We bump, rattle and roll downward. It's true that you don't have to do a lot of pedaling, but it would be a mistake to think that going downhill is easy. It takes concentration. One wrong move really could be fatal. The mud makes it slippery, but it's those big rocks -- some the size of your fist -- that make it really perilous.
"It's quite nerve-wracking," says Studdart during one of the breaks. "It doesn't get any easier. I had a bit of a fall back there. I decided to say hello to the ground."
She confesses her pride was hurt, but she's determined to make it to the bottom.
For the adrenaline junkies it is a thrill. Especially when the clouds lift and the spectacular scenery reveals itself. But there's no time to let the scenery distract you: Keep those eyes on the road.
Jeff Klassen is smiling from ear to ear.
"It's unbelievable, more than I hoped for actually," he says. But even the veteran adventurer admits the corners are pretty frightening.
"It gets your heart rate going, but it's lots of fun."
Despite Matthew's admonitions, we don't run into a lot of cars. Because this is no longer the main road to La Paz.
In December 2006, a brand new highway opened, connecting La Paz to the Yungas Valley. It was a feat of engineering financed with millions of dollars of foreign aid. (But as Bolivians know all too well, Mother Nature just doesn't seem to want roads in these valleys. As we saw on our return journey, the concrete reinforcements that hold back the mountainside on the new road refuse to stay where they've been placed. The road is plagued with rock falls and mud avalanches. We drove the road at night in the fog -- not a good idea -- and dubbed it "the World's Second Most Dangerous Road.")
With the new road open the fabled old road is now the preserve of local farmers and a thrill for thousands of bikers. But just in case you're feeling cocky and overconfident, there are those small crucifixes lining the roadway in memory of the thousands who have died cascading over the cliff sides.
The road is still a killer. Since mountain biking began here 10 years ago, 11 bikers have died, two in the last year.
"The first person who ever died biking the road with a company pretty much drove straight off the edge," says Matthew as he points to a cliff next to the winding road below us.. "They'd been complaining pretty much all day that their brakes weren't working well. The group that they were with was well ahead of them. It had stopped there for her to catch up. She pretty much just rode straight off an 800 foot cliff right in front of them."
Matthew was quick to add that his company -- the largest and not the cheapest -- has never lost a cyclist. But they do fall. Regularly.
As I turned a muddy corner I came across Australian Paul Cheal sprawled in the mud. I stopped to help him up.
"I came around the corner and hit a loose rock," he tells me. "I tried to recover but couldn't. I was going too fast."
Cheal is OK. He's not bloodied, but he's bruised and little humbled. "I was finally having fun on the bike ride and then I fell. Too relaxed maybe."
At its best, a ride down this road is the essence of exhilaration: a Bolivian Rocky Mountain high. The adrenaline pumps, the clean air courses through the lungs (the breathing gets noticeably easier as you get lower) and the scenery is spectacular.