built on the work of Ole Rømer, whom he had met earlier. In Rømer's scale, brine freezes at zero, water freezes and melts at 7.5 degrees, body temperature is 22.5, and water boils at 60 degrees. Fahrenheit multiplied each value by four in order to eliminate fractions and make the scale more fine-grained. He then re-calibrated his scale using the melting point of ice and normal human body temperature (which were at 30 and 90 degrees); he adjusted the scale so that the melting point of ice would be 32 degrees and body temperature 96 degrees, so that 64 intervals would separate the two, allowing him to mark degree lines on his instruments by simply bisecting the interval six times (since 64 is 2 to the sixth power).
That’s some serious complication to the system.

The Celsius scale makes much more sense. It’s based on the freezing point of pure water...something that is readily available around the universe...and the boiling point of pure water at sea level...again something we have in abundance around the universe...and making 100 division between the two. Easy to grasp and easy to calculate with.

Kelvin would be okay but doing the arithmetic is a little more complicated because of the scale. Our clever monkey brains prefer smaller numbers.
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Old 11-06-20, 10:28 AM
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For me, GIs is a PERFECT scale. Because it completely mimics the human body dynamics.
An average walking step is say 30". For steep hills it would be 20". One day in Vancouver I was pushing my 120 lb bike up a really steep hill. I was down to a seemingly ridiculous 5" steps.
For bigger GIs like 100", then obviously you need to be running fast.
I don't otherwise think in terms like going 30 feet with 1 rev. For that I would think I am doing a 4 or 3 minute mile.
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Old 11-06-20, 10:34 AM
  #23  
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I can't see how that "mimics" the human body dynamics other than "larger numbers = more speed/longer steps". That would be true of just about any scale.
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Old 11-06-20, 10:59 AM
  #24  
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Originally Posted by Russ Roth View Post
For my personal use gear inches hasn't been that significant. But my kids ride track, road and BMX and knowing gear inches has proven invaluable. I'm building a road bike for next year for my daughter and was trying to decide on the crank options 50/34 or 48/36. A quick look at gear inches and the 48t with a 12t small cog on the cassette with a 26" wheel with 28c tire will be only a couple inches under the max rollout allowed by USA cycling for junior riders. Saved me buying the wrong gearing. With track and BMX having a nice big gear inch chart printed out means I can pick the next closest gearing without having big changes to fine tune what's best for them.
Now this is a really good use for gear inches. I imagine, depending on how much rabbit holing a person wants to do, someone could calculate wheel size and gearing to get as close to max rollout with least weight.

John
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Old 11-06-20, 11:05 AM
  #25  
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Just from a memory perspective I've always used 108 as the start of my GI scale, since it was the old 52/13 gearing that was common with a 27" wheel. I tend to think of everything else in relation to that. But what the numbers really mean still doesn't equate to anything beyond the comparison, so now that I'm at 50/13 I know I start lower. With 9/10/11 speed the charts became a little less relevant too, since they were used in the old days to select 5 or 6 or 7 cogs so you wouldn't get overlapping gearing between your big and small chainrings. But with 10 cogs still within that same range of big and small, and also with indexing, it just isn't as much of a need. You are going to have overlap in any case and you still have plenty of gears. And the double shift is no longer necessary when you have 13,14,15,16,17,18,19,21,23,26 like I do. It is already a fine enough shift staying on one ring. If you go 11 or 12 you have even more to choose, with an extra large one thrown in. And did anybody really double shift anyway? I think I had 13/15/17/20/23/27 on one old freewheel and never found it necessary, but also had a corn cob on another bike, except that the lowest gear was still way too high.

In my mind though GI was always just for comparisons, not as a measure of anything useful.
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