27.5 Single Speed Gearing- Mtbr.com
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  1. #1
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    27.5 Single Speed Gearing

    Does anyone have suggestions on what gearing to run on a 27.5 Single Speed that will be used for steep off road terrain? What are you running and what is your terrain like?

  2. #2
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    32/20 is a pretty common starting point for 29 inch wheeled bikes, a 32/19 or 30/18t combo would be a close match to that in a 27.5 wheel size.

  3. #3
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    Look here....


    Sheldon Brown's Bicycle Gear Calculator


    ...if you want to play with the numbers, but I would agree with JB above, albeit "steep off road terrain" can be one thing to one rider and another to somebody else ...

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  4. #4
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    If you have a feeling which ratio works for you - put ratio second and calculate your gear inches and distance travelled. There is whole bunch of online calculators to do it - I just created excel spreadsheet where I track all my bikes GI and DT. Result looks like this:


    Ratio - divide chainring tooth count by cog tooth count
    GI - multiply your ratio by actual wheel diameter (actual is the key here - measure with tires on)
    Distance travelled - multiply your GI by 3.14

    Keep in mind that tire tread, rolling resistance, wheel and crank weight, frame geometry, etc will affect the feel. But for general estimate what would work - does its job very well for me.

    It only kind of let me down once - I tried to do the same for my fixed fatty and sheer traction of Bud&Lou threw all the numbers off. Should have thought of it before and ask for advice but well - live and learn

  5. #5
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    If you want to do deep dive and take into account crank length, etc - can't recommend Sheldon's site enough Sheldon Brown's Bicycle Glossary G

  6. #6
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    Ok, first off, I don't think that crank length affects gear inches. I'll enjoy being told I am wrong, of course.

    Secondly, Sheldon Brown doesn't have mtb width tires for 650b. Somewhat problematic, but I think that 700x38mm has the same diameter as a 650b x 2.2 or something, so you can just use that. Yes, I am going off a shit ass memory, and no, I'm not going to look it up and make sure I'm right.

    Lastly, aim for 45 gear inches for mtbing. I used that ratio when I was in Oregon for 4-10k climbing days. It is the same ratio I used for this ride. The ride is neither super steep or long, but it'll give you an idea of what I'm riding.

    Anywho, hope this helps mildly.

  7. #7
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    I usually don't use what Sheldon or other sites have for wheel measurement. Same tire can have slightly different profile on wider/narrower rim, etc. So I just measure actual tire on the wheel I am going to use.

    As for crank length - I never went that deep. As I mentioned - my little spreadsheet works for me.

    45 GI - interesting that you mention this number. I decided to give it a go on my Moonlander with Bud&Lou about a week ago. No snow rides on this GI yet but feels much better then ~49GI I had before.

  8. #8
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    There you guys go again with all that gear inch crap. It is a total waste of time. If he already had a bike with a gear ratio he liked, you could do a simple ratio to calculate the equivalent gear for a different wheel size. But he doesn't, so the best thing for him to do is to pick a gear, try it, and change it as needed. That's why he is asking for recommendations. So here's mine - try a 32 chainring with a 19 or 20 tooth cog. That's a good all around gear for a single speed with 27.5 tires.

  9. #9
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    Quote Originally Posted by Thor29 View Post
    There you guys go again with all that gear inch crap. It is a total waste of time. If he already had a bike with a gear ratio he liked, you could do a simple ratio to calculate the equivalent gear for a different wheel size. But he doesn't, so the best thing for him to do is to pick a gear, try it, and change it as needed. That's why he is asking for recommendations. So here's mine - try a 32 chainring with a 19 or 20 tooth cog. That's a good all around gear for a single speed with 27.5 tires.
    He did not say he does not have one. And GI help me personally a lot when choosing what ratio to run.

  10. #10
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    I'm using 28:13 on a rigid 14 pound 13 oz. 650B here in Houston. Ill keep this gearing for some of the Hill country trails.
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  11. #11
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    Running a 32x17 on flatter terrain, 18t for hillier stuff with occasional use of a 19. With your trail description you may want to start with a 20.
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  12. #12
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    Quote Originally Posted by umarth View Post
    Ok, first off, I don't think that crank length affects gear inches. I'll enjoy being told I am wrong, of course.
    You are correct - it doesn't affect gear inches. However, crank length does have an effect because of leverage, so it should not be overlooked. 5 mm difference in crank length makes more difference in torque than adding (or subtracting) one tooth from the chainring. What makes things a bit difficult is that cranks are levers attached to a bunch of other levers - our legs - so there are anatomical and personal preferences to consider. With chainrings and cogs we can easily calculate things but the outcome of such calculations regarding cranks are not directly applicable.

