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Gear inch question

Old 09-04-20, 07:41 AM
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kombiguy
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Gear inch question

My bike currently has a 50-39-30 front ring set, ans an 11-36 sprocket set. Using those numbers, I get a gear inch low of 22.6
If I were to change to a 26 tooth low gear on the front, that would result in a gear inch of 18.1
Is that really a significant difference? Or perhaps my question might be rephrased as, how low a tooth count would I need on a front ring to make a significant difference?
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Old 09-04-20, 08:09 AM
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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 calclations.

I don't know if that's significant or not. It's about a 13% reduction in gear ratio and gear inches.

Glenn
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Old 09-04-20, 09:39 AM
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gear ratio x wheel diameter in inches = gear inches.
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Old 09-04-20, 09:53 AM
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Originally Posted by kombiguy View Post
My bike currently has a 50-39-30 front ring set, ans an 11-36 sprocket set. Using those numbers, I get a gear inch low of 22.6
If I were to change to a 26 tooth low gear on the front, that would result in a gear inch of 18.1
Is that really a significant difference? Or perhaps my question might be rephrased as, how low a tooth count would I need on a front ring to make a significant difference?
Itís a ratio of 13/15 or a reduction of 13.3%. About like one gear shift on a 7-speed cassette.

Otto
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Old 09-04-20, 12:26 PM
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Originally Posted by kombiguy View Post
My bike currently has a 50-39-30 front ring set, ans an 11-36 sprocket set. Using those numbers, I get a gear inch low of 22.6
If I were to change to a 26 tooth low gear on the front, that would result in a gear inch of 18.1
Is that really a significant difference? Or perhaps my question might be rephrased as, how low a tooth count would I need on a front ring to make a significant difference?
You can play with different gear combinations and see what kind of difference they make using this tool:

Bicycle Gear Calculator

That page will show you how each combination moves when you change chainrings. But it seems your question is more about how the different gear combinations will feel. I think a difference of 4 gear inches will feel significantly easier to pedal, but you might only need it for very steep climbs or for loaded climbing on long grades.

You're also going to have less redundancy in your gears if you only change the 30t to a 26t. On your current setup, your middle ring only has 3 gears above your small ring. If you reduce the size of your small ring, you will have 4 gears above your small ring when you are in the middle ring. That said, you have a lot of redundant gears already in your drivetrain. It probably makes sense to space out your chainrings more to get the most out of your cassette. I would consider a 24t small ring, a 36t middle ring, and keep the 50t big ring. Here's a link to what that would look like versus your current setup:

Bicycle Gear Calculator

Good luck!

Edit: I just realized my suggested setup require a derailleur capacity of 51. That might be hard to setup.

Last edited by BoraxKid; 09-04-20 at 12:31 PM.
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Old 09-04-20, 12:53 PM
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One thing to remember is that each derailleur was designed to have a range of gears it will traverse. If you know the brand and model number of the derailleur you can usually find what that range is. It is the # of teeth on the large chainring minus the # of teeth on the small chainring. If you exceed this value with your new setup, the derailleur will not work correctly.
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Old 09-04-20, 01:04 PM
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It's the same difference as dropping your highest gear, the 11 to a 13. Quite real. (Or moving your 36 rear up to a 42.) SO, yes, the 30 to 26 front IS significant.

Ben
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Old 09-04-20, 01:14 PM
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Not having a lot of redundancy is good unless you can ride in the 39t most of the time with the 50t on flats and downhill and 30t/26t for hard climbing.

With a triple redundancy isnít that important to me as the middle is more all purpose and inner and outer only use part of the cassette. It depends on how much chainring shifting you like to do to find the optimum ratio.

John
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Old 09-04-20, 02:27 PM
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13% is significant, and you will notice it!
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Old 09-04-20, 02:30 PM
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Just keep the 30 ring and remove 4 teeth from it. That's how it works, right?
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Old 09-04-20, 02:49 PM
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Originally Posted by livedarklions View Post
Just keep the 30 ring and remove 4 teeth from it. That's how it works, right?
No, that is not how that works at all. To make a difference in the gear size, you have to actually change the diameter of the chain ring, which will necessitate changing the number of teeth to maintain compatibility with the chain. Since chain links and chain ring teeth are pretty much standardized, most people simply refer to the chain rings by the number of teeth on them, and it is assumed that there are no missing teeth on the chain ring.

