Motorcycles specifically intended for enduro competition first appeared at the International Six Day Trial (ISDT) now called the International Six Days Enduro (ISDE). The ISDE was first held in 1913 at Carlisle, England. The ISDE requires an enduro motorcycle to withstand over six days and upwards of 1250 km (777 miles) of competition; repairs are limited to those performed by the rider with limited parts. The ISDE has occurred annually, apart from interruptions due to World War I and World War II, at various locations throughout the world. The early events were a test of rider skill and motorcycle reliability. The earliest ISDE courses used the dirt roads common in that era. Today, most of the routes are off-road. In 1980, the ISDT was renamed the International Six Day Enduro (ISDE). Until 1973, the ISDE was always held in Europe. In 1973 it was held in the United States, and since then it has been held outside Europe more frequently: twice in Australia (1992 and 1998), again in the USA (1994), Brazil (2003), New Zealand (2006) and Chile (2007). The ISDE has attracted national teams from as many as 32 countries in recent years. In the 1970s the term was used in US marketing applied to
dual-purpose motorcycles regardless of their suitability for competition. In the U.S., enduro motorcycles appeared in light and heavyweight classes during the Greenhorn Enduro hosted by the
Pasadena Motorcycle Club (PMC). The Greenhorn Enduro was a nationally recognized 500-mile, two-day desert off-road competition that pounded both rider and machine. Veterans of the early Greenhorn Enduro included Bud Ekins and
Steve McQueen. Many current enduro motorcycles are built along the basic lines of a World Championship (WEC) machine, as used in the World Enduro Championship. The WEC is a time-card enduro, whereby a number of stages are raced in a time trial against the clock over a course of at least 200 km (124 miles) consisting of both paved and unpaved trails and roads (up to 30% of the course may be on public or private asphalted roads). Another popular type of enduro competition that has spurred enduro motorcycle development is
endurocross, a hybrid event combining enduro and
supercross. In the UK, most enduro clubman bikes were 2-strokes, but many
ACU events had a separate class for 4-stroke machines, such as the
Honda XR series. Such 4-strokes tended to be effectively "sporty trail bikes" rather than de-tuned scramblers, but their much improved fuel economy (compared to 2-strokes) meant that they could complete lengthy laps of thirty miles or more without refuelling. This obviated the need for a back-up team to man refueling stops, allowing 4-stroke clubmen to compete without a support entourage. ==Technical developments==