Gear Ratio Calculations for the SickBikeParts Shift Kit

Fabian

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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.
 
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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.
 
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.

 
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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?

and

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.
 
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.
 
Fabian,

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.

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

The front input chainring is a 36t, out is a 32t. I also played with front input with a 48t,
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?

and
The 22t sprocket is the standard input sprocket from the AGK jackshaft, which also has a standard 9t out to the shift kit. The shift kit input is 17t, and out is a 10t.
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.

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

 
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