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Thread: Gearing

  1. #1
    Senior Member George's Avatar
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    Gearing

    I don't know if I'm having a senior moment or not. I'm trying to figure speed out in gear inches. I'm in the middle ring 36 and the 18 on the cog with a 27" wheel, that gives me 48.84422. How or where, do I go with this figure to figure my speed.
    George

  2. #2
    feros ferio John E's Avatar
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    Actually, 36/18 = 2.0, and 2.0 * 27" = 54", meaning that every time you turn the cranks one full revolution, you advance 54 * pi inches along the pavement. If you turn the cranks at 90 RPM, that's 1.5 revolutions per second, and you are traveling 1.5 * 54 * pi = 81 * pi inches per second, or about 21 ft/sec = 14.5 mph.

    speed (mph) = (cadence in rev./min.) * (1 min / 60 sec) * (60 mph / 88 ft/sec) * (gear inches = wheel diameter * chainring teeth / cog teeth) * pi * (ft/12 in)

    Simplifying, we get: speed (MPH) = cadence (RPM) * gearing (gear-inches) * 0.003.

    Thus, if you can spin the cranks at 120 RPM in a 96-inch gear (50/14 on a road bike, 48/13 on a mountain bike), you can hit a speed of just over 34 mph. An easy calibration point is 100 RPM in a 100-inch gear = 30mph.
    Last edited by John E; 09-18-07 at 08:24 PM.
    "Early to bed, early to rise. Work like hell, and advertise." -- George Stahlman
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  3. #3
    Senior Member SaiKaiTai's Avatar
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    That's about as nice an explanation of how gear inches work as I've ever seen.
    And I've looked a LOT
    '13 Felt Z3 - '08 Jamis Aurora Elite - ('07 Giant OCR C2)

  4. #4
    Senior Member George's Avatar
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    Thanks John, that's great, I was wondering what was/is happening to my average. It must be slowing down for turns and stop signs, but I sure don't average that much and I can't understand why.
    George

  5. #5
    Senior Member George's Avatar
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    One more question, John, what do you mean by pi? thanks.
    George

  6. #6
    feros ferio John E's Avatar
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    Quote Originally Posted by George View Post
    One more question, John, what do you mean by pi? thanks.
    Pi, the transcendental ratio of the circumference of a circle to its diameter, is approximately 3.1416. To understand pie (with the silent "e"), you need to read elsewhere in this forum.
    "Early to bed, early to rise. Work like hell, and advertise." -- George Stahlman
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    Peugeot: 1970 UO-8, S/N 0010468
    Bianchi: 1981 Campione d'Italia, S/N 1.M9914
    Schwinn: 1988 Project KOM-10, S/N F804069

  7. #7
    Erect member since 1953 cccorlew's Avatar
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    Pi is roughly
    3.14159265358979323846264338327950288419716939937510582097494459230781640628620899862803482534211706 7982148086513282306647093844609550582231725359408128481117450284102701938521105559644622948954930381 9644288109756659334461284756482337867831652712019091456485669234603486104543266482133936072602491412 7372458700660631558817488152092096282925409171536436789259036001133053054882046652138414695194151160 9433057270365759591953092186117381932611793105118548074462379962749567351885752724891227938183011949 1298336733624406566430860213949463952247371907021798609437027705392171762931767523846748184676694051 3200056812714526356082778577134275778960917363717872146844090122495343014654958537105079227968925892 3542019956112129021960864034418159813629774771309960518707211349999998372978049951059731732816096318 5950244594553469083026425223082533446850352619311881710100031378387528865875332083814206171776691473 0359825349042875546873115956286388235378759375195778185778053217122680661300192787661119590921642019 8938095257201065485863278865936153381827968230301952035301852968995773622599413891249721775283479131 5155748572424541506959508295331168617278558890750983817546374649393192550604009277016711390098488240 1285836160356370766010471018194295559619894676783744944825537977472684710404753464620804668425906949 1293313677028989152104752162056966024058038150193511253382430035587640247496473263914199272604269922 7967823547816360093417216412199245863150302861829745557067498385054945885869269956909272107975093029 5532116534498720275596023648066549911988183479775356636980742654252786255181841757467289097777279380 0081647060016145249192173217214772350141441973568548161361157352552133475741849468438523323907394143 3345477624168625189835694855620992192221842725502542568876717904946016534668049886272327917860857843 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    WANTED: Not a darn thing. I've got it all. Life is good.
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  8. #8
    Senior Member turtleguy54's Avatar
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    George, gear inches relate back to the early 1900's ordinary bikes, those with the huge front wheel and no gears. 54 gear inches means you are riding your modern bike just like someone then with a front wheel of 54" diameter. So, one rotation of your pedals turns the 54" wheel one rotation or approximately 170".

    From there you can extrapolate how fast or far you are traveling by minute or hour. I do this all the time on rides over 2 hours by myself (I quit using a bike computer in the 80's). It keeps my mind busy on looooong West Texas roads.

  9. #9
    His Brain is Gone! Tom Bombadil's Avatar
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    Pi?

    Did someone mention Pi?

  10. #10
    OnTheRoad or AtTheBeach stonecrd's Avatar
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    The problem with the gene pool is that there is no lifeguard and the shallow end is much too large

    2013 Noah RS

  11. #11
    Senior Member George's Avatar
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    cccorlew, I tried to work out your answer and I'm sorry you're wrong. Thanks everybody!
    George

  12. #12
    Small Member maddmaxx's Avatar
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    Quote Originally Posted by cccorlew View Post
    Pi is roughly
    3.14159265358979323846264338327950288419716939937510582097494459230781640628620899862803482534211706 7982148086513282306647093844609550582231725359408128481117450284102701938521105559644622948954930381 9644288109756659334461284756482337867831652712019091456485669234603486104543266482133936072602491412 7372458700660631558817488152092096282925409171536436789259036001133053054882046652138414695194151160 9433057270365759591953092186117381932611793105118548074462379962749567351885752724891227938183011949 1298336733624406566430860213949463952247371907021798609437027705392171762931767523846748184676694051 3200056812714526356082778577134275778960917363717872146844090122495343014654958537105079227968925892 3542019956112129021960864034418159813629774771309960518707211349999998372978049951059731732816096318 5950244594553469083026425223082533446850352619311881710100031378387528865875332083814206171776691473 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0126456001623742880210927645793106579229552498872758461012648369998922569596881592056001016552563756
    Its generalities like this that lead to crashed mars landers!..

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