A Derny is a motorized bicycle

Motorized bicycle

A motorized bicycle, motorbike, cyclemotor, or vélomoteur is a bicycle with an attached motor and transmission used either to power the vehicle unassisted, or to assist with pedaling. Since it always retains both pedals and a discrete connected drive for rider-powered propulsion, the motorized...

 for motor-paced cycling events such as during six-day

Six-day racing

A six or six-day is a track cycling race that lasts six days. Six-day races started in Britain, spread to many regions of the world, were brought to their modern style in the United States and are now mainly a European event. Initially, individuals competed alone, the winner being the individual...

 and Keirin

Keirin

is a track cycling event in which racing cyclists sprint for victory. Keirin originated in Japan in 1948; the first Olympic competitions in the sport occurred in 2000....

 racing, or motor-paced road races

Road bicycle racing

Road bicycle racing is a bicycle racing sport held on roads, using racing bicycles. The term "road racing" is usually applied to events where competing riders start simultaneously with the winner being the first to the line at the end of the course .Historically, the most...

. 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

Burdin

Burdin may refer to the following people:*Claude Burdin , a French engineer*Sergey Burdin , a Russian football player...

, 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

Drafting (racing)

Drafting or slipstreaming is a technique where two vehicles or other moving objects are caused to align in a close group reducing the overall effect of drag due to exploiting the lead object's slipstream...

 or slipstream

Slipstream

A slipstream is a region behind a moving object in which a wake of fluid is moving at velocities comparable to the moving object . The term slipstream also applies to the similar region adjacent to an object with a fluid moving around it...

ing 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

Keirin

is a track cycling event in which racing cyclists sprint for victory. Keirin originated in Japan in 1948; the first Olympic competitions in the sport occurred in 2000....

 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

Classic cycle races

The classic cycle races are one-day professional cycling road races in the international calendar. Most of the events, all run in western Europe, have been fixtures on the professional calendar for decades and the oldest ones date back to the 19th Century. They are normally held at roughly the same...

 was motor-paced using dernys from half-distance. Other important races on the road behind dernys have included the Critérium des As

Critérium des As

The Critérium des As was a cycle race that was generally held at the end of the season, with entry by invitation only, for the leading riders of the season. Competitors rode behind pacers on tandems or motorcycles. It was held from 1921 until 1990, mostly in Paris, France but also in Switzerland...

 in Paris

Paris

Paris is the capital and largest city in France, situated on the river Seine, in northern France, at the heart of the Île-de-France region...

.