Gear ratio guru

roccosbike

roccosbike

#1
#1
Ok All you gear ratio specialists if I have a 12 tooth clutch going to two 15 tooth gears on a jackshaft back to a 59 tooh gear what is the ratio:shrug: the rear tire is a 130/50-8 if you need that too. looking for a high speed gear on a light bike.I can only change the jackshaft sprockets.Thank you for any help you can give me on this.
 
Motra

Motra

Active Member
#2
#2
If the jackshaft has an input of 15 and an output of 15 then there's no reduction there, so essentially your gear ratio is 4.9:1, that's 59 divided by 12. If you want your top speed then you need to measure the outer circumference of the drive wheel and max engine RPM.
 
roccosbike

roccosbike

#3
#3
I don't know a lot about this stuff but does the rear wheel gear ratio change when you go from 12 to 15 teeth before the rear sprocket:shrug: if not this gear ratio should be pretty good.the rear wheel outside dia is 13"
 
roccosbike

roccosbike

#5
#5
Hey Thanks Minibikedude, Those two sites are the $#it. Motra Thank you also. I will be going to a 14 tooth after the clutch and it will give it a 4.59 gear, for this bike it will be perfect with its tire size.
 
65ShelbyClone

65ShelbyClone

Well-Known Member
#6
#6
There is a sticky thread (the only sticky thread, actually) at the top of this forum with top speed and gear ratio calculators including one for a jackshaft setup.

To do it manually, you just divide the driven spocket size by the drier sprocket size. If there is a jackshaft with different size spockets on either side, find the engine-to-JS ratio, then find the JS-to-rear wheel ratio, and multiply the two ratios together. Here's an example with random numbers:

13t clutch -> 18t JS in -> 12t JS out -> 50t at the wheel.

18/13 = 1.39:1

50/12 = 4.17:1

1.39 x 4.17 ≈ 5.77:1 total.
 
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