Of the many cool things in “Race Across the Sky,” a documentary about this summer’s Leadville Trail 100 mountain bike race won by Lance Armstrong, one thing in particular sticks out. As Armstrong and the other elite riders in this endurance race reach the turnaround, a dirt cul de sac atop a barren dome, a 12,750-foot summit reached after 7 miles and 3,000 vertical feet of climbing, they must pass the other riders as they head back to the finish in Leadville. Superhuman racers such as Armstrong and six-time LT 100 winner Dave Wiens, about who will both complete the race in under 7 hours, will brush shoulders with folks who will take nearly twice as long to finish, not to mention the 500 or so who will have to abandon. As the two distinct worlds pass one another, words of encouragement are exchanged in both directions.

“Go, Lance!” yells a weakend rider who, at this lung-shriveling altitude barely has air to breathe, let alone cheer on an athlete whose VO2 max — a measure of getting oxygen to the muscles — is roughly twice that of a normal human.

And a similar reply of encouragement in return from the seven-time winner of the Tour de France, road racing’s most treasured prize.

“Everybody’s encouraging everybody,” Lance says at one point in the film. “You don’t see that on the Tour.”

It’s a sharp contrast to the not-so-elite marathoners who take umbrage at sharing asphalt with their slower brethren, a phenomenon described in a recent New York Times article picked up and reprinted in Raleigh’s The News & Observer last week. In that article, some veterans of the 26.2-mile run sniffed that their endeavor has been cheapened by an influx of slow runners and — gasp! — walkers.

It’s a curious contrast, this exclusion by the marathoners vs. the inclusion of the elite endurance mountain bike racers. In the case of the Leadville Trail 100, that sense of inclusion starts at the top. At the prerace meeting the night before the LT 100, race  founder and organizer Ken Chlouber tells a packed gym of 1,400 anxious riders: “This race isn’t about Lance Armstrong. This race isn’t about Dave Wiens. This race is about YOU!” That point is reiterated moments later by the soft-spoken Wiens, whose trophied past includes a bronze medal at the 1996 Olympics.

To underscore that every rider in the gym belongs, Chlouber, a former miner, offers his credo for the race: “You’re better than you think you are, you can do more than you think you can.”

Thus, it’s no surprise that the highlight of “Race Across the Sky” isn’t the battle between Lance and Dave Wiens for first, but rather the stories about the back-of-the-packers. About the female cyclist who had been hit by a car and completely broken, but fought back to ride and finish this year’s race. About the female racer diagnosed with MS in 1984. About the guy who tries to fend off a hail storm wearing a trash bag with holes cut out for his arms. About dads and moms slowing to cross the finish so their kids can run across with them.

And perhaps most poignantly at the Twin Lakes support station when Chlouber and a handful of race volunteers have the unenviable task of stopping riders who don’t make the time cut-off. Riders such as Tom Bryan, a 69-year-old friend of Chlouber’s who recently had both knees replaced and, says Chlouber, “trained like hell for this race.” Most of the flagged racers seem relieved. Others, including Bryan, are devastated.

The Leadville Trail 100 isn’t an isolated case in the world of mountain biking. In 2004, I was on a team at the old 24 Hours of Snowshoe. I was poking along in the middle of the night when I heard someone pull up behind and politely ask, “When you get a chance, if I could pass … .” It was Chris Eatough, the best 24-hour racer in the country. “Thank you,” he said as he zipped past. And as I anchor every race I enter in the local Triangle Off-Road Cyclists races, the story is always the same: The faster they are, the more politely they pass.

“Race Across the Sky” debuted last Thursday with one-night showings nationally, including the Triangle. It’s not a general-release film; rather, it’s shown via satellite by Fathom, which shows everything from classic moves (“The Wizard of Oz” is coming up Nov. 17) to live concerts and sporting events in local theaters. It’s an new distribution vehicle but one that seems to discourage widespread viewing. Fortunately, the response to last Thursday’s “event” was such that Fathom has scheduled a second nationwide showing, on Nov. 12. (Tickets go on sale Nov. 7; for information, go here.

If you missed Thursday’s show,you really should try to catch this one. It helps explain what drives us to, in the words of pro mountain biker Travis Brown, “do impossible things.”

And if you do go, take a friend. Preferably a whiney marathoner.

Photo: LT 100 founder and organizer Ken Chlouber loves his racers, no matter how fast they are.