The grueling mountain stages of the Tour de France, now under way, may humble the best bicycle racers in the world, but none of those scenic European peaks looms as large as Haleakala, the 10,000-foot Maui summit that sings a siren song each year to cyclist Mike Zagorski.

For three years in a row, Zagorski has won or tied for first in the leg-torturing bicycle race to the top of Haleakala — the Ka Lima O Maui Cycle to the Sun.

The August race is strictly for the fittest of cyclists.

Riders start at sea level and cover 36 miles before finishing atop the dormant volcano. To get there, they have to cope with finish-line temperatures that can be 30 degrees cooler than when they started and an atmosphere with nearly 5 percent less oxygen.

Zagorski said the thin air is the most difficult thing to deal with — and this after the hot, dry flanks of Haleakala and the windy, near-freezing upper slopes.

"You are still pushing yourself, but the higher you go, the harder it becomes," he said. "At the end of the race, you try to make a hard effort, and your legs are saying: 'No.' "

This is the point where a lot of people who simply walk at that altitude begin to experience a painful headache.

Not Zagorski, a 28-year-old McCully resident who spends more time on a saddle than most people do in an easy chair. He'll ride 300 to 500 miles a week.

"I think the last half hour of the climb, the last couple thousand feet, you start to cramp," he said. "Your heart is definitely beating harder. Your muscles are needing oxygen, and there is not much. Your heart is trying to cram in as much as it can find."

Zagorski began racing when he was 15, hopping on a mountain bike for events in Scotland, Wales and England. He'd race nearly 40 times a year and flirted with burnout.

By 2001, he had moved to Hawai'i and decided that racing road bikes was easier. He's been a fixture on the road-racing scene ever since, notching a slew of victories, including the state time-trial championship the past three years.

The race up Haleakala comes near the end of the racing season. To prepare, Zagorski will log hundreds of miles riding up and down Tantalus, the climb that most closely resembles Haleakala but is by no means the steepest ride on O'ahu, he said.

"Halekoa Drive is probably the worst," he said. "It's nasty. It is just steep and it never flattens out. On Kamehame Ridge, when you get to the residential area it rolls a bit. On Halekoa, you never get to relax. I think it averages a 14 percent grade or more. It is twice as steep as Tantalus."

Perhaps the biggest attraction of a bicycle ride that climbs for 36 miles is the trip down. But organizers of Cycle to the Sun won't allow that, Zagorski said. Still, the champ found a way last year after completing a training ride before the race.

He zipped along at 50 mph.

"It wasn't crazy fast," he said.

That distinction goes to a ride on the Big Island. Last year, Zagorski raced from Hilo to the top of Mauna Kea, then turned around.

"I think descending there toward the Saddle Road I was going 70 mph," he said. "Seventy on a bicycle gets a bit scary. I think that is the fastest I have ever gone. If you fall at that speed, you lose skin pretty fast."

Reach Mike Gordon at mgordon@honoluluadvertiser.com.