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Gear inch question

Old 09-06-20, 09:43 AM
  #26  
Pop N Wood
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Originally Posted by cyccommute
Most people don’t know what gear they ride in.
But I would think even you could figure out when you run out of gears on the top or bottom end?
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Old 09-06-20, 01:15 PM
  #27  
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The upshot is that I've ordered a new crank and bottom bracket. Based on Sheldon Brown's gear calculator, that should move me from a low gear of 22.6 gi to one of 18.1 gi.
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Old 09-06-20, 02:29 PM
  #28  
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Originally Posted by Pop N Wood
But I would think even you could figure out when you run out of gears on the top or bottom end?
That has nothing to do with knowing which gear you are in. I really don’t think while ratios while riding and I think about gearing far more than most people do. I know what the high is and the low is but I honestly don’t think “I’m riding in the 4th out of 9 gear so that means I’m riding in the 72” gear”. I don’t think anyone does that. We generally ride by “feel” because doing calculations on the fly is just too difficult.
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Old 09-06-20, 07:12 PM
  #29  
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This (Bicycle Gear Calculator) is my favorite resource for toying around with gear inches and development, development being in European inches. It's an invaluable resource. Also, kudos to the poster who pointed out the derailleur capacity issue.

All too often in discussions about gearing, people seem to forget that drivetrains are systems and that derailleur capacity is a thing.

For Shimano you can get all the info you need from their product information website (https://productinfo.shimano.com/#/) either from the current section, or more likely, the archive.
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Old 09-07-20, 02:48 AM
  #30  
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Originally Posted by pdlamb
Back to the original question: yes, it'll make a difference on steep hills (>12%, perhaps), or if you're touring with a load. In other words, if you run out of gears with the setup you've got, that change will give you about 1.5 extra low gears, and that's often significant.
I concur with pdlamb - If you do loaded touring in mountains, you will appreciate the lower granny gear.
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Old 09-07-20, 12:26 PM
  #31  
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Originally Posted by kombiguy
The upshot is that I've ordered a new crank and bottom bracket. Based on Sheldon Brown's gear calculator, that should move me from a low gear of 22.6 gi to one of 18.1 gi.
Follow up and let us know if you feel the difference, please. I'm a high gear guy myself so it's not something I'd contemplate, but I'm always curious how stuff like this works out for people, and it might be helpful for people planning rides like yours.
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Old 09-07-20, 06:03 PM
  #32  
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Originally Posted by livedarklions
Follow up and let us know if you feel the difference, please. I'm a high gear guy myself so it's not something I'd contemplate, but I'm always curious how stuff like this works out for people, and it might be helpful for people planning rides like yours.
Will do!
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Old 09-08-20, 10:10 AM
  #33  
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It sounds like you have a Shimano Tiagra triple set-up. I bought a bike with the express notion of changing the small chainring to a 26 tooth. I'd had success with doing that with a Campy triple (24 tooth) in the past. It didn't work with Tiagra. If I adjusted things just right, it would only work marginally. even with a Jump Stop installed, it dropped the chain way too much for my liking. Tiagra is set up to work with a limited range of chainrings in each position. In my opinion, they picked some bad choices for a triple. I wound up changing the whole crankset to a Sugino Alpina II with 48-36-24 chainrings and using a bar-end friction shifter and Alpina derailleur. I'm a shift-o-holic and I had to have STI for the rear but friction shift on the chainrings is fine (if not preferable). For the rear, I replaced the cog set (11-28) with one that has 11-34. This necessitated in my changing the rear derailleur for one that would wrap the extra chain. I used a Deore 9 speed mountain bike derailleur which has the same pull ratio as the Tiagra 10 speed road derailleur. It works flawlessly. You can't use Shimano 10 speed mountain bike derailleurs with Shimano 10 speed road shifters.


