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  1. #1
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    Trying to understand gear ratios for a 29er. Please help!

    I must preface this by saying that I went to public school and studied art! Math ain't my game!

    I've done a lot of searching in the threads, but am still seeking an easy to understand answer to my question......

    How do I determine the best chainring/cog combo for my type of riding?
    29er with a current set up of 34t/18t.


    I'm more concerned with climbing. In the time that I have had my SS, I've yet to come across a trail that I was spinning on the downhill sections, but have gotten my butt kicked on some long climbs.
    I'm good with getting out of the saddle on hills, but just want to make it a little easier on some races I have coming up.

    I've read a lot about trying different configurations, but I'm a lowly fireman with kids and don't have a lot of extra $$ to "try" things out.

    Just hoping someone can point me in the right direction as I'm pretty lost when it comes to the calculators and such.

    Thanks!
    Brian
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  2. #2
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    More information is required re your terrain. Climbing doesn't indicate a hell of a lot. How much elevation change? One big climb or many short pitches?

    If you are wanting an easier gear for a race with a fair amount of climbing, then consider a 34 x 20/21. You will definitely spin out on the flats/downhills, but you should be able to get up climbs a lot easier than with a 34 x 18.

    FYI
    34 x 18 on a 29er gives a gear ratio of 1.89 (with a cadence of 90, this will put you at 23.4 kph)
    34 x 21 on a 29er gives a gear ratio of 1.62 (with a cadence of 90, this will put you at 20.0 kph)

    If that makes things easier to understand.

  3. #3
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    Go here:

    Sheldon Brown's Bicycle Gear Calculator


    This chart might be helpful to determine if you are geared too high.


  4. #4
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    Short pitches, lots of up and down!
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  5. #5
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    Quote Originally Posted by michaelscott View Post

    Been there, but it doesn't help if I don't have a clue what the numbers correlate to.

    Eg, the reason for this thread.
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  6. #6
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    Front chainring (#teeth) X wheel diameter (in inches)/ rear cog (#teeth) = gear ratio (in inches) or distance traveled (in inches) for every revolution of the cranks. Ride a geared bike on your local route(s) and see what gear ratios you use when climbing and then adapt that to your singlespeed setup.

  7. #7
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    The ratios that work well for 29er single speed range from 1.5 to around 2.5. You are in the upper end and should consider going down. I personally think 1.6 is a good starting point for most people. All you need is a pocket calculator.

    If you want to keep your chainring the math is simply 34/1.6=21.25 (round down to 21). The 34/21 is about the same as the ratio you would get with a 32/20 or a 36/22.

    In theory you could also use a 30/18 for a similar ratio but little rings suck and most cranksets don't allow you to run anything smaller then 32.

    My advice is the same as Shalom. Give 34/21 a try and see how it works out.
    Mark Farnsworth, Raleigh, NC
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  8. #8
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    Perfect! Thanks everyone for the info and dumbing it down for me!!
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  9. #9
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    Most stock SS comes with 32/18. I found that's a good starting point - fast enough on flats, quite challenging on some climb but doable. I recently went 32/20, and it made the climb whole lot easier. There is this one long stretch of climb in my neighborhood full of rocks and roots, and with 32/20, I climbed up no problem (with 32/18, I would end up walking 2 out of 10 times, and would require some rest to catch my breath the other 8 times).

    So, ya, try 21t cog.
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  10. #10
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    Quote Originally Posted by FirefighterMTN View Post
    29er with a current set up of 34t/18t.
    I read that and immediately thought "a bit tall", but of course, I don't have your legs and I don't ride where you ride.

    As a guide, my rides are usually around 3hrs length, covering 50km or thereabouts on a mix of firetrail and singletrack. Some long sustained climbs, like 6km with a gain of 350m and some short sharp ones like 1 in 4 for about 50m. Also some steeper sustained ones like the stupid road climb at the end of my ride which gains 80m over 800m.

    I have 2 26" singlespeeds, one being 33:16 and the other 33:17.

    My 29er runs 32:19.

