Fabian
Well-Known Member
G'day all
+++NOTE+++
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.
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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
+++NOTE+++
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
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