how to calculate gearing?

[Thewurst]

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?

 

[WightBoy]

Here is a great ratio calculator. Download the exe file and use the program. http://jimsitton.net/ratiocalc/

 

[Thewurst]

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]

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.

 

[Thewurst]

yes it is, it has a 36t I believe. I may end up welding on a larger 80t sprocket. I can't find any bolt-ons

 

[WightBoy]

can you post some pics of the bike the way it is now? 80t is massive IMO what top end does that give you at 95k?

 

[WightBoy]

Maybe try a 64t? are you using the rubber rag joint?

 

[Thewurst]

yea it has the rubber rag mount, I will post pictures tomorrow. 64 will net me 46mph

 

[WightBoy]

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.

 

[Thewurst]

where do I get it? I looked under 49cc bike sprocket on ebay and couldn't find anything higher than 60

 

[WightBoy]

http://www.kingssalesandservice.com/motorized-bike-sprockets/

 

[Thewurst]

how do I know if they have the right type of chain? are there any cheaper options? $60 a piece is quite a bit

 

[WightBoy]

Duane (thatsdax.com) has a 72 tooth for $25. Says its good for 3:1 and 5:1 gear boxes. Its near the bottom of this page. http://www.thatsdax.com/ENGINE_KIT_PARTS_PAGE_2.html

 

[5-7HEAVEN]

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?

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.

 

[5-7HEAVEN]

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?