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Thread: Math quiz

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    Senior Member sgtdirt's Avatar
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    Math quiz

    is there a difference in gearing ratio between a 29er and 26" wheel. a friend has a SS 29 with a 32 chainring and a 22 freewheel. my 26" is geared with 36 chainring and 17 freewheel. he says they are about the same ratio (taking in wheel size difference).

    can this be true?

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    Quote Originally Posted by sgtdirt View Post
    is there a difference in gearing ratio between a 29er and 26" wheel. a friend has a SS 29 with a 32 chainring and a 22 freewheel. my 26" is geared with 36 chainring and 17 freewheel. he says they are about the same ratio (taking in wheel size difference).

    can this be true?
    No.

    For the 29 (700C: 56-622), one crank rotation results in 1.45 wheel rotations (32/22). Each wheel rotation results in about 230 cm travel so each crank rotation gives 1.45 * 230 = 335 cm.

    For the 26 (50-559), one crank rotation results in 2.12 wheel rotations(36/17). Each wheel rotation results in about 205 cm travel so each crank rotation gives 2.12 * 205 = 434 cm.

    434/335 = about 1.3 so they are not almost the same ratio.

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    Two-Wheeled Aficionado ColinL's Avatar
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    Quote Originally Posted by desconhecido View Post
    No.

    For the 29 (700C: 56-622), one crank rotation results in 1.45 wheel rotations (32/22). Each wheel rotation results in about 230 cm travel so each crank rotation gives 1.45 * 230 = 335 cm.

    For the 26 (50-559), one crank rotation results in 2.12 wheel rotations(36/17). Each wheel rotation results in about 205 cm travel so each crank rotation gives 2.12 * 205 = 434 cm.

    434/335 = about 1.3 so they are not almost the same ratio.
    I agree with this, except it's weird that you did the math based on a 29x2.25 and a 26x2.0. I didn't see the tire width mentioned in the OP?

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    Quote Originally Posted by ColinL View Post
    I agree with this, except it's weird that you did the math based on a 29x2.25 and a 26x2.0. I didn't see the tire width mentioned in the OP?
    You have to assume a tire circumference (or diameter or radius) to do the calculation. I have no experience with big 700C tires so I just looked up tire circumferences for these tires and it appears that 230 cm is a reasonable number to use in such a calculation. Likewise with the 559 mm tires.

    Are these not reasonable tire sizes to use to answer the question? The ratio resulting comes out very close to 29/26 -- could have just used nominal diameters and gotten the same result. Or, rim diameters (622/559).

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    Senior Member sgtdirt's Avatar
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    those tires sizes are correct. nice SWAG!

    although, i was incorrect on my friends gearing... he's running 32X20 mine is still the same (34X17).
    we are both getting ready to switch to a 650 bike. haven't decided on gearing yet.

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    Two-Wheeled Aficionado ColinL's Avatar
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    Quote Originally Posted by sgtdirt View Post
    those tires sizes are correct. nice SWAG!

    although, i was incorrect on my friends gearing... he's running 32X20 mine is still the same (34X17).
    we are both getting ready to switch to a 650 bike. haven't decided on gearing yet.
    that was lucky.

    32x17 on the 29x2.25 would be close to 36x17 on the 26x2.0 assuming average volume & diameter tires.

    wait .. you wrote 36T on your bike twice, including the first post, and now you said 34x17. if you have 34x17, his closest equivalent gear would then be 32x18.

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    Senior Member sgtdirt's Avatar
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    oops...again. yes, i am riding 36X17. so he was correct.

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    Two-Wheeled Aficionado ColinL's Avatar
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    Quote Originally Posted by sgtdirt View Post
    oops...again. yes, i am riding 36X17. so he was correct.
    not really, 32x22 is way different than 32x18.

    32x22 does sound wrong, though. That would give a top speed of something like 12-14 mph depending on how fast you can spin. (100 - 120 rpm) 32x18 sounds more realistic on a singlespeed 29er.

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    Quote Originally Posted by sgtdirt View Post
    oops...again. yes, i am riding 36X17. so he was correct.
    No. The easiest formula to compare gearing is probably gear inches. Gear inch= Wheel diameter (inches) x Chain ring Tooth# / cog tooth # Using nominal wheel diameter (26 and 29 inch in this case) is usually close enough.

    For the gear ratios you have given:
    32x20 on a 29r equals 46.4 gear-inch
    36x17 on a 26r equals 55 gear-inch
    Even if you are running 34x17, you are still not close. He is running a low gear (hill friendly) and you, if you are rrally running 36x17, a pretty tall gear.

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