Derny

A derny is a motorized bicycle for motor-paced cycling events such as during six-day and Keirin racing and motor-paced road races. It is driven by a 98 cc (6.0 cu in) Zurcher two-stroke engine and by being pedalled through a fixed gear, typically of 70 teeth on the front chainring and 11 on the sprocket on the back wheel. The combination allows for smooth acceleration and slowing, important when the rider taking pace is centimetres from the pacer's shielded back wheel. A coupling between the motor and the back wheel ensures the derny will not stop dead if the motor seizes. Top speed, with rider pedalling, is up to 50 km/h (31 mph), depending on gearing.