There some skip-tooth chain rings out there, as well as some non-standard chains and cogs, but generally speaking, chain ring teeth and chains conform to an expected standard size.
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Old 09-04-20, 03:11 PM
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While @BoraxKid's suggestion might be physically impossible, it has merit as an idea.

As many others have said, yes, dropping to a 26 up front will be a noticeable change. I would call it a significant a opposed to an incremental change.

I agree with @70sSanO that redundancy is not a terrible thing, and in fact is inevitable in almost any multi-ring set-up unless a person designs specifically to eliminate at, beyond other considerations. The keys for me are useable ranges---progressions of gears on the cassette where I can ride without needing a front shift. Nothing is worse than being halfway up a tough climb and realizing you need to drop to the lowest ring---the momentum loss isn't fatal but it can be painful. Similarly I don't like to be riding rollers and constantly shifting between rings to get both good climbing and good descending speed.

Going to the smaller chain ring will be noticeable, and if you want, look at getting a smaller middle ring but do pay attention to derailleur capacity (which you can find online I am sure.)

Keep us posted.
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Old 09-04-20, 03:17 PM
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Originally Posted by Maelochs View Post
While @BoraxKid's suggestion might be physically impossible, it has merit as an idea.
Well thank you, for that bit of recognition! I'm glad to see civility isn't lost on this board.

And not to be too pedantic, but my idea is only impossible with stock parts (the largest capacity rear derailleur I could find was 47t). I bet some bored engineer or mechanic could find a way to tease an extra 4t of capacity from a long-cage derailleur.
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Old 09-04-20, 03:23 PM
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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.
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Old 09-04-20, 03:26 PM
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Originally Posted by kombiguy View Post
My bike currently has a 50-39-30 front ring set, ans an 11-36 sprocket set. Using those numbers, I get a gear inch low of 22.6
If I were to change to a 26 tooth low gear on the front, that would result in a gear inch of 18.1
Is that really a significant difference? Or perhaps my question might be rephrased as, how low a tooth count would I need on a front ring to make a significant difference?
So what is the problem that you are attempting to solve? Are you planning heavy haul touring? Climbing the Himalayas? Inquiring minds wants to know.
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Old 09-04-20, 03:37 PM
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Originally Posted by kombiguy View Post
My bike currently has a 50-39-30 front ring set, ans an 11-36 sprocket set. Using those numbers, I get a gear inch low of 22.6
If I were to change to a 26 tooth low gear on the front, that would result in a gear inch of 18.1
Is that really a significant difference? Or perhaps my question might be rephrased as, how low a tooth count would I need on a front ring to make a significant difference?
Hereís what your gearing would look like with a 26 tooth cog. Assuming a 700 C wheel, youíd drop from a 23Ē low to a 20Ē. It might not seem like much but it is lower. If you can go to a 24 tooth inner, you would get a 18Ē gear.

This gear calculator is very powerful in that it allows you to easily compare two systems. You can even change the cassette between systems if you like. Iíd suggest playing around with the calculator with other cranks. For example a 48/38/22 would give a very good low and itís not that much different from what you have now. I doubt youíd have to do much to make the derailer work. Iím running a 46/36/20 crank on one of my bikes with a road front derailer. I donít have any shifting issues.
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Old 09-04-20, 04:59 PM
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Originally Posted by CAT7RDR View Post
So what is the problem that you are attempting to solve? Are you planning heavy haul touring? Climbing the Himalayas? Inquiring minds wants to know.
Cycling from St. Augustine to LA via Birmingham, Branson, OKC, and Williams Az.
One fat guy on a Surly LHT, self sufficient camping mostly, with a once a week motel.

BTW, 62 years old.
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Old 09-04-20, 05:01 PM
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The problem is that it appears to get that small ring on the Surly, I'd have to change the bottom bracket and cranks. It may have to wait until I'm done with my trip. Thanks to all who gave me the low down!
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Old 09-04-20, 05:26 PM
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Originally Posted by BoraxKid View Post
No, that is not how that works at all. To make a difference in the gear size, you have to actually change the diameter of the chain ring, which will necessitate changing the number of teeth to maintain compatibility with the chain. Since chain links and chain ring teeth are pretty much standardized, most people simply refer to the chain rings by the number of teeth on them, and it is assumed that there are no missing teeth on the chain ring.

