People are modifying bicycles by adding a gasoline engine (similar to a chainsaw motor) to the frame. A second chain drive is usually rigged to move the back wheel through the hub assembly. These standard bicycles were not designed or manufactured to support the weight of the motor, additional power through the hub, and increased pressure on the brakes. This in turn causes stress on the frame and other parts of the bicycle – ultimately resulting in failure and breakage.
Additionally, adding a gasoline engine to a standard bicycle now transforms it into a motorcycle. This means the “motorcycle” has to be Certified, properly plated with a permit, class M license is required to operate, the proper helmet worn, signal lights/brake light/headlight required, and valid insurance is required.