Max Power for a Bicycle
Dalton asked: Are there restrictions on what size of engine you can put on a bicycle or tricycle?
That depends on what you are trying to accomplish. If you want the vehicle to remain a bicycle and not register it or have a driver’s license, it must meet the following statutory definition of a bicycle.
s. 316.003 – Definitions
(4) Bicycle – Every vehicle propelled solely by human power, and every motorized bicycle propelled by a combination of human power and an electric helper motor capable of propelling the vehicle at a speed of not more than 20 miles per hour on level ground upon which any person may ride, having two tandem wheels, and including any device generally recognized as a bicycle though equipped with two front or two rear wheels. The term does not include such a vehicle with a seat height of no more than 25 inches from the ground when the seat is adjusted to its highest position or a scooter or similar device.
Note that only electric assist motors are permitted in that definition.
If you want to use a gasoline assist motor on a bicycle, it must be registered as a moped and requires a driver’s license. See this post.
See the embedded link for the DMV Procedure RS 68 for the details.
If you want to have a higher-powered engine, either gas or electric, the vehicle must be meet the requirements and be registered as a moped or a motorcycle.
Low-Speed Electric Bicycles sold commercially in the US are regulated by the Consumer Product Safety Commission. Its rule limits the output of their motors to 750 watts and requires the motors to be so regulated that when the bicycle is propelled solely by the motor its maximum speed is less than 20 mph.
Section 316.003(4) of the Florida statutes (which Geo quoted) requires the motorized bicycle’s “electric helper motor” to not be capable of propelling the bicycle faster than 20 mph, but places no restriction on the output (“size”) of said motor. At first blush, this statute seems to limit motorized bicycles to 20 mph when their motors are providing propulsion, but a careful reading reveals that, like the federal law, the speed restriction only applies while the motor is providing 100% of the propulsion (sometimes known as “throttle”).
If the rider provides some amount of propulsion (sometimes known as “pedal assist” or “pedelec”), the statute is silent about the bicycle’s maximum speed. Consequently, motorized bicycles that can exceed 20 mph when propelled by rider and motor power simultaneously are commercially available in Florida. These typically can not exceed 28 mph because of a federal and international legalities.
Some motorized bicycles permit the rider to switch between throttle and pedal assist modes, and I’ve talked with one person whose motorized bicycle even has a cruise control feature.
Based on my understanding of the law and how the industry is marketing motorized bicycles, I believe the answer to your question is that there is no legal limit on the size (output) of the electric motor as long as there is some way to control the motor so it does not propel the bicycle faster than 20 mph when it is operated in throttle mode. This is, of course, assuming a DIY project being operated in Florida.