how to calculate gearing?

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Thewurst

New Member
Mar 23, 2013
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Wisconsin
I put my already motorized bike together, with a different motor and transmission reduction. (the one motor originally was a 7:1, the transmission I put on was a 3:1. As I expect it just stalls. Is there some sort of calculator that I can use to find the right sized sprocket for the rear?
or formula?
 
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Thewurst

New Member
Mar 23, 2013
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Wisconsin
Ok thank you for that, I found a 60t sprocket on ebay that should bolt right up. This engine is off of a cateye pocket bike. I put a walboro carb with adjustable jets on it, so that should give it an edge of power. The calculator says it should go 49mph at 9500 rpms (2-stroke) with this sprocket, is it still geared too high? I assume so
 
WightBoy

WightBoy

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Aug 30, 2012
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Michigan, USA
Is this the bike from your previous thread, just with a different motor/trans? What size sprocket do you have on now? If 9500 rpm is your redline, I personally would shoot for a 40mph top speed. That should give you a comfortable 30ish mph cruising speed. I don't care what anybody says, 30 mph is fast on a bike that only has 2" wide tires.
 
WightBoy

WightBoy

Member
Aug 30, 2012
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Michigan, USA
That may be a good size. The 64 tooth is the largest I know of that is commonly available. I am running a 49cc 4 stroke on my build so I don't exactly know the power band of the little 2 strokes.
 
5

5-7HEAVEN

Well-Known Member
Aug 2, 2008
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Thewurst said:
I put my already motorized bike together, with a different motor and transmission reduction. (the one motor originally was a 7:1, the transmission I put on was a 3:1. As I expect it just stalls. Is there some sort of calculator that I can use to find the right sized sprocket for the rear?
or formula?
Click to expand...
Is this a pocket bike engine? Any pictures on your setup? If your transmission has 3:1, you might need 6:1 more reduction. So 3:1 x 6:1 = 18:1, which should work ok.

It'd be easier to use T8F pocket bike chain and sprockets. A 72t sprocket is about the size of a 48T bicycle sprocket. Use an 12t trans sprocket with 72t big sprocket to get 18:1. They have Chinese rear hubs with left side freewheel sprocket. The 4-hole 72t sprocket will bolt right on. Good luck.
 
5

5-7HEAVEN

Well-Known Member
Aug 2, 2008
2,661
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At 45mph with 9500rpm, your gear ratio is 16.34:1. With this same ratio @ 30mph, rpms is at 6300rpm. Not bad. What size is your transmission sprocket?
 
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