    Yeah, that's a lot of numbers for one email, but as a comparison it might help you out. I'm an average rider, exceedingly average. Just your regular worker drone with a family who gets out when he can to ride and pushes himself when he does to fit as many kms into each ride.

    Also, sorry for the metric measurements, but my people are an evolved race.

    Grumps

  11. #11
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    21t it is!! Thanks for these replies! I think I'm finally getting it!

    Grumps--Short and squatty, so anything "tall" is generally not in my vocabulary!!
    Thanks!!
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  12. #12
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    One last dumb question........

    If a 1.6 ratio is the goal, does it matter if it is achieved with changing the chainring or changing the cog?

    Ie, is there any difference in a drivetrain that had the chainring switched to achieve a ratio rather than the cog to do the same other than ground clearance with the smaller chainring?
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  13. #13
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    Not really. But it's easier to change the cog (remove wheel, unscrew lockring, replace) than changing chainring (remove crankset, unscrew chainring bolts, replace).
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  14. #14
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    Just to add to the confusion out there - I am a hardcore, long time single speeder and I run 32/21 on my 29er. That's a lower (easier gear) than a 34/21. I can outrun most geared riders I encounter with the rare exception that we hit a long flat section where I spin out. On the downhills I just don't use as much brake and let gravity do it's thing. So I would actually recommend a 22 tooth cog which would give you about the same gearing. Regardless, a bigger cog will definitely help.

    And by the way, all that gear inches talk is crap. If you are just looking at one bike it makes no sense to talk gear inches since your wheel and tire combo are not changing. Just compare gear ratios instead.

  15. #15
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    Quote Originally Posted by FirefighterMTN View Post
    One last dumb question........

    If a 1.6 ratio is the goal, does it matter if it is achieved with changing the chainring or changing the cog?

    Ie, is there any difference in a drivetrain that had the chainring switched to achieve a ratio rather than the cog to do the same other than ground clearance with the smaller chainring?
    Rings smaller than 32 tooth are a bad idea in terms of wear, drive train, and concentricity. In theory you could run a 22 front ring with a 12 tooth rear cog but it would ride like sh*t. A 38/22 drivetrain will run much better than 22/12.

    For the most part clearing logs is about technique. Log clearance has never been an issue for me with large rings. I am not great on logs but I am good enough and going down to smaller front ring would not help.

    Ideally you would want to use the largest chainring that will fit on your frame and then just adjust the cog size to reach the ratio you need. For many frames this would be a 38 or 40 tooth ring and something like a 22 or 24 tooth cog. There is no need to go any bigger than around 40 tooth.

    Since you already own a 34 tooth ring just stick with that ring and replace the 18 tooth cog. You don't need to replace both very often. When your drivetrain wears out, you can migrate to larger rings and cogs.

    The other idea is to go with the 22 tooth cog for now and if it turns out to be too low you can later upsize the chainring. Cogs and rings with even numbers also wear better. Many manufacturers do not provide single speed cogs larger than 22 tooth. Running 22 tooth cog makes sense because it's a size that is easy to acquire. For me this means running 38/22 as my go to gear with 36 tooth ring for big days in the mountains.

    If you have a four bolt cranks and you run two sized chainring bolts it is pretty quick to swap rings. You just need a 5mm and 6mm allen wrench to do the swap and it only takes a few minutes. You don't need to remove the cranks from the bicycle unless you have the silly spiderless rings.
    Mark Farnsworth, Raleigh, NC
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  16. #16
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    Awesome info! Thanks so much!!!!

    Checked out your site, too!

    Very nice!!!
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  17. #17
    NedwannaB
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    I agree with febikes(and a couple others) about keeping the 34 and bumping up a tooth or 2 on the rr cog. And the "general rule" is 2 more teeth on rr cog for a 29r would ~equal what you would/would have run on a 26r. I.e. 32/16 is 2:1 on a 26r, 32/18 for the 29r. Explore up/down from there for your terrain/riding style accordingly. Obviously different type/brand of tire sizes will net a little different overall roll out/diameter but you get my point. If you haven't jumped from small to big wheel SS then it's a mute point. You mentioned up and punchy climbs on your trails, even tho you are usually out of the seat when climbing another thing to consider for gear choice is if you mash or rev spin.

