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.

The first Derny 'Entraineur' or 'Bordeaux–Paris' models, with their characteristic petrol tank across the handlebars, were built by Roger Derny et Fils of the Avenue de St Mandé, Paris, France in 1938. A fleet of Dernys was maintained for the long-established 'Bordeaux-Paris' road-race and the Derny was used for many other track and road events and for endurance training. Derny also built a touring adaptation called the 'Solo' and tandems and mopeds.