NICE, France—Their fingers were too numb to grip a water bottle, let alone the brakes. As the freezing rain turned to hail on a Saturday afternoon in March, the riders in the peloton shivered their way up the Col Saint-Roch. At the top, there was no relief. As hard as the ascent was, the worst part of the stage was ahead of them: They now had to plunge down the hill. French rider Tony Gallopin of team Lotto-Soudal, who was chasing the leaders’ group in the Paris-Nice cycling race, knew the famously steep drop in the hills behind Nice would be hair-raising. But like a lot of modern riders, he had learned that seconds could be mined on these mountain descents. Chasing world road race champion Michal Kwiatkowski, he tucked his body into the most aggressive aerodynamic position he knew. He moved his backside off the saddle and practically rested his chin on the handlebars, where he clung on for dear life. On the slick, narrow roads, any debris, any jerky twist of the wheel, would cause his inch-wide tires to skid at more than 50 miles an hour. From there, he could be thrown into the side of a cliff, a guardrail or into sheer oblivion. Mr. Gallopin pulled away when Mr. Kwiatkowski ran out of gas, and he eventually swooped down the final descent alone to win the stage. His body and bicycle had made it in one piece. His nerves, not so much. “I took a lot of chances,” Mr. Gallopin said in French at the finish. “And the brakes weren’t working at the beginning. I scared the daylights out of myself.” Not often discussed, riding downhill is cycling’s supreme test of derring-do. Even seasoned pros avoid going downhill full tilt if they can help it. And as the Tour de France sets off Saturday with two-time Tour champion Alberto Contador of Spain, 2013 winner Chris Froome of Great Britain, 2014 winner Vincenzo Nibali of Italy and the 25-year-old hotshot Nairo Quintana of Colombia battling for the lead, the difference, cycling experts and riders say, may very well come on the way down. “There’s been a flurry in the last couple of years of guys like Nibali, who have seen that there are gains to be made in the descents,” said Dave Brailsford, the architect of Team Sky’s victories in the 2012 and 2013 Tours. Bicycle makers continue to produce lighter machines with better grip and more powerful brakes. Riders have been flirting with unheard of speeds during descents, touching 70 miles an hour. And race organizers have been responding with more challenging mountain stages than ever. The past four editions of the Tour all featured at least 11 downhill sections so steep that the road lost 50 meters in vertical elevation per kilometer. Ten of the descents in this summer’s race are even stiffer, averaging at least a 6% downhill, including the notoriously technical Col d’Allos in the race’s final week.

Col D’Allos: The Descent Where the Tour de France Could Be Won or Lost After 86 miles, four climbs and nearly four hours on the bike, Stage 17 of the Tour de France pushes the peloton off the notorious Col d’Allos in the Alps. Col D’Allos 7,382 feet The first half of the 10-mile downhill stretch averages a 7% grade Lowest Point 3,983 feet Begins a 3.9-mile climb averaging a 6.5% grade Pra Loup 5,314 feet Summit finish after 100 miles, end of stage 17 Select points to change view 1 2 3 Graphic by Hani Lim and Renée Rigdon. Source: Amaury Sport Organisation; NASA; Google

