As well as a safety risk of attaching a petrol motor to a bicycle, the legal risk is highlighted in two Irishexaminer.com articles covering a court case — one from last month and the second from last week.
Basically, in legal terms, a bike with a petrol engine attached is no longer a bicycle. Once a 50cc engine or similar is attached, then your bicycle legally becomes a light motorcycle. Then tax, insurance, and a driving licence are needed to use it on a public road or street.
A standard bicycle is a vehicle in law, but become a motor vehicle (aka a mechanically propelled vehicle) once an engine is attached.
Often heard excuses like “you have to pedal for the engine to engage, so, it’s still a bicycle” are unlikely to work in court, as was the court case covered in the Irish Examiner articles. Saying the engine is only “as strong as a leaf-blower” will also not work.
Electric bicycles are different
There is an exception for electric bicycles which meet requirements set out in law. Electric bicycles (aka pedal-assist electric ebikes) are classed as bicycles and this is supported by law (S.I. No. 614/2015 by its reference to an EU definition and in the Finance Act).
The legal definition of allowed electric bicycles is defined with fairly plain English as:
“pedal cycles with pedal assistance which are equipped with an auxiliary electric motor having a maximum continuous rated power of less than or equal to 250 W, where the output of the motor is cut off when the cyclist stops pedalling and is otherwise progressively reduced and finally cut off before the vehicle speed reaches 25 km/h”
The Road Traffic (Miscellaneous Provisions) Bill 2019 might update or expand this definition, but the details of it has yet to be published. The bill is currently undergoing pre-legislative scrutiny.