Re: Why do mopeds use smaller wheels rather than larger wheels?

Hi

My answer to this question does not mean that I advocate building such a 
contraption.  In my opinion you would be better off buying a commercially 
available unit and not deal with the liabilities associated with a 
conversion.  Having said this, you might want to look through old issues 
of Popular Mechanics and Mechanics Illustrated.  Back in the 1960's there 
were many similar conversions and detailed plans and descriptions were 
published in these magazines.  Your local library should be able to help 
you locate the issues of interest.

The ratio question is pretty simple: The reason minibike manufacturers use 
small wheels is because they are less expensive than big wheels.  In 
addition, small wheels allow the use of smaller driven sprockets since the 
wheel rpm is much higher for a given speed, this also reduces the cost of 
the components.  For example:  A 26" bicycle wheel is 81.6" in 
circumference (26"*pi), a 10" diameter minibike wheel is only 31.4" 
(10"*pi) in circumference.  A minibike wheel will rotate 2.6 times for 
every rotation of the bicycle wheel.  Consequently, in order for both 
wheels to travel at the same velocity, the gear reduction  on the 26" 
wheel has to be 2.6 times that of the smaller wheel, if the minibike has a 
50 tooth sprocket on the rear wheel, the 26" wheel would need a 130 tooth 
sprocket in order to travel at the same speed.  To determine gear ratios 
for chain drives, you need to divide the number of teeth on the driven 
sprocket by the number of teeh on the driver sprocket.  The driver is the 
one found on the motor clutch, and the driven is the one on the rear wheel.

I hope this is helpful.
Greg