“In bike racing, there are 90% of the riders saying, ‘That descent is too dangerous and we shouldn’t be doing that,’ ” said Jonathan Vaughters, a former U.S. Postal Service and Crédit Agricole rider who now runs the Cannondale-Garmin team. “And the other 10% are thinking, ‘This is how I’m going to win this race.’ ” In the sport’s rampant doping years, all the focus was on the climb—the area where riders with enhanced blood could make up the most ground. But today, with doping believed to be far less prevalent, one of the biggest advantages seems to be a matter of guts. Descending barely requires pedaling, just seriously advanced bike handling. The only limit is the rider’s appetite for risk. “Sometimes you’ve got to do bad stuff to get down a hill quickly,” said Cannondale-Garmin rider Alex Howes. “Stuff your mom wouldn’t watch you doing on TV.” “ Sometimes you’ve got to do bad stuff to get down a hill quickly. ” — Alex Howes Testing boundaries This summer’s Tour marks two significant anniversaries in its tumultuous love affair with the mountains. It is the 40th birthday of the iconic red-polka-dot jersey, awarded to the Tour’s King of the Mountains. But it is also 20 summers since the death of Fabio Casartelli, who crashed on the descent of the Col de Portet-d’Aspet in the Pyrenees, a section of road that still appears on the Tour. Mr. Casartelli isn’t cycling’s only victim of the downhill. More recently, the 2011 Giro d’Italia claimed the life of Wouter Weylandt ; the 2009 Giro left Pedro Horrillo in a coma after he plummeted down a 200-foot ravine; and, in May, Domenico Pozzovivo had to quit the Giro after a gruesome crash while aggressively rounding a corner. So when organizers hear fans demand more daring stages, more made-for-television drama, these considerations weigh on their minds. “I see sometimes on social networks, ‘They don’t dare try anything. Shouldn’t we have a time-trial in a descent?’ ” Tour de France director Christian Prudhomme said, referring to stages when the riders go out one-by-one and race the clock. “We would never do that. It would be irresponsible. It’s insane. Yes, it could spice things up, but to what extent: killing people? It makes no sense.” Still, organizers test the boundaries. They know that mountains drive ratings. At the 2009 Giro, after Mr. Horrillo’s downhill crash, the peloton protested a course for being too dangerous. “The boys in the bunch are livid,” Lance Armstrong wrote on Twitter at the time. Two years later, also at the Giro, the peloton considered another protest after seeing the gravel-road descent off the infamous Monte Crostis. “There is not just one or two but 10 men that could ride off the edge there,” one racer said at the time. Women cyclists are also squeezing seconds out of downhill stretches, although their top stage races, such as the Giro Rosa in Italy and Women’s Tour of Britain, are much shorter. Marianne Vos, the world’s top female cyclist, has worked closely with mountain bikers for years to hone her technique riding downhill, most recently at her team’s January camp in Spain. It is no coincidence that Ms. Vos, also a multiple world champion in cyclocross, has used her speed in technical descents to build leads in all three of her women’s Giro wins. “When you’re actually looking at mountain stages in the Tour de France and doing reconnaissance of them, you’re reconning the descents more than the climbs,” Mr. Vaughters said. “Because at the end of the day, the climbing, that’s just a physical effort. A climb is a climb is a climb.” Each mountain crest brings a set of potentially life-or-death questions. Riders’ heart rates slow from around 200 beats a minute to under 100, but their focus can’t drop. Vital downhill skills include precisely timed braking, efficient cornering and a mastery of aerodynamics.

Anatomy of a Descent Pro rider Andrew Talansky demonstrates his technique on the downhills. PHOTO: ROBERT LIBETTI/THE WALL STREET JOURNAL

“How do you cut the corner?” said Trek Factory Racing’s Fabian Cancellara, a veteran of nine Tours and one of the best downhill riders in the professional peloton. “Do you brake? Not brake? How much do you trust the tires?” Some questions are less technical and more existential. They all affect how hard a rider is prepared to go, Mr. Cancellara said. “Are you young? Are you a bit older? What are your goals? Do you have a family? Do you have kids?” “ Are you young? Are you a bit older? What are your goals? Do you have a family? Do you have kids? ” — Fabian Cancellara It can take months to restore confidence after even a small crash downhill, Mr. Cancellara explained. Lars Boom, a Dutch rider for the Astana team, said he had to coach himself back to descending at full speed after his first daughter was born. The problem is that descending is almost impossible to practice at top speed. It is too dangerous for teams to send their best riders hurtling down mountains with nothing at stake. Plus, they don’t get roads to themselves in training the way they do on the closed course of a race. “You either have it or you don’t have it, this skill,” Mr. Cancellara said. For those who do, technology is constantly giving them the tools to maneuver faster. Carbon-fiber bike frames made the first impact when they began replacing steel and aluminum in the late-1990s, instantly making bikes lighter. The frames used by professional riders now weigh around 2 pounds, making them faster—but also more finicky. At the same time, manufacturers have made advances in the development of rubber compounds for tires, improving grip at high speeds. But the biggest jump has come in the brakes department. Improvements in the materials and precision of caliper brakes—two pads on each wheel squeezing the rim, a design used for a century—have given the riders the ability to carry huge speeds into bends. “The stopping power and modulation has improved a lot, even though it’s still just within the caliper-brake realm,” said Cannondale-Garmin team leader Andrew Talansky. Brakes are about to improve more with a major innovation: disc brakes. Commercially available on road bikes, they operate by giving the brake pads a cleaner surface to act on. Instead of pinching the rim of the wheel—which also needs to be light, strong and aerodynamic—the brakes squeeze a disc at the wheel’s center. The bike manufacturers’ lobby has been urging the sport’s governing body to approve disc brakes for use in the three Grand Tour competitions—the Tour de France, Giro d’Italia and the Vuelta a España—for years. In April, the Union Cycliste Internationale approved testing in races below the top-tier beginning this August, with a view to officially introducing them at the highest level in 2017. Nowhere will they have a bigger impact than in descents. With more stopping power, riders will be able to carry more speed into corners and brake harder a fraction of a second later. The problem is, in a sport where one rider’s mistake can take out a whole pack, Mr. Cancellara said, “You can’t trust everybody.”

