I wrote a coupla introductory posts awhile ago but never saw them posted. So I'll try again.
The shiny object I'm currently obsessed with is in the title above and picture below. The stark simplicity of that silhouette reminds me of board track racers from a hundred years ago. They were the bare essentials. A motor and 2 wheels. Nothing extra. But hey, what else do you need?
The burning question at the moment is gearing. One revolution of the back tire results in 7 ft of forward travel so we're gonna need a lot of reduction. Doing the math, 32:1 indicates 15mph at 6,000 rpm. Does that seem reasonable?
Sounds kinda feasable but you have to take into consideration that the pedal arms adds fulcrum to the equation..Im thinking without going into mathematics youre gonna have a 5-6mph torque monster on your hands...
Pedal bikes differ slightly from motor bikes as the motorbike has the smaller sprocket on the drive side and a larger sprocket on the power[pedal] side no matter the size of wheel and the bicycle has a large power sprocket and a smaller drive sprocket...Typically theyre geared 2:1/3:1 with even with 26" tires as the length of the crank arms determines things too..
This is known as gain ratio and is alot more accurate to getting a proper ratio for your bicycle.Gain Ratio, which is a calculation that utilizes gear inches and also figures in crank length, since crank length affects leverage.
Almost anything thats motorized has a rule of thumb should be geared between 10-12:1..That should produce enough engine torque to get the bike up to 28-30mph.
For that bike if you plan on using the original crank n chain system and running a sprocket off the leftside front of the crank youre gonna need a sprocket same size as the one thats already on it..
If you do plan on keeping the pedal system too just know on a fixed bike it will be very laborious trying to pedal it on anything but a flat or downhill surface too..Mountainbikes make better candidates for dual moders..
If youre keeping the pedal mode and are adding a rear gear finding a #25 or TF-8mm sprocket or even a #415 chain pinion to work with the rear sprockets for motorized bicycles is gonna be your biggest obstacle..
The engine youre using is a clone of the Robin FL-411NB..They used to sell a 50cc 2 stoke DAX bicycle kit that was called a 2 stroke Dax50 I believe so you may be able to find the drive parts you need..
Last one I built had a Techumseh 3hp 2 stroke snowblower engine with a centrifugal Comet clutch that was modified...
My latest Frankenstein build consists of a similar bicycle with an 8hp Military OHV 4 stroke made by the Chrysler Outboard division built in 1952 and has maybe 10hrs of runtime on it despite its appearance...LOL..Its the very first OHV 4 stroke gasoline engines way ahead of Briggs and Honda..Same design was built by the Chrysler Outboard Corp and used by the military from 1952 until 1987...It has an oilpan similar to a car engine and has an oilpump to circulate the oil too..
Its gonna be a military themed single mode motorized version when Im done...Plans are to add fatter tires.
Good Luck with yours.