There some skip-tooth chain rings out there, as well as some non-standard chains and cogs, but generally speaking, chain ring teeth and chains conform to an expected standard size.
Wooooshhhh!!
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Old 09-04-20, 07:38 PM
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Originally Posted by kombiguy View Post
Cycling from St. Augustine to LA via Birmingham, Branson, OKC, and Williams Az.
One fat guy on a Surly LHT, self sufficient camping mostly, with a once a week motel.

BTW, 62 years old.
You are going to want that 26 tooth chain ring around Birmingham and Branson. Very steep hills in those areas. 18 gear inches is what you want for all that weight. I can speak from experience. Good luck. The difference between 21 g.i. and 18 g.i. is significant.
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Old 09-05-20, 06:10 AM
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Originally Posted by BoraxKid View Post
No, that is not how that works at all. To make a difference in the gear size, you have to actually change the diameter of the chain ring, which will necessitate changing the number of teeth to maintain compatibility with the chain. Since chain links and chain ring teeth are pretty much standardized, most people simply refer to the chain rings by the number of teeth on them, and it is assumed that there are no missing teeth on the chain ring.

There some skip-tooth chain rings out there, as well as some non-standard chains and cogs, but generally speaking, chain ring teeth and chains conform to an expected standard size.
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Old 09-05-20, 06:38 AM
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How are you guys calculating gear inches without knowing the tire size?

Simple answer is ride your bike fully loaded. if you ever find yourself trying to shift into a lower gear that isn't there, than consider swapping out rings.
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Old 09-05-20, 02:28 PM
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Originally Posted by Pop N Wood View Post
How are you guys calculating gear inches without knowing the tire size?

Simple answer is ride your bike fully loaded. if you ever find yourself trying to shift into a lower gear that isn't there, than consider swapping out rings.
By making some assumptions. A 50/39/30 crank isnít usually found on mountain bikes so that means 700c tires. Touring is usually 32mm to 37mm. If you did the calculations with a 23mm tire, the gearing isnít going to be radically different. The gear calculator makes it easy to change the wheel parameters.
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Old 09-06-20, 09:20 AM
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And by my guess different people are making different assumptions leading to arguments about different numbers

Gear inches can be hard to translate into real world. One way around that is to keep an excel spreadsheet with all of your bikes in it. Most people know what gears they like to ride on each of their bikes and which ones they never use. When it comes time to upgrade gearing the spread sheet helps you select the best set of rings.
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Old 09-06-20, 09:39 AM
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Originally Posted by Pop N Wood View Post
And by my guess different people are making different assumptions leading to arguments about different numbers
Itís okay to make educated guesses. The size of the wheel has an impact but it easily fixed.

Arguments about gearing are also usually of the ďno one needs gears that lowĒ nature. Iím a firm believer in having the lowest gears I can engineer with the equipment I have. If that means modifying the crank or adding a derailer extender, Iím willing to go that far. Iíve never found myself saying ďmy gear is too lowĒ.

Gear inches can be hard to translate into real world. One way around that is to keep an excel spreadsheet with all of your bikes in it. Most people know what gears they like to ride on each of their bikes and which ones they never use. When it comes time to upgrade gearing the spread sheet helps you select the best set of rings.
Most people donít know what gear they ride in. I doubt most people know which cog number they are riding in most of the time, much less what the ratio is. I think of gearing and gear ranges a lot but while Iím riding I canít tell you which specific gear Iím riding at any particular point. The closest I can get is along the lines of ďbig chainring and something in the middle of the cassetteĒ because thatís what I use most of the time.

One of the things I do is to provide a link to gear-calculator.com whenever I post about gearing on here so that people can look at what their gearing is. If I made an incorrect assumption, itís easily corrected in the calculator. But, honestly, I only use the gear-calculator for planning gearing. I donít use it while I ride nor is there any need to do so.
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Last edited by cyccommute; 09-06-20 at 09:43 AM.
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