Tour de France: How to cycle as fast as a pro
The professional cyclists tackling Europe's biggest mountains as they wend their way to the Tour de France's finale in Paris seem superhuman. But how do they do it - and can ordinary riders learn a few lessons?
There aren't many opportunities to compare pros and amateurs in a like-for-like race.
But in 2011, Bradley Wiggins rode the British national 10-mile time trial championships in Hull, the global cycling star taking on national professionals and amateurs.
Wiggins won with a time of 19 minutes 44 seconds - just missing out on the British record at the time.
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In 59th place was Sean Eden. He was the middle-placed finisher in a high-quality race, so probably a good representative of all the decent amateur cyclists taking part. He finished in 22 minutes 33 seconds. So Wiggins was around 15% quicker.
Cyclist magazine has done a similar calculation using the Etape du Tour, a race for amateurs that covers one of the stages of the Tour de France a few days before the pros do it.
In 2015, Vincenzo Nibali won the stage in 4 hrs 22 mins 53 secs. The first person over the line in the amateur race was William Turnes, who took 5 hrs 2 mins 56 secs. So again, the pro was around 15% faster.
But there's a complicating factor. Cycling is all about power, and much of that power is used to overcome air resistance - about 90% of a cyclist's effort if they are going at 40km/h.
The problem for someone trying to emulate a pro is that, the faster you go, the more air resistance you are up against and the more power you need. If you want to cycle twice as fast, you actually need to pedal eight times as hard.
Former British time trial champion and cycling journalist Michael Hutchinson also raced that day in 2011 against Wiggins, coming second. He has done some calculations on how many watts the riders were pedalling at.
"Bradley Wiggins won that event pedalling at 470 watts, which - as anyone who has ever looked at the wattage on a gym bike knows - is a big, big number to ride at for 20 minutes," he says. "Sean Eden, according to my maths, would have ridden at about 312 watts, so he would actually need to pedal 50% harder to go at the same speed as Bradley Wiggins.
"To put that another way, 312 watts is the kind of training pace Bradley Wiggins could hold for six or seven hours while holding a conversation."
Even for a time trial specialist like Hutchinson, the extra power needed to match Wiggins' speed was immense.
"I was 30 seconds behind Wiggins. I was probably doing about 435 watts to Wiggins' 470. That's the sort of margin that doesn't sound like a lot but that's the kind of gulf I was never going to cover."
In a time trial, there's not much you can do about this battle between the cyclist and the air in front of them.
But the other major way that professional cyclists go much faster than the rest of us in a race like the Tour de France is their expertise in drafting or slipstreaming.
This is where they cycle "on the wheel" of the rider in front so they are protected from some of the air resistance. If they are riding in a big pack, known as the peloton, the front of the group can form a wall against the air and wind for all the riders behind.
The Tour de France sees plenty of sprint finishes where stars like Mark Cavendish were "led out" out over the last few hundred metres, a teammate riding directly in front of them right up until the last minute. This leaves the sprinter with far less air to pedal through and plenty of energy to put into the scramble for the line.
Earlier this year, an experiment at Eindhoven University of Technology in the Netherlands in collaboration with KU Leuven, ANSYS and Cray, revealed that drafting is even more effective than previously assumed.
Aerodynamics expert Bert Blocken lined up 121 quarter-sized ceramic models of cyclists on bikes in a peloton formation in a wind tunnel. He found that a cyclist riding on a flat road at the mid-rear of the peloton experiences only 5% to 10% of the air resistance of those at the front.
This