Jackshaft Pictorial flowchart for SickBikeParts Jackshaft Shifter (shift) Kit

Discussion in 'Transmission / Drivetrain' started by Fabian, Sep 27, 2009.

  1. Fabian

    Fabian Well-Known Member

    G'day all

    I've edited the post below to fix a maths typo error - the information you are reading is now corrected.
    The notes from the moderators are previous to the maths in this post being fixed.


    I don't have a tacho fitted to my bike (yet), but have a speedo.
    Was curious as to what kind of rpm's my motor is turning so i've decided to flowchat (ok, a real crappy flowchart sketched up in MS Paint) the sprocket and gear combinations to work out ratios and reverse engineer known speeds calculated back to engine rpms

    I've started with a given speed of 100 kilometers per hour as that makes all calculations easily divisable by 10 and 1, besides the "base 10" numbering system works for me as i've been lucky to be born with 10 fingers and if i run out of those i can use my 10 toes as a backup calculator.

    Here it goes (please feel free to correct any poorly calculated arithmetic).

    100 kilometers per hour = 100,000 meters per hour
    A typical 26 inch mountainbike wheel measures 2075mm around it circumference - 2.075 meters
    100,000 divided by 2.075 = 48,192 wheel revolutions per hour
    48,192 divided by 60 = 803.2 wheel revolutions per minute.

    Reverse engineering 1st gear speeds to engine rpm's
    In my case i have a 1st gear sprocket size of 32 teeth
    36 divided by 32 = 1.125
    803.2 divided by 1.125 = 713.95 pedal crank rpm
    48 divided by 11 = 4.364
    713.95 multiplied by 4.364 = 3115.68 jackshaft rpm
    17 divided by 10 = 1.7
    3115.68 multiplied by 1.7 = 5296.66 engine output shaft rpm
    82 divided by 20 = 4.1
    5296.66 multiplied by 4.1 = 21,716 engine rpm at 100 kilometers per hour
    21,716 divided by 100 = 217.16 engine rpm @ 1 kilometer per hour.

    I have to input pedal power at 8 kilometers per hour in 1st gear, towing my trailer with a gross weight of 60 kilos up one of the third steepest hills in my area.
    217.16 multiplied by 8 = 1737.28 rpm

    Now onto top gear
    In my case i have a top gear sprocket size of 11 teeth
    36 divided by 11 = 3.273
    803.2 divided by 3.273 = 245.41 pedal crank rpm
    48 divided by 11 = 4.364
    245.41 multiplied by 4.364 = 1070.97 jackshaft rpm
    17 divided by 10 = 1.7
    1070.97 multiplied by 1.7 = 1820.65 engine output shaft rpm
    82 divided by 20 = 4.1
    1820.65 multiplied by 4.1 = 7464.66 engine rpm at 100 kilometers per hour
    7464.66 divided by 100 = 74.64 engine rpm @ 1 kilometer per hour.

    74.64 multiplied by 50 = 3732 rpm @ 50 kilometers per hour
    74.64 multiplied by 35 = 2612 rpm @ 35 kilometers per hour

    This calculation only works if using the optional 11 tooth jackshaft output sprocket.
    The Standard sprocket is 10 tooth so if wanting to work out accurate figures the sprocket ratio will be 4.8:1

    By these calculations you can work out any engine rpm for any rear cassette gear using any of the available jackshaft output sprockets.

    To work out miles per hour divide kilometers per hour by 1.6
    50 kilometers per hour = 31.25 miles per hour.

    Have fun playing with numbers

    Cheers Fabian

    Attached Files:

    Last edited: Sep 27, 2009

  2. Fabian

    Fabian Well-Known Member

    I cant remember where i got this dyno graph from, but someone decided to dyno their Chinese motorised bicycle.
    I have no information as to what engine size he was using, nor what gearing he was running so i'm unable to calculate the rpm and correlate those figures against the power curve.

    Maybe someone can provide this information.
    From experience my motorised bicycle engine feels like it makes maximum torque at around 12 kilometers per hour in 1st gear which works out to around 2500 rpm

    I just want to see if the dyno graph backs up my own seat of the pants feeling.


    Attached Files:

    Last edited: Sep 27, 2009
  3. srdavo

    srdavo Active Member

    you should be able to edit your own posts. the edit box is in the lower right hand corner of your posts.

    If not.... send me a PM with exactly what you want your post to say....& I'll paste it in place.

    then, afterwards...I'll remove the off topic posts from your thread.

    I could easily fix your typo.... but since the rest of your formula is 'off'.... you need to do the math.
    My kids used to try & trick me into doing their homework!! hahahaha :grin5:
  4. 210061741

    210061741 Guest

    48 tooth sprocket rpm

    what would the rpm of the 48 tooth sprocket be if the enginw was at 10000 rpm???
    I'm looking into a freewheel for a jackshaft and need to tknow max rpm with a factor for safty.

    You seem to have all the figures and thought you could save me some work.