The Butler Petrol Cycle was a three-wheeled vehicle, with the rear wheel directly driven by a 5⁄8 hp (0.47 kW), 40 cc (2.4 cu in) displacement, 2 1/4 in × 5 in (57 mm × 127 mm) bore × stroke, flat twin four-stroke engine (with magneto ignition replaced by coil and battery) equipped with rotary valves and a float-fed carburettor (five years before Maybach) and Ackermann steering, all of which were state of the art at the time.
Starting was by compressed air. The engine was liquid-cooled, with a radiator over the rear driving wheel. Speed was controlled by means of a throttle valve lever. No braking system was fitted; the vehicle was stopped by raising and lowering the rear driving wheel using a foot-operated lever; the weight of the machine was then borne by two small castor wheels. The driver was seated between the front wheels. It wasn’t, however, a success, as Butler failed to find sufficient financial backing.