Photograph by Velo Steveon Flickr.
The drawbacks of using bike engines in cars is the lack of reverse gear, lack of low end torque (both characteristics making parking difficult) and that the engines get a short lifespan since they have to pull more weight than they Bike-engined car were designed for. More recently, conversions of small cars such as the Mini have started to emerge, since conversion kits have become more available.
A bike-engined car is a small or light weight car that is powered by an engine that was designed for use in a motorcycle. The advantage for car builders is that small car engines in high states of tune are relatively rare whereas large motorcycle engines are frequently designed for high specific outputs.
Most of these cars use the sequential gearbox from the motorcycle, allowing for faster shifting than a traditional automotive gearbox. Early examples include the Bond Minicar from 1949 which used a 122 cc Villiers motorcycle engine.