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Please explain gearing on 26" vs. 29" to me

Discussion in 'The Shop' started by mudgirl, Jul 6, 2012.

  1. mudgirl

    mudgirl Molester of monkeys

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    I have a 26" Cannondale RZ and a 29" Giant Anthem. Why is it that I can crank up this one particular long hill I climb several times per week in my middle ring (32) and large ring in back (34) on my C-dale, but on the Anthem, I have to shift down to small ring in front (24) and next to largest ring in the back (34, still). I have been reading a bunch of threads on mtbr.com about this, but it's still not quite clear to me, other than I am getting the vague impression that it just takes more power to spin those wagon wheels. But somehow, even though I have to expend more power (?), I feel less tired on the 29er and feel like I can go faster, and this is somewhat backed up by my speeds on my gps tracks. I guess because on the 29er, each pedal stroke propels me just a bit further than the same pedal stroke would on a 26"? Even if I'm in a much lower gear combo? But maybe it's not that much lower due to the different wheel sizes? Both bikes have 170mm cranks and I'm running Stan's Arches stock wheelsets and Nobby Nics on both bikes, so there's no difference in wheels/rolling resistance or any of that stuff aside from 26" vs. 29". I'm really confused about how all this works.
     
    #1 -   Jul 6, 2012

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  2. sanjuro

    sanjuro Tube Smuggler

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    The first concept how far your wheel travels per crank revolution, i.e. how far do you go every time you pedal once around.

    With a 26x2.1 tire, it would be 1.9 meters per revolution with a 32/34 gear ratio, and with a 29x2 tire, it is 1.6 meters with a 34/34. BTW, I am using gain ratios as described by Sheldon Brown, "Development can be calculated by dividing the chainwheel size by the rear sprocket size, multiplying the result by the wheel diameter and by pi (3.1416)"

    With all other things equal, the farther you traveled per revolution, the more work you need to apply to turn the wheel. Of course, there are several factors to consider.

    For example, if your climb is fairly rough, the larger wheel should roll over the obstacles more easily. The 29er wheel though is going to be heavier, by almost a full pound. If your climb has a steady, moderate slope, the heavier 29er wheel is going to hold momentum better, but if the grade varies, the lighter 26 wheel will accelerate with less effort to accommodate the sharper grades.

    Another thing to consider is your own pedaling style. If you are a spinner, i.e. 80-100rpm, which a shorter crank arm length lends itself to, the lower gear of your 29er in the 24/34 suits your style. If you prefer to mash, 60-80rpm, the 26 in the 32/34 is harder.

    I would try using Sheldon Brown gear calculator, http://sheldonbrown.com/gears/ and print out the meters development for 26 and 29 bikes, then try several gears to see which ones you like.

    Also, here is a simple explanation with some weights.

    http://www.mbaction.com/Main/News/Shootout_Wheel_Wars_29_vs_275_vs_26_5159.aspx
     
    #2 -   Jul 6, 2012
  3. Quo Fan

    Quo Fan don't make me kick your ass

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    Read and learn about gear inches. It also takes more energy to get the larger wheels turning, but will continue turning with less effort. Still clear as mud?
     
    #3 -   Jul 6, 2012
  4. -BB-

    -BB- I broke all the rules, but somehow still became mo

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    Ok... Most simple answer you can get...

    Sanjuro explained it well, but not in as simple of terms.

    With the SAME exact gearing on a 26 and a 29, the 29r will travel FURTHER for each rotation of the cranks. So because you are traveling FURTHER with the 29r you will have to put more POWER into it.

    Or... To travel the same distance, a 26in bike will need MORE pedal/crank rotations. The same energy is required to do so, but that energy is spread out to more rotations on the 26in than on the 29in, so LESS power is required on the 26in per rotation of the cranks.
    :thumb:
     
    #4 -   Jul 6, 2012
  5. mudgirl

    mudgirl Molester of monkeys

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    Thanks, guys. It's starting to get a bit clearer. I also forgot about the fact that my 29er weighs 2lbs less than my 26er... I suppose that has a little to do with it, as well. The hill is a 3/4 mile long, somewhat steep but fairly smooth, dirt road I ride to get to the trailhead, so that probably just keeps the momentum going. QF, math makes my head hurt.... ack!!! :) But I get what you're saying: perpetual motion and all that.

    I am definitely a spinner and not a masher, coming from a roadie background. I notice that I am usually one or two rear cogs higher on my 26er than on my 29er on flat/slightly uphill terrain, but using the gear calculator, I see that's still not high enough to compensate for the distance I lose with 26" wheels vs. 29". Interesting stuff.
     
    #5 -   Jul 7, 2012