This was kind of a round-about way to do it, but I now have the bike that I wanted.
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Old 09-08-20, 12:36 PM
  #34  
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Mine is a Sora group, although it appears that the bottom bracket is Tiagra. If they break them out that way.
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Old 09-08-20, 02:10 PM
  #35  
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Originally Posted by cyccommute
That has nothing to do with knowing which gear you are in. I really don’t think while ratios while riding and I think about gearing far more than most people do. I know what the high is and the low is but I honestly don’t think “I’m riding in the 4th out of 9 gear so that means I’m riding in the 72” gear”. I don’t think anyone does that. We generally ride by “feel” because doing calculations on the fly is just too difficult.
I think the advantage to Gear Inch is as a comparative value, for two reasons: One, it's a relatively simple number. Use one of the gear calc programs, and you get a table that's populated by 2-digit numbers, rather than 1:1.XXX ratios. It's a lot easier to understand for the less math savvy.
Two, it also takes into account tire size, which since most bikes these days run 622mm isn't more than noise, but it you've got a 571 (26") MTB, or something with 20" like a folder or mini-velo, along with 700c bikes, it can be easier to see where they stack up against each other.

I see that a lot when someone switches from a hybrid / comfort bike, to a more sporting road bike, and complains that it's 'so hard to pedal' the new bike. They're used to using mostly 'the 6 or 7' (thanks Gripshift ) which is like a 77 or 80, so when they use the same lineup on their new bike, it's like a 100+ You can put the gear-inch tables of the old and new bikes next to each other, and easily see where they compare to each other.
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Old 09-09-20, 06:45 AM
  #36  
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Originally Posted by GAtkins
At 26-36 I get gear inches of 19.5 if I did the calculation correctly. I get rollout of 61.26 inches at 26 versus 70.69 inches at 30.

Gear ratio is .72 at 26 versus .83 at 30. At a cadence of 80 I get a speed of 4.64mph at 26 versus 5.35mph at 30.

Somebody please check the calculations.
Yes, the OP’s calculation back-calculates to a wheel size of 27” so the 26-36 combination produces roughly 19.5 gear inches. It’s still a 13/15 ratio or a 13.3% reduction.

Otto
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Old 09-13-20, 03:58 AM
  #37  
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Well, I got my new crankset installed. Now, as soon as I start my trip, I'll update on how well it works! Thanks to all!
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Old 09-13-20, 08:41 AM
  #38  
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Originally Posted by cyccommute
I know what the high is and the low is but I honestly don’t think “I’m riding in the 4th out of 9 gear so that means I’m riding in the 72” gear”. I don’t think anyone does that. We generally ride by “feel” because doing calculations on the fly is just too difficult.
I actually do this..... I know what gear inch I am riding at any time. It's always been a way for me to calculate mph while riding. Never had a speedometer and don't use strava or a gramin or any electronic device. I usually ride in a certain gear inch per various roads, and it's easy to spin 90 rpm on the crank because that is one second of time per 1.5 revolution per crank.
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Old 09-13-20, 09:33 AM
  #39  
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Originally Posted by trailangel
I actually do this..... I know what gear inch I am riding at any time. It's always been a way for me to calculate mph while riding. Never had a speedometer and don't use strava or a gramin or any electronic device. I usually ride in a certain gear inch per various roads, and it's easy to spin 90 rpm on the crank because that is one second of time per 1.5 revolution per crank.
You are way out on the tail of the distribution curve then. Most people aren’t going to keep up to 30 numbers in their head, know which gear they are in at any given moment, count the number of revolutions, count the seconds, and then plug all that data into this formula

speed = π * (diameter + (2 * tire_size)) * (chainring/cog) * cadence
And do all that on a constant basis since the speed is changing constantly based on a whole bunch of different factors while also dealing with the whatever the world throws at you while trying to balance a relatively unstable vehicle on a fairly narrow contact patch.

