Gear Ratio Calculations for the SickBikeParts Shift Kit

Discussion in 'Transmission / Drivetrain' started by Fabian, May 21, 2013.

  1. Fabian

    Fabian Well-Known Member

    From the 10T clutch output shaft sprocket to the Jackshaft 17T sprocket = 1.7:1

    Jackshaft right hand side sprocket is a 9T which drives the SickBikeParts optional 48T pedal crank sprocket = 5.33:1

    In low range gear (selected by the front derailleur) i am running the optional SickBikeParts 24T sprocket
    In Midrange gear i am running the SickBikeParts optional 30T sprocket
    In High range gear i am running the SickBikeParts standard 36T sprocket

    The rear cassette is a jail broken set of (9 speed) Shimano style cassette sprockets made up from two individual cassette clusters, giving me the following sprocket sizes:

    11 - 12 - 13 - 14 - 16 - 19 - 23 - 28 - 36

    In low range and with 1st gear selected the sprocket ratio is 1.5:1
    In high range and with the top gear selected the sprocket ratio is 1:3.272

    The total (chain driven) gearing value in low range first gear is 13.592:1
    The total (chain driven) gearing value in High range top gear is: 2.764:1

    Engine primary gear ratio = 4:1

    Low range first gear - 13.592 * 4 = 54.368
    High range top gear - 2.764 * 4 = 11.056

    Wheel circumference = 2.065 meters

    Low range first gear = 54.368 engine rpm per wheel revolution.
    High range top gear = 11.056 engine rpm per wheel revolution.

    5mph = 8.05 kilometers per hour
    30mph = 48.3 kilometers per hour

    8,050 meters / 60 = 134.166 meters per minute
    48,300 meters / 60 = 805 meters per minute

    134.166 / 2.065 (wheel circumference) = 64.971 (rear wheel rpm per minute) - 1.082 wheel rpm per second
    805 / 2.065 (wheel circumference) = 389.830 (rear wheel rpm per minute) - 6.497 wheel rpm per second

    64.971 * 54.368 = 3,532 engine rpm in low range first gear @ 5mph - - - - - 58.866 engine revolutions per second
    389.830 * 11.056 = 4,309 engine rpm in high range top gear @ 30mph - - - - - 71.816 engine revolutions per second

    High range first gear = 7.514mph (12.097km/h) @ 3,532 engine rpm

    The rear cassette has a 327.2% over drive between first gear and top gear, if you like playing with numbers and want to work out gear ratio combinations.
    Last edited: May 23, 2013
    5-7HEAVEN likes this.

  2. 5-7HEAVEN

    5-7HEAVEN Member

    Fabian, that is an EXCELLENT explanation! At what rpm is your engine most comfortable?
    Fabian likes this.
  3. Fabian

    Fabian Well-Known Member

    The engine is virtually free of vibration at 3,200 rpm and maximum torque is around 3,500 rpm with maximum power at around 4,800 rpm.
    The reed valve intake will make usable power down to 2,500 rpm and the engine will stall at 1,600 rpm.

    It seems happy to sit in the 3,800 rpm range with gear changes at around 4,100 rpm for maximum comfortable power.
    If i'm trying to squeeze every last ounce of power out of the engine, i'll change gears at 5,100 rpm.
  4. magna_503

    magna_503 New Member

    I made a chart here for a shift kit with a 99cc, and it looks like i better not go over 4th gear, final gear ratio of 9.92. From the look of it, it's crazy fast with anything over 4th gear. Looks like i can run it in first gear 32t til 4525rpm/25mph and drop down to 4th gear 21t and cruise @26mph @ 3065rpm.

    Looks like we only need a 5speed in the rear or so.

    I haven't built this yet to verify the numbers though.

    Last edited: Jun 5, 2013
    Fabian likes this.
  5. Fabian

    Fabian Well-Known Member

    Some of the numbers look incorrect, because the standard "clutch out" sprocket is a 10 tooth, not an 11 tooth.

    The standard and optional SickBikeParts sprockets for the front input are 24T, 30T and 36T.
    I am not sure where you get the 32T sprocket?


    i am not sure where you get the 22T input shaft sprocket when the only sprocket that is available from SickBikeParts is a 17T

    If you are using your own custom sprockets or sprockets from a different vendor then that will chage the situation and make your chart a correct description of the available ratios.
  6. Fabian

    Fabian Well-Known Member

    It is an impressive and well organised gearing chart.
  7. Fabian

    Fabian Well-Known Member

    Interestingly your 1st gear is equivalent to my high range top gear.
  8. 5-7HEAVEN

    5-7HEAVEN Member

    Impressive research. maybe enough for a thesis, lol.

    Can you pare it down? Max TQ @ 3200rpm, max power @ 4800rpm. Shift points for a standard SBP shift kit w/single chainring and 11t/34t cassette.
  9. magna_503

    magna_503 New Member


    The chart is for a AGK jackshaft kit mated with a SBP shift kit. The 11t sprocket is their standard one, I might be wrong on that. But changing the clutch to a 10t would change increase the final gear ratio by approximately 1. I made the chart for various configuration.

    To keep a safe speed on the bike, and keeping both AGK and SBP kit standard, here it is :)

  10. Fabian

    Fabian Well-Known Member

    Thanks for correcting me. I just made an incorrect assumption.
  11. hellbound212

    hellbound212 New Member

    this chart is for built motors correct because most motors govern at 3600rpm? correct?
  12. magna_503

    magna_503 New Member

    yes this is with a governor removed. Anyhow, mph is a 12% too high due to high tire circumference guestimate. I lost the spreadsheet on my comp for it though.
    Last edited: Jun 14, 2014
  13. magna_503

    magna_503 New Member