# Question About The Science: Shift Kit vs Standard - Gears And Sprockets And Torque And ...

#### yuckfoo

##### Well-Known Member
I'm looking for a basic (but not too basic) explanation of how a shift kit changes the way a build gets its torque and speed, etc. I hear people talking about using 44 tooth gears on their standard builds. I think on my bike (with shift kit) I'd go straight up a tree with a 44 tooth as my first gear but I wouldn't be able to go very fast down the road. How does the power transference change as the motor drives a 17 tooth gear, transferring to a 10 tooth gear, then transfers to my 44 tooth sprocket, then on to my bike's 7 speed freewheel.

If you have a PHD level understanding of this I'd love to hear from you, but if you can dumb it down a bit that would be better. I just want to understand it better, not do my own calculations and ponder going from an x tooth gear to an x +/- 1 tooth gear.

#### GreasyChris

##### Well-Known Member
Gear ratios are decided by deviding the driven gear by the drive gear. Your engine has an internal ratio of roughly 4:1 giving you a 4. Your engine/clutch gear has 10 teeth. The jackshaft sprocket on that side has 17. So you devide 17 by 10 giving you 1.7 the other side of your jack shaft you have another 10 tooth driving your 44tooth pedal crank chainring so 44÷10= 4.4 then the other chainring on your pedal crank is I'm guessing a 36 tooth so now you would devide what ever gear your in by 36. Say 5th gear has 20 teeth on that sprocket you'd devide that by the 36. 20÷36= .555556
Now you multiply all your gear ratios together. So 4x1.7x4.4x.5555 giving you a 16.66:1 gear ratio for 5th gear. You have to devide the driven gear by the drive gear for the remaining speeds and then multiply that number by 4x4.4x1.7 to get the ratio for each gear. You then take your ratio and enter it into the calculator and it will give you speed for a specific rpm. In 5th gear at 5000rpms you'd do 23 pmh. Here's a link to the calculator. Hope that helps. http://www.advanced-ev.com/Calculators/TireSize/

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#### yuckfoo

##### Well-Known Member
Gear ratios are decided by deviding the driven gear by the drive gear. Your engine has an internal ratio of roughly 4:1 giving you a 4. Your engine/clutch gear has 10 teeth. The jackshaft sprocket on that side has 17. So you devide 17 by 10 giving you 1.7 the other side of your jack shaft you have another 10 tooth driving your 44tooth pedal crank chainring so 44÷10= 4.4 then the other chainring on your pedal crank is I'm guessing a 36 tooth so now you would devide what ever gear your in by 36. Say 5th gear has 20 teeth on that sprocket you'd devide that by the 36. 20÷36= .555556
Now you multiply all your gear ratios together. So 4x1.7x4.4x.5555 giving you a 16.66:1 gear ratio for 5th gear. You have to devide the driven gear by the drive gear for the remaining speeds and then multiply that number by 4x4.4x1.7 to get the ratio for each gear. You then take your ratio and enter it into the calculator and it will give you speed for a specific rpm. In 5th gear at 5000rpms you'd do 23 pmh. Here's a link to the calculator. Hope that helps. http://www.advanced-ev.com/Calculators/TireSize/
Perfect. I'm not too sure I'll use this science for anything, but having a basic glimpse of it at work is exactly what I was looking for.

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