    The classic SS MTB gearing for 26" bikes is 32/16 and 32/18 for 29ers, so it would make most sense that 32/17 is a starting point for 650B bikes. For hilly terrain, it might be a good idea to add a tooth to the cog so 32/18 might be the ticket.

  13. #13
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    27.5 Single Speed Gearing

    Quote Originally Posted by Saul Lumikko View Post
    You are correct - it doesn't affect gear inches.
    Yup correct, crank length does not affect gear inches. But it DOES affect the gain ratio.

    And tire size affects both gear inches and gain ratio.
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  14. #14
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    If he already had a bike with a gear ratio he liked, you could do a simple ratio to calculate the equivalent gear for a different wheel size. But he doesn't, so the best thing for him to do is to pick a gear, try it, and change it as needed. That's why he is asking for recommendations. So here's mine - try a 32 chainring with a 19 or 20 tooth cog. That's a good all around gear for a single speed with 27.5 tires.
    I like how you are sure your shit smells better than others. I gave him gear inches as a number because 1) he didn't say he had a ss and 2) I have no ****ing clue what sort of crank he owns, nor which chainrings/cogs are already in his collection, if any. If he wanted to shove something on his bike, he wouldn't have posted. In future, don't be so immediately butt hurt about gear inches if it is a better tool than your proposal.

  15. #15
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    For climbs that are 9-12% sustained for 1-2mi or 5-8% for up to 7mi i will run 32x19. 32x18 if i've been riding more than once a week. Anything steeper and/or longer than that i start thinking about an easier gear while i walk. I'm not strong by any means. So, if i can run this gear in SoCal any average rider should be able to as well.

  16. #16
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    Quote Originally Posted by umarth View Post
    I like how you are sure your shit smells better than others. I gave him gear inches as a number because 1) he didn't say he had a ss and 2) I have no ****ing clue what sort of crank he owns, nor which chainrings/cogs are already in his collection, if any. If he wanted to shove something on his bike, he wouldn't have posted. In future, don't be so immediately butt hurt about gear inches if it is a better tool than your proposal.
    Yikes. You sure are cranky. Gear inches is not a very useful tool for most people no matter how angry you get about it. Most mountain bikes have tires that are fairly close in circumference for a given wheel size and small variations like going from 2.1 to 2.3 will not affect gearing very much. Do the math. (When comparing bikes with different wheel sizes, a simple ratio is enough with no need to worry about tires or cranks).

    What he asked was what gear ratio for steep terrain on a bike with 27.5 inch wheels. I recommend 32/19 or 32/20. My answer has nothing to do with gear inches, crank length or what my bowel movements smell like.

  17. #17
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    Quote Originally Posted by Thor29 View Post
    Gear inches is not a very useful tool for most people. I recommend 32/19 or 32/20. My answer has nothing to do with gear inches, crank length or what my bowel movements smell like.

    Your answer has everything to do with gear inches. Your recommendation of 32/19 is right about 45 gear inches for a 27.5 wheel which is exactly what umarth suggested.

    Quote Originally Posted by umarth View Post
    Lastly, aim for 45 gear inches for mtbing.
    I (and many others) find gear inch charts handy because if I want to run a 45 in. gear but don't have, or like a 32/19 combo I can quickly see that I could also use a 22/13, 28/17, 30/18, 34/20, 40/24, etc., etc. Also, if I wanted to use that same gearing on a 20 in. wheeled bike I could see that a 32/15 would do it. Yes, there are other ways but for me gear inches are an easy reference.

  18. #18
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    Thanks all for the responses. I typically pay attention to gear inches for BMX racing because the track I ride at usually never changes and certain ratios work very well; however, depending on strength there will also be some room to adjust. Typically for single speed MTB I try to achieve a "happy medium" that will allow me to climb most of the hills and still be able to accelerate through the more flowing sections of single track that have combinations of smaller climbs, decents, and semi-flat features. With this being said it would be hard for me to determine exactly what gearing will be appropriate by utilizing calculations since riding areas will vary and strength having its large role as well. I have decided to run a 32/20 combination to begin with (as suggested by a few) since my riding area is (Northern California SF Bay Area) Skeggs ECDM and surrounding areas which present with grades that can sometimes exceed 12%. I believe thats around 43 GI. Interestingly enough it looks like calculations are a great way to achieve a starting point! Thanks, I will add to this post when I try it out.
    Last edited by trailrider70; 12-10-2013 at 03:35 PM.

