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It's an annual amateur ultra-distance mountain biking race traversing the length of the Rocky Mountains, from Banff, Alberta Canada to Antelope Wells, NM.... it's the world's longest off-pavement cycling route.
The race format is strictly self-supported, and it is not a stage race - the clock runs continuously from the start until riders cross the finish line, more than two weeks later.
The race has a very low profile, has no entry fees, no sponsorship, and no prizes. Although "letters of intent" from likely starters are encouraged, any rider may turn up on the day to participate.
Challenges along the route include mountains, great distances between resupply towns, risk of mechanical failure or injury, bears, poor weather, snowfall, and significant unrideable sections that require pushing the bike.
Riders usually adopt a "bikepacking" style, carrying minimal equipment sufficient for camping or bivouacking, and only enough food and water to last until the next town. In this way, riders ride huge distances each day.
The Tour Divide has been raced and completed on both single speed bicycles and tandem bicycles. It usually starts on the second Friday in June
https://en.wikipedia.org/wiki/Tour_Divide
http://tourdivide.org/
It was tirelessly mapped over a 4 year span, and published in 1998, highlighted by long dirt roads and jeep trails that wend their way through forgotten passes of the Continental Divide. It travels through Canadian provinces of Alberta and British Columbia, and the United States of Montana, Idaho, Wyoming, Colorado, and New Mexico