Sprockets Speed with different sprockets. Rpm confusion?

Discussion in 'Transmission / Drivetrain' started by Charles Laypool, May 5, 2016.

  1. Charles Laypool

    Charles Laypool Active Member

    Today I was riding my bike around and I looked at my GPS when I got home and it said my top speed was 36 mph with a 44t sprocket so i wanted to get a even 40 mph so i did some math
    These engines stock run 5500rpm
    With a 1:4 reduction so final drive rpm is 1375
    With a 44t sprocket the math says im only going 24 mph? So if this thing was pushing me 36 mph then its rpm would have to be 8000 correct me if im wrong please
    this engines stock no mods at all
    I figure my gearing for each diffrent sprocket is

  2. FurryOnTheInside

    FurryOnTheInside Well-Known Member

    We don't know your tyre (actual measured rollout) circumference. I thought reduction was 4.1:1 too. I'm not sure gps is accurate either unless you go in dead straight lines on a dead level road.
  3. butre

    butre Well-Known Member

    the internal reduction is 4.1:1 and the 44t gives an additional 4.4:1. overall is 16.4:1

    newer stock engines push more like 7000 on flat ground and you likely hit that top speed downhill
  4. Charles Laypool

    Charles Laypool Active Member

    I use bmi go cart gear calculator
    First i figure out my drive rpm with the reduction 8k÷4.1=1951
    Then i put my rpm my tire size 26.5 my axle sprocket teeth 44 and my clutch/drive sprocket teeth 10 in the calculator

    so speed would be 34.957mph

    I sure did its good to know these things are improving somewhat over time
  5. Rob H

    Rob H Member

    I think there are too many variables too use just math for figuring exact speed of each gear but it could be a generalized speed
  6. Charles Laypool

    Charles Laypool Active Member

    I agree
  7. butre

    butre Well-Known Member

    the only variable is slippage, which you're not going to get under the no load situation of a top speed run
  8. Steve Best

    Steve Best Well-Known Member

    Charles, my math is similar to yours.
    I get 58-60 kph on the level with a 44t sprocket on a 26" tire with a 2015 Grubee GT5A.
    Figures around 8000 rpm under load. Had another post somewhere where I figured it out.
    WAY past 5500 rpm.

  9. Charles Laypool

    Charles Laypool Active Member

    Thank u steve good to know im on the right track all of this is because im working on a dual engine bike jackshafted to a 36t rear sprocket got the mount done alot more to do tho