  19. #19
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    Quote Originally Posted by trailrider70 View Post
    Thanks all for the responses. I typically pay attention to gear inches for BMX racing because the track I ride at usually never changes and certain ratios work very well; however, depending on strength there will also be some room to adjust. Typically for single speed MTB I try to achieve a "happy medium" that will allow me to climb most of the hills and still be able to accelerate through the more flowing sections of single track that have combinations of smaller climbs, decents, and semi-flat features. With this being said it would be hard for me to determine exactly what gearing will be appropriate by utilizing calculations since riding areas will vary and strength having its large role as well. I have decided to run a 32/20 combination to begin with (as suggested by a few) since my riding area is (Northern California SF Bay Area) Skeggs ECDM and surrounding areas which present with grades that can sometimes exceed 12%. I believe thats around 43 GI. Interestingly enough it looks like calculations are a great way to achieve a starting point! Thanks, I will add to this post when I try it out.
    Good choice. My main riding area is Skeggs and I run a 32/21 on a 29er. The hardest climb that I ever do there is Methuselah from the bottom of North Leaf. Near the top it gets a little painful, but it is totally ridable. There are some fire road climbs that are pure evil at Skeggs (like the Fir trail from Methuselah to Resolution) but there's really no good reason to ever ride those trails and they suck on a geared bike too.

  20. #20
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    Hope it works out great for you. Remember, if you have to walk the first time out, you will still have to walk the second time out.

  21. #21
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    Thanks Thor29, I have a 29er ss as well which I run a 32/22 on. I would be running a 21 but it was difficult to find a 21 cog so I went with a 22. I can climb well with it. Im looking forward to the 27.5.

  22. #22
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    Quote Originally Posted by umarth View Post
    Hope it works out great for you. Remember, if you have to walk the first time out, you will still have to walk the second time out.
    Reasoning? There is a thing called progression. What about the 3rd?.....

  23. #23
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    Wow 45 gear inches sure is a tall gear, don't think I could push that one up the hills here in SoCal, but more power to you. I am running more like 39 inches and I stand most of the way up. Of course my SS is 24 years old and probably weighs a bit more than the newer bikes....

    oh and 26 inch wheels....


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  24. #24
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    Quote Originally Posted by lewisfoto View Post
    Wow 45 gear inches sure is a tall gear, don't think I could push that one up the hills here in SoCal, but more power to you. I am running more like 39 inches and I stand most of the way up. Of course my SS is 24 years old and probably weighs a bit more than the newer bikes....

    oh and 26 inch wheels....


    Steven
    Nah. I usually run 45-46 gear inches here in SoCal. I do walk once in a while (i'm not too proud to say that), but, i was able to complete the Counting Coup (44mi/8000+ft) on it. And, i'm not strong by any definition.

  25. #25
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    It seems that the 32/20 combination works very well where I ride. I suspected this mainly because I know what worked and didn't work with my 26 and 29 single speed. A 32/19 would also be good but for now 32/20 is the ticket. The 32/20 turns out to be about 43 gear inches with the 650 B wheel (at least thats what I came up with, could be wrong). From all of the input that I have read in this thread in addition to my own experience, I would say that are many variables to consider when choosing the most desirable gearing (terrain, strength, stamina, skill level, and riding style to name a few).

  26. #26
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    Quote Originally Posted by jetboy23 View Post
    Nah. I usually run 45-46 gear inches here in SoCal. I do walk once in a while (i'm not too proud to say that), but, i was able to complete the Counting Coup (44mi/8000+ft) on it. And, i'm not strong by any definition.

    Ooops my mistake,

    I thought I was running a 30 x 20, turns out it is 32x18, or 45.8 gear inches. In my own defense It has been almost ten years since I built the thing. He tries to hide....


    Steven

  27. #27
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    Quote Originally Posted by lewisfoto View Post
    Ooops my mistake,

    I thought I was running a 30 x 20, turns out it is 32x18, or 45.8 gear inches. In my own defense It has been almost ten years since I built the thing. He tries to hide....


    Steven
    10 years and no replacing drivetrain to know what setup is? Don't ride much....

  28. #28
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    Quote Originally Posted by JMac47 View Post
    10 years and no replacing drivetrain to know what setup is? Don't ride much....

    Good call. More correctly, haven't ridden it much lately. About 5 year ago I started getting interested in vintage MTB and ride those bikes now. (Although my single speed is 20+ years-old as well). Still it is kind of embarrassing that I did not know the gearing, oh well

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