# gear ratio?

Discussion in 'Frame Mounted Engines' started by matt420, Sep 17, 2008.

1. ### matt420New Member

I'm tryin to find out what gear ratio I would have , I'm using 20in tire ,6 tooth front sprocket and a 72 tooth back sprocket. What will my ratio and speed be. Will this work? Thanks ,Matt

Last edited: Sep 18, 2008

2. ### HoughMadeGuest

Your gear ratio is simply the rear sprocket teeth divided by the front- 72/6= 12- so with that alone, it's 12:1. The wheel size changes the effective gear ratio when compared to the same sprockets on wheels of a different size, but it does not actually change the gear ratio. What I mean by the convoluted sentence is that your 12:1 is 12:1. However, because your wheel is 6" smaller than a 26", the wheel is 23% smaller than the 26", therefore, with a 20" wheel, your setup acts like a 26" with 23% lower gearing.

Your 20" at 12:1 acts like a 26" at 14.76:1. That's usually not quite enough to get the engine in the power band. You probably want a reduction of closer to 19:1 on a 26" bike which is equivalent to 14.63:1 on a 20".

3. ### matt420New Member

O.K I'm understandind it now , so I'm going to need a bigger sprocket on the back. I'm using a 49cc pocket bike motor w/6 tooth front sprocket, I didn't know what kind of sprocket to use on the back, I might try to go with an 86 tooth then if I can find one. The motor revs to 10,000 rpm so this thing should fly when I build it, I'm going to make it as light as possable. Thanks HoughMade