    I was running a 32/19 setup on my 29r and just switched out to a different set of cranks with a 34t (from previous 26"/650B setup) front ring thinking I'd bump to a 20t rr cog later. But so far the 2:1 is working better than with the 32. The bigger ring/longer chain seems to smooth things out. I'm a light/small in stature rider 5'-7"/135#s so climbing is somewhat in my favor and I can work with a bit taller gear.
    Wait,who did he tell you that?....

  18. #18
    rho
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    I gotta say i'm a big fan of 34/21 on a 29er. Or even 34/20 on days where a bit more speed is desired.

  19. #19
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    Once again, thanks for the replies! Truly have a good feel for what I need!!!
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  20. #20
    CB2
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    If you're riding an 18 now, and it's mostly okay, I'd only go to a 20. I like to look at in gear inches:

    34 / 18 x 29 = 54.77"
    34 / 21 x 29 = 46.95"
    34 /20 x 29 = 49.3"

  21. #21
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    Thanks!

    What would the gear inches total relate to?

    Is there an optimal # for hills or flats?
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  22. #22
    NedwannaB
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    Quote Originally Posted by FirefighterMTN View Post
    Thanks!

    What would the gear inches total relate to?

    Is there an optimal # for hills or flats?
    1-Wheel rotation.

    2-Uhm, you on your trails are the optimal.
    Wait,who did he tell you that?....

  23. #23
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    Quote Originally Posted by CB2 View Post
    If you're riding an 18 now, and it's mostly okay, I'd only go to a 20. I like to look at in gear inches:

    34 / 18 x 29 = 54.77"
    34 / 21 x 29 = 46.95"
    34 /20 x 29 = 49.3"
    This is exactly why you don't need to use gear inches - notice that in every equation there is a 29. Since that doesn't change, it doesn't help at all with the comparison. Instead, just do the ratio and compare. Bigger number means taller (harder) gearing, smaller number means lower (easier) gearing.

  24. #24
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    New chain data for new ratio

    Hope you don't mind me jumpin in on this thread.

    Carried my 29er Civilian Luddite with me from Afghanistan to China, where I can now ride year round on the South China Sea door step. Roads are mostly flattish with several 100' inclines, but not many really steep hills, so I was thinking about changin my gears. Currently comes with a 32/18. A 36/18 would make it a 2.0 ratio from a 1.77. Might try that one first.

    My question to all you old timer pros is how do I know how many links I will need in the new chain and secondly exactly what do I need to buy to break my old chain and to assemble my new chain. Thirdly what size chain should I be ordering. (width)

    Thanks boys and girls
    Attached Thumbnails Attached Thumbnails Trying to understand gear ratios for a 29er.  Please help!-talibanbiker.jpg  

    Trying to understand gear ratios for a 29er.  Please help!-2013-03-17-17.16.09.jpg  

    Trying to understand gear ratios for a 29er.  Please help!-2013-03-18-12.23.57.jpg  


  25. #25
    NedwannaB
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    Actually with 32/18 you are at 2:1 29r gearing currently.
    You could switch up gearing easily/economically by just changing your rr cog on the hub unless its a thread-on type integral bearing freewheel (can't tell from pix). And with sliding dropouts you "may" be able to adjust tension without modifying chain. If not a simple chain tool or bicycle multi-tool with chain tool is all you need. You can also get a Power Link (master link) if you do need to shorten. If you can't identify what type chain then chain tool way to go. Line up drift pin on tool to the pin on chain link and crank away until pin pops out. Repeat to next link(s) needed to get correct length but only pop pin out enough to release the un-needed link(s). You do have some play on where you want the rr wheel positioned fore/aft in back which can be beneficial depending on how you want bike to ride. Closer to pedals(shorter wheelbase) makes bike more lively to raise front end (manual/wheel) and more responsive to turns. Farther out in sliders(longer wheelbase) little more stable.
    If you end up getting new front ring and chain, typically you can get an SS specific (bmx) 8 or 9 SPD chains.
    Last edited by JMac47; 09-08-2013 at 10:20 AM. Reason: typo
    Wait,who did he tell you that?....

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