Inside the Peloton Check the speed and heart rate of Cannondale-Garmin rider Nathan Haas as he descends 1,200 feet over 3.4 miles in a stage of the Tour de Romandie in Switzerland. His bike-mounted camera captured the view. Elevation ft Speed mph Heart Rate bpm Distance mi Distance mi Heart Rate bpm Elevation ft Speed mph Your browser doesn't support video. Please upgrade your browser to see the example. Source for video, data: Team Cannondale-Garmin

‘Walking a tightrope’ Cornering at high speeds is where cycling meets Formula One. The difference is that Formula One drivers know what the full corner looks like. Cyclists, most of the time, are whipping into the unknown. They look for clues in the final yards: loose gravel or slippery patches on the road surface; traffic signs advertising a chicane or a switchback. More valuable is the timing of the brake light on the police motorbike leading the pack. (Tour veterans say the trustworthiness depends on the type of officer. Local cops? They’re OK. But the motorcycle-mounted French Republican Guard, battle-hardened regulars of the Tour, are better.) As they approach the corner, riders feather the brakes—mostly the front to avoid skidding—and begin to scrub some speed. Touch here is so important that many riders keep their fingers exposed, even in frigid conditions, to feel every vibration as the brakes judder beneath them. Shifting their weight to the outside pedal, they lean the bike in to corner, tracing an efficient line across the apex. Locking the brakes here is asking to crash. When you lay it on the line and have your flow, ‘you can just feel that you’re not making mistakes,’ said Mr. Talansky. Photo: Daniella Zalcman for The Wall Street Journal “When you really do lay it all on the line and you have your flow,” Mr. Talansky said, “then it is like walking a tightrope, with that same mental engagement and focus…You can just feel that you’re not making mistakes.” On the exit of the curve, riders begin to accelerate again and squirm back into the tuck. With their chests parallel to the ground, they absorb every bump in the road in the small of their back. It boils down to technique, concentration—and cojones. Take Janier Acevedo, a young Cannondale-Garmin rider from Colombia who is indiscriminately referred to as insane by his teammates. “I like the descents when they’re wet and slippery, when it’s more difficult for everyone else,” he said. He goes one step further than most on the straightaways, by taking his hands off the handlebars and gripping the stem below them. His face is practically stuck to the speedometer, which showed he once cleared 75 miles an hour. “Sin miedo,” he said in Spanish. No fear. Which makes him an exception. At the 2013 Tour, French rider Thibaut Pinot reached the summit of the Col de Pailhères near the top contenders in the overall competition. As a 23-year-old flag bearer for the FDJ outfit, this was his moment for a statement. All he had to do was keep up on the technical descent. The problem was he was petrified. Due to a severe crash in a downhill as a junior, he told reporters later, “I’m scared of speed the way others have phobias for spiders and snakes.” By the end of the day, he was six minutes behind the overall leader, an eternity in cycling. He was still mentally shattered when he lost 25 more minutes the next day. It took him months to get over the performance—his team eventually hired a professional rally driver to race him around and re-educate him in matters of speed. Riders can smell fear in the peloton—rivals braking at awkward times, or timidly taking corners—and it feeds competition. “There are…a lot of guys who cannot descend. Or maybe who think they can descend, but they cannot,” said Mr. Boom, the Dutch rider, who credits his bike-handling skills to a background in cyclocross. There are two kinds of descenders in the professional peloton. At the front, the Contadors, the Nibalis, the riders scratching out every last second in the general classification—the competition to win the overall race. And at the back, the guys who couldn’t climb out of a paper bag. Usually, bigger than the flyweight Grand Tour contenders, they drop out of the main group by going too slowly uphill. At the top, the stragglers, known as the gruppetto, face a decision: shoot downhill to rejoin the peloton, or risk elimination by falling behind that day’s cutoff time. A veteran usually organizes the gruppetto. He knows exactly how to calculate that day’s cutoff time and how to push the group home. But accidents are unavoidable in a crowd desperate to make time. Knowing how to crash properly is crucial—like keeping your head out of oncoming traffic. “You just watch it happen,” Cannondale-Garmin’s Mr. Howes said. “Here we go, I hope I find a good spot to land.”

Keeping the Wheels on Team Cannondale-Garmin Alex Banyay, one of the mechanics on the Cannondale-Garmin cycling team, works on Andrew Talansky ’s bike during the Paris-Nice race in March. Daniella Zalcman for The Wall Street Journal 1 of 11 • • • • • 1 of 11 Show Caption Alex Banyay, one of the mechanics on the Cannondale-Garmin cycling team, works on Andrew Talansky ’s bike during the Paris-Nice race in March. Daniella Zalcman for The Wall Street Journal