some of the best cyclists in the world. They're here in the back.

They're keeping up but also resting while their teammates in the front do most of the work.

It's a technique called drafting and it's what helps them survive the

three-week race around France. But drafting only really makes a difference

on these parts of the route: the long flat and hilly stages. Eventually these

riders reach stages where they won't be able to rely on others. They have to rely

on their own strength and endurance to win the race. That happens here on the

climbs. They're the most brutal and exciting parts of the race and they're

what makes the Tour de France the most famous bike ride in the world...

The Tour de France began as a desperate ploy to sell more copies of the sports

newspaper, L'Auto. The paper was struggling, so 1903 its editor, Henri Desgranges

organized a 19 day, 2,400 kilometer bike race around the country.

It went on to become such a success that L'Auto made it an annual event. Year after

year they added new routes to make the tour more challenging and also more

interesting for people to follow. By 1908, L'Auto's sales had more than doubled. Then

in 1910, one of Desgranges' writers, Alphonse Steines, suggested adding a new

twist to the route: the Tourmalet. It was a brutal 19 kilometer uphill climb

ascending 1400 meters to the summit. To see if this was even possible

Steines jumped in his car to make the climb. He drove up, but his car got stuck

in the snow at the top, he suffered from hypothermia and nearly

died. But nonetheless he sent a telegram saying: Tourmalet crossed. Very good road.

perfectly passable. So the Tourmalet made its debut in the 1910 Tour. French

cyclist Octave Lapize was the first to make the climb successfully. But he had

to walk up some parts and famously called officials assassins when he

reached the summit. But he went on to win the whole tour and his statue was placed

at the top of the Tourmalet. Since then, climbs became a staple of the Tour de

France. This year the route is made up of 21 stages over 23 days. It features 30

major climbs; seven of them are in the tour's most difficult category.

Including the Tourmalet, which is part of the event for the 86th time. More than

any other climb in the Tour's history. These climbs are where the tour is

ultimately won or lost. These are the long, flat, and hilly stages that are

usually about 200 kilometers long. Here cyclists ride together in a

formation, called a peloton. It allows cyclists to save energy by sitting

behind another rider or drafting. At high speeds, riders use most of their energy

to pedal against wind resistance. But when a rider stays close behind another,

they're sheltered from much of that. So, pedaling becomes much easier and they

can keep up with the front riders. The way to measure this is to look at how

much power a cyclist generates. Here at the front of the peloton, a Tour de

France rider will generate at least 300 watts of power. I jumped on a bike to see

what that feels like and just two kilometers holding 300 watts was really

really hard. By comparison when a rider is behind the lead in the peloton, they

only need to generate about 240 watts to move at the same speed. Holding 240 watts

for two kilometers felt remarkably easier. So even though these two might

finish a 200 kilometer flat stage at the same time, one is going to be way less

tired than the other. That's why you see some of the tour's best riders here in

the back. They're drafting up their teammates whose job it is to do the hard

work now so that the team's best cyclist is rested for the most difficult part;

the mountains. Where they'll have to be on their own.

When the peloton starts pedaling uphill, it slows down. At this stage the race is

less about fighting wind resistance and more about gravity, which affects all

riders the same way in the peloton. So, now every rider in the front and the

back needs to generate an extraordinary amount of power to keep up pace. In 2010

for instance, Danish cyclist Chris Anker Sorensen made it to the front of the

pack on the Tourmalet climb. "Look at the face on Chris Anker Sorenesen. Dishing out the

pain at the front end. As the lead, he dictated the pace of the whole group.

This chart shows his power output on the final climb. He averaged 415 watts for

more than 11 minutes. "...face of Chris Anker Sorensen now, really dishing out the pain.

And around here he peaked at an incredible 590 watts. "magnificent riding by

Chris Anker Sorenson, but how long can he keep this up? "Now look at American cyclist

Chris Horner. Even though he was several positions behind Sorenson, his power

output was almost the same. The faster Sorenson climbed, the harder it was for

the rest of the peloton to keep up. And so the formation started to break up as

weaker riders fell behind. This is the moment in the race when the tour's best

riders switch from drafting to relying on their own strength to get ahead.

Andy Schleck has finally attacked and Alberto Contador has gone with him.

The race is breaking up behind them like two of the best riders here:

Luxembourgian Andy Schleck and Spaniard Alberto Contador, who were coasting in the back

of the peloton during the long flat stages. But here they are halfway up the

Tourmalet, breaking away for the win. each likely generating well over 400 watts

for the final eight kilometers. Contador in the yellow jersey, was the overall

Tour de France leader but only by eight seconds. In second place was Schleck, who

would try and lose him on this climb.

"Andy Schleck is riding like a man possessed"

It was neck-and-neck to the very top...

It's Schleck on the right. Contador! Schleck wins! Contador takes second!

Schleck edged Contador by a hair at the top of the Tourmalet to win the stage. But since

he didn't lose him, Contador kept his overall lead and went

on to win the Tour de France. This kind of drama is only possible in the

mountains and this year's route makes climbs particularly important.

This year's Tour is being called the highest in history because of how many climbs

there are over the course of three weeks. Just one day features seven climbs. Even

after more than two weeks in the race, riders climb to 2,770 meters

above sea level where the thin air makes climbing even harder.

That's what makes the Tour de France the most grueling and

prestigious race in this sport. The winner isn't simply the strongest rider

but the one who endures the most pain and ultimately has what it takes to

conquer the mountains.