If you have more than one bike with more than one wheel size, you multiply the gear ratios that number of bikes. I have 8 bikes and the drivetrains are only duplicated in a couple of them. That’s at least 6 different gear ratio charts I have to keep in my head. That’s just not going to happen for me nor for the vast majority of bicyclist.
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Old 09-14-20, 07:30 AM
  #40  
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Originally Posted by cyccommute
That has nothing to do with knowing which gear you are in. I really don’t think while ratios while riding and I think about gearing far more than most people do. I know what the high is and the low is but I honestly don’t think “I’m riding in the 4th out of 9 gear so that means I’m riding in the 72” gear”. I don’t think anyone does that. We generally ride by “feel” because doing calculations on the fly is just too difficult.
You and I are a lot alike. I think in gear-inches often when I ride. Folks who build and ride fixed gears learn to do this to compare how a particular setup will work for them. And, 72" is the sweet spot for me, so most of the time, I'm mentally comparing the gear that I'm in to what I would be experiencing at 72". Gear-inches are the great normalizer between all types of [road] bike riding.
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Old 09-15-20, 10:06 AM
  #41  
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Originally Posted by Phil_gretz
You and I are a lot alike. I think in gear-inches often when I ride. Folks who build and ride fixed gears learn to do this to compare how a particular setup will work for them. And, 72" is the sweet spot for me, so most of the time, I'm mentally comparing the gear that I'm in to what I would be experiencing at 72". Gear-inches are the great normalizer between all types of [road] bike riding.
72” also makes a decent choice for single speed. One of my bikes is running 42/16 SS at 71”, which is about as close as I can get at the moment.

When I run a multi gear cog set, it will have a combination with relatively straight chain-line that produces about 71-72 gear inches and I will spend a lot of time in that gear.

The other bike has the same 42/16 but ends up at 68” on 26” tires. I wouldn’t mind getting that one up to about 71 to 72, but not exactly sure how to make it happen.

Otto
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Old 10-03-20, 06:52 PM
  #42  
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Originally Posted by livedarklions
Follow up and let us know if you feel the difference, please. I'm a high gear guy myself so it's not something I'd contemplate, but I'm always curious how stuff like this works out for people, and it might be helpful for people planning rides like yours.
Big difference. I haven't been out of the saddle since getting it. I may go slow, but I get there!

Last edited by kombiguy; 10-07-20 at 05:27 PM.
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Old 10-03-20, 07:10 PM
  #43  
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Originally Posted by trailangel
I actually do this..... I know what gear inch I am riding at any time. It's always been a way for me to calculate mph while riding. Never had a speedometer and don't use strava or a gramin or any electronic device. I usually ride in a certain gear inch per various roads, and it's easy to spin 90 rpm on the crank because that is one second of time per 1.5 revolution per crank.
+1 I always liked that 100 RPM in 67 gi is 20 mph. (67 gi - 42 X 17, a very common fix gear ratio.) Easy to go from there and calculate all sorts of stuff in my head while riding. 100 RPM - 10 pedal revs n 6 seconds. Useful back when we wore watches.
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Old 10-04-20, 10:55 AM
  #44  
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Yes 13% is a noticeable difference. That happens to be the step size on the Pinion 18 speed gearbox. Since gear inches = chainwheel to cassette ratio times wheel diameter, it is really only the ratio that is important, the wheel diameter will stay constant. But convention and comparing different setups may make it convenient.
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Old 10-05-20, 10:14 PM
  #45  
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What new crankset did you go with, OP?

I started with a 3x9 (30-39-50 & 11-32) Sora groupset on my Specialized AWOL. I swapped the rear derailleur out for a Deore 9 speed, which worked perfectly with the STI shifters (Mega 9), and put on a 12-36 cassette. That worked for awhile, until I swapped the stock drops out for Jones bars and went with full 3x9 (22-32-44 & 12-36) Deore. I'm still running that set up, but I'm a bit sick of it right now and looking to go 2x or 1x.
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Old 10-07-20, 05:26 PM
  #46  
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Originally Posted by PDKL45
What new crankset did you go with, OP?

I started with a 3x9 (30-39-50 & 11-32) Sora groupset on my Specialized AWOL. I swapped the rear derailleur out for a Deore 9 speed, which worked perfectly with the STI shifters (Mega 9), and put on a 12-36 cassette. That worked for awhile, until I swapped the stock drops out for Jones bars and went with full 3x9 (22-32-44 & 12-36) Deore. I'm still running that set up, but I'm a bit sick of it right now and looking to go 2x or 1x.
I went with a Velo Orange triple. Looks great and works as well.
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Old 10-07-20, 06:09 PM
  #47  
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Originally Posted by kombiguy
I went with a Velo Orange triple. Looks great and works as well.
26-36-48 JIS (square taper)?
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Old 10-12-20, 04:51 PM
  #48  
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Originally Posted by pdkl45
26-36-48 jis (square taper)?
48/34/24, jis
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