ss gearing, chainstay length variabllity?- Mtbr.com
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  1. #1
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    New question here. ss gearing, chainstay length variabllity?

    in essence, is there much difference in a 32x20 set up on a, say, 'medium' frame bike with a certain chainstay length versus a 'small' frame equivalent set up and, of course, short chainstay?

    and, to add to the mix, is there a reason cyclocross ss setups are so much more 'different' (42x17, 39x18 etc) than the usual mtbike setups? is it again CHAINSTAY length?

  2. #2
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    Perhaps I'm misunderstanding you... but chainstay length has nothing to do with gear ratios unless you are aiming for a "magic gear" with no chain tensioner. If you mean crank length, then this will have an effect on gain ratio (longer cranks = lower effective gear ratio at your legs).

    I would hazard a guess that cyclocross bikes have longer gearing because they run on flatter courses with higher average speeds than a MTB?

  3. #3
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    yeah, i don't even know if i understand my own question, ha! well, thinking of a Salvador Dali stretch (if you know what i mean), let's say a single speed bike that's 30 feet long (that's a long chain) versus a clown bike of 3 inches.....same cog sizes...32x20....but what's the difference where it counts? pedaling up a hill.

    hope that makes it less obtuse. a short chainstay will equate to a shorter chain. i wonder how this all translates to climbing effort/resistance (as opposed to spinning, going downhill)

    p.s. my area cyclocross courses (pennsylvania, delaware, new jersey, new york) are flat in a few area, but tons of elevation...very xc as opposed to cx....!)

  4. #4
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    There will be no difference in your example, as long as the cranks are the same length. Torque at the rear wheel, by definition, is torque at the BB x gear ratio. So, if you have the same crank length, you will be applying the same torque around the front chainring/BB. Then, if you have the same gear ratio, no matter the distance between them, you'll have the same torque at the rear wheel and pedal "resistance." Same is true if you had a 10 ft diameter chainring up front and the same gear ratio to the back chain ring, but 175 mm cranks.

  5. #5
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    well, alright....that's all i needed to know. my LBS guys, great mechanics, weren't sure, but then, there's few ss'ers around here. we're still oddballs for some reason...!

  6. #6
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    Quote Originally Posted by UBUgoat View Post
    well, alright....that's all i needed to know. my LBS guys, great mechanics, weren't sure, but then, there's few ss'ers around here. we're still oddballs for some reason...!
    No prob home skillet! Not that it really means anything, but I am a mechanical engineer, so this type of crap is running through my brain all the time. Just so you don't think I'm a troll spitting out info that doesn't mean anything. I could find you link, but if you search "rear wheel torque" or something similar, you should be able to find the equations.

  7. #7
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    Quote Originally Posted by jnbqm9 View Post
    No prob home skillet! Not that it really means anything, but I am a mechanical engineer, so this type of crap is running through my brain all the time. Just so you don't think I'm a troll spitting out info that doesn't mean anything. I could find you link, but if you search "rear wheel torque" or something similar, you should be able to find the equations.
    ha! i'm NOT a mech, but i do like to ruminate on this lovely sort of mulch but this question of chainstay length extending to mars versus extending a couple of angstroms kept haunting me...i knew a few here could give me specifics. appreciate the help!

  8. #8
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    Quote Originally Posted by jnbqm9 View Post
    There will be no difference in your example, as long as the cranks are the same length. Torque at the rear wheel, by definition, is torque at the BB x gear ratio. So, if you have the same crank length, you will be applying the same torque around the front chainring/BB. Then, if you have the same gear ratio, no matter the distance between them, you'll have the same torque at the rear wheel and pedal "resistance." Same is true if you had a 10 ft diameter chainring up front and the same gear ratio to the back chain ring, but 175 mm cranks.
    While the torque may be the same, traction is different. A SS with shorter CS of around 16-17" will often give you better traction on steep hills than a bike with say 18" or more (generalization). Even moreso with the extremes the OP used. When I ride our tandem solo, I can't get virtually any traction on the climbs because there's no weight back there!
    May the air be filled with tires!

  9. #9
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    Would those supershort CS like 14~15" be better for traction as well?

  10. #10
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    yeah, i guess the cs length becomes more of a variable when one leaves theoretical flat, uneven ground, gets out of the saddle...perhaps weight (of bike and body weight shift) is another factor at one of those extreme points when you are trying to get up and over a big rock on a steep climb....i experienced that pivot(al)-moment yesterday on my GT Peace small frame. the problem a few times was simply tire choice and wet roots. my elbow can attest to that.

    i guess beyond all this conjecture and science is really the art of climbing. technical ability, frame, tires (etc etc) all increase or decrease that 'window' of possible success whether it's a short punchy, root-n-rock strewn section of trail or just a long steady grinder.

    again, appreciate the insights. heading out to test the notions now...

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