Derny

Derny publicity material

650cc Triumph TR65 Thunderbird Derny first used at Leicester Velodrome in 1982 and now at Herne Hill Velodrome

Derny Taon 125, 1955/56

A Derny is a motorized bicycle for motor-paced cycling events such as during six-day and Keirin racing, or motor-paced road races. It is driven by a 98cc 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 machine will not stop dead if the motor seizes.

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. That firm closed in 1957, though another company, Derny Service of rue de Picpus serviced and rebuilt machines into the 70s. Derny also built a street adaptation called the 'Solo' as well as tandems and mopeds.

The name derny is now applied to all such vehicles, regardless of manufacturer. It is used by the Larousse dictionary as a generic term for a small pacing motorcycle used in cycle races. There have been several attempts to copy or improve the original. One, the Burdin, was briefly successful but proved not strong enough for repeated fast riding on the steep tracks used in six-day racing. Modern machines are made by a small company in Neerpelt, Belgium, and dernys used on the track now are either new or have the original frames but new 90cc engines.

On a derny, the driver sits close to the back in an upright position to provide an envelope of low wind resistance for the cyclist drafting or slipstreaming behind. The machine has to be bump-started. It can then pace riders up to 90 km/h, although races rarely exceed 80 km/h.

For most derny races, the cyclist sits in the slipstream of the derny for the duration of the event. In keirin races, common in Japan and familiar elsewhere, the derny brings several riders up to speed, at which point it pulls off and the race finishes in a sprint without the pacer.

Some riders also train behind a derny on the road.

A small group of semi-professional pacers travels around Europe for the winter six-day season. Most are in their 60s and 70s and some have been pacing for more than 40 years.

In most of its later history, 560 km Bordeaux–Paris Classic road race was motor-paced using dernys from half-distance. Other important races on the road behind dernys have included the Critérium des As in Paris.