Benefits of larger cogs/chainrings? (34x21 vs 32x20)- Mtbr.com
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  1. #1
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    Benefits of larger cogs/chainrings? (34x21 vs 32x20)

    Is there any real difference between the two ratios above on a 29er with 175 crank arms. I know they're not exact but for the sake of argument lets assume the ratios are identical... is there any difference between the two besides in theory longer wear life with the bigger cogs? would the 32t be more 'spinny' than a 34?

  2. #2
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    I know the correct answer, but you're going to get lots of opinions on this question, many of them from people who seem to ignore the scientific facts (they must be Republicans).

    Going to get a bowl of popcorn now...
    It never gets easier, you just go faster. -Greg LeMond
    I'm not as fast as I think I am. -JeffL

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    LOL!!! Better than Dems..... I got my corn popped and watching myself. I ran 34x20 on my 650B. Then this winter I built a SS specific frame 650B bike and it has a 32 chainring. I started with a 20 rear cog and am not sure I like it? So I got an 18 tooth cog on order. My LBS single speeder told me that 32x18 is a pinch higher than 34x20 but not much. Close enough that they are almost the same. So gain 2 teeth and drop 2 teeth and not exactly the same...... You would think they would be but I guess not. That give you a little to think about??

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    iowamtb - 32:18 is not "almost the same" as 34:20. It is almost the same as 34:19. The rough rule is add or remove two teeth on the front and 1 tooth on the rear, to keep the same ratio. The closer you get to a 2:1 relationship (e.g. a gear of 32:16) the closer is the approximation for this "2 tooth 1 tooth" rule.

    But the original question was, for two equivalent gear ratios (say, 32:16 and 34:17), is the smaller diameter chainring going to be more "spinny"? For the same gear ratio. People don't agree on the answer to this. Really. I have my popcorn bowl...
    It never gets easier, you just go faster. -Greg LeMond
    I'm not as fast as I think I am. -JeffL

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    So you are saying that if you have 2 650B bikes, one with 34x20 and one with 32x18, the 32x18 is going to be considerably higher geared? I had 32x20 and just ordered a 18 tooth rear cog and have no idea yet if I will like it or not.

  6. #6
    Stateline Falls, Watauga
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    Yes, 32:18 is a steeper gear than 34:20. Basically, 1 cog tooth harder in comparison. Be patient with the new 32:18 gear on your 650b - it's still easier than 32:18 on a 29er. You're in Iowa - how hard can the climbing be?
    It never gets easier, you just go faster. -Greg LeMond
    I'm not as fast as I think I am. -JeffL

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    I went from 34/18 to a 32/16. According to any calculator it is indeed a bit harder. However, I'll be interested on the facts that may be given to the OP. What is the right answer?

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    Quote Originally Posted by JeffL View Post
    Yes, 32:18 is a steeper gear than 34:20. Basically, 1 cog tooth harder in comparison. Be patient with the new 32:18 gear on your 650b - it's still easier than 32:18 on a 29er. You're in Iowa - how hard can the climbing be?
    That's what I asked Blake at the bike shop. He said for all the trails in the midwest he has found 32x 18 to be ideal for him. His shop is in Missouri an hour and half south of me. Terrain is rocker but overall the grades are similar. 45 minutes to the west of me is the loess hills and they are very steep and hilly. A lot of the trails in that area, around the iowa and nebraska border are built in these hills.

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    Quote Originally Posted by JeffL View Post
    But the original question was, for two equivalent gear ratios (say, 32:16 and 34:17), is the smaller diameter chainring going to be more "spinny"? For the same gear ratio. People don't agree on the answer to this. Really. I have my popcorn bowl...
    I love playing around with my ratios. Popcorn is also nice. In terms of feel and performance on the bike any 2.0 ratio will feel the same. You could ride 28/14 or 36/18 and it would feel the same. The difference comes when you deal with chainwear, dirt, wear on the cog or ring, etc. In all cases of wear and dirt the 36/18 will out perform the 28/14 setup. For the most part this is an issue when chain stretch starts causing the chain to damage and eventually pop off the cog but provided you replace the chain, ring, and cog when needed the performance is the same.

    Another factor is different setups allow different effective chainstay lengths for people using sliders. I like to keep my bike in a pretty narrow range and sometimes play around with the chainstay length implications of gearing just to be a geek.

  10. #10
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    The higher toothed pair will require more chain and is heavier but it will wear slower. The smaller toothed pair is lighter and stuff will wear faster.

  11. #11
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    Quote Originally Posted by JeffL View Post
    I know the correct answer, but you're going to get lots of opinions on this question, many of them from people who seem to ignore the scientific facts (they must be Republicans).

    Going to get a bowl of popcorn now...
    that was funny!
    Rigid SS 29er
    SS 29+
    Fat Lefty
    SS cyclocross
    Full Sus 29er (Yuck)

    Stop asking how much it weighs and just go ride it.

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    I used an older ring and cog over the winter months (not that it mattered up here as we didn't get much riding in with all the blizzards and the like) that was a 34/21 setup. My normal setup is a 32x20 (mainly for ground clearance under the ring). I didn't notice anything different when riding.

  13. #13
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    Some of you guys need a little help with simple math. Here's how you do it - divide the number of teeth on the chainring by the number of teeth on the cog and compare that number:

    34/21= 1.62
    32/20 = 1.60

    So they really are pretty much the same, only a 1% difference that you probably couldn't feel in a blind test.

    And yes, theoretically the bigger chainring and cog will last longer but will also be heavier. Not sure why this would induce a popcorn crunching slugfest. Is it really that controversial? Seems straightforward to me. No need to invoke dumbocrats or retardocans.

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    Thor29, some people claim that the bigger ring/cog combo is more efficient and pedals easier. E.g., you would need less energy to push a 36x18 compared to a 32x16 despite the same 2:1 ratio. This is where the debate lies. I agree, 34x21 is the same as 32x20 for all practical purposes.

    Personally, I find poor chain line is a bigger impact on efficiency than the size of the ring and cog. Plus, I like lighter weight and more ground clearance.

  15. #15
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    Quote Originally Posted by Thor29 View Post
    And yes, theoretically the bigger chainring and cog will last longer but will also be heavier. Not sure why this would induce a popcorn crunching slugfest. Is it really that controversial? Seems straightforward to me. No need to invoke dumbocrats or retardocans.

    That's exactly what I was thinking


    Quote Originally Posted by solo-x View Post
    Thor29, some people claim that the bigger ring/cog combo is more efficient and pedals easier.

    There's no debate as to whether or not bigger rings are more efficient, they are, but unless you're using really small cogs the difference is inconsequential.

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