On mile 41 of the 50 Mile Ride for Rwanda, Dave Hartstein pedals toward a wide stream in Mission Viejo and wonders if exhausted muscles can navigate water flowing over rocks — terrain so slick there is no traction.

To steady his nerves, Hartstein, a 54-year-old aerospace engineer, sucks in air. Then he and several other mountain bikers hurtle down a 45-degree slope. Water soaks pedals, shoes, shorts. Wet sand sucks at knobby tires. Mud splatters glasses and stings faces.

The group rockets up the other side, navigates over loose rocks, steers through a 6-inch gap in a sandstone outcropping.

“It’s something you have to work up to,” Hartstein said. “It take a lot of preparation.”

But the thrill is about far more than testing himself on one of California’s epic mountain bike rides. On this trail, Hartstein joined nearly 1,300 riders sharing, cheering one another on and — best of all — helping others.

The Ride for Rwanda is like no other mountain bike ride. It is not a race. It’s a nonprofit event run by volunteers to raise funds to buy transportation bikes for people in Africa.

The idea was born a decade ago during a 50th birthday mountain bike ride. But mile by mile, rider by rider, the effort matters.

Celebrating its 10th anniversary, the annual 50 Mile Ride for Rwanda has raised nearly $900,000.

Along with paying for event expenses such as sheriff’s deputies, that money has funded national mountain bike teams in Rwanda, Eritrea and Ethiopia. More importantly, it has paid for nearly 1,000 utility bicycles, allowing farmers to move cargo over monsoon-soaked roads in central Africa.

Founder Doug Grant says, “Whether you’re a rider, a sponsor, a donor or a volunteer, you’re helping us make an impact well beyond the borders of Orange County.

“Durable cargo bikes are helping impoverished farmers in Rwanda be more productive and earn more income for their families; rugged bikes are providing a way for children in remote villages to attend school and get an education that will help them rise above poverty.”

This year I join hundreds of 50 Mile Ride for Rwanda mountain bikers in a vast parking lot in Rancho Santa Margarita as the sun rises. The asphalt is wet from an overnight shower. But no one cares. Some stretch. Some gulp last-minute coffee. But most fist-bump and chat about old memories and the new ride.

There is electricity in the air. It is similar to a race day, but different.

Adam Guerrero, a 31-year-old certified bicycle mechanic, is a veteran racing cyclist and helps clean and fix bicycles during the ride. “Races are more individualistic, more tense,” he says. “This is a very difficult event, but this is more laid back.”

Grant, CEO and president of Westamerica Communications, stands at the starting line. Instead of a gunshot, Grant merely calls for the ride to begin. And instead of a crush of cyclists, Grant strolls through the crowd waving, thanking many riders by name and high-fiving just about everyone.

For Grant, the hundreds of hours he donates every year are paid for in smiles. More than 1,400 volunteer hours go into each event.

To reduce congestion, riders pedal off in waves. After a few miles, Arroyo Trabuco Trail morphs from smooth dirt road to rock-garden single track. Some riders struggle. Others blast it. Soon, a trail of riders more than a mile long snakes up and over the steepest hills in O’Neill Regional Park.

Down a series of sharp switchbacks, an over-caffeinated rider hugs my rear tire. Squeezing by, he nearly scrapes my front tire. For a moment, I’m annoyed and a little scared. Then I’m reminded of myself during past and, probably, upcoming rides.

With the positive attitude that is infectious in these parts, I can’t help but wave, “All good.”

Hartstein, quality manager for Parker Aerospace in Irvine, Calif., says that in a way he’s trained for this ride ever since he started mountain biking four years ago.

Every few months, he goes a little longer, tackles ever-more technical trails. Still, he admits he kicked up training about three months ago.

At mile 20, he and other riders hit an aide station in Caspers Wilderness Park. Most stop, rest, munch on oat bars and hydrate.

Susie Lam is a U.S. immigration assistant and one of dozens of volunteers. This is her first time with Ride for Rwanda and she is with her 25-year-old daughter, Michelle, to support a friend. Lam pours jug after jug of water into riders’ water bottles.

“I enjoy it so much,” says Lam. “It’s a lot of fun seeing all these people really healthy and strong.”

But a few miles farther, Lam might reconsider.

At mile 22, muscle-throbbing, heart-pounding uphills loom. At first, everyone tries to ride the beasts. But halfway up, most are defeated. They hike over loose rocks and deep ruts, pushing their bicycles higher and higher.

At the entrance to Dove Canyon, Hartstein still has 15 miles to go. But he knows the area well. The worst is over. Mostly.

After a series of streams and a few more gnarly hills, Hartstein crosses the finish line in Rancho Santa Margarita. He has been on trails more than six hours. Yet the first thing out of his mouth is to ask how a friend feels.

That’s typical of Hartstein, just as it’s typical of Rwanda riders. Jacke Van Woerkom is founder of the Trail Angels, a women’s mountain biking club of more than 300 members. Seeing a friend and with true care, she says, “Good to see you. How are you doing?”

Hartstein looks around at work buddies, neighbors and friends of friends. “It’s fun to meet different people,” he says, “to have that many people together.

“But what’s really cool is to be part of something that’s doing so much good. That’s the thing that makes this event what it is.”

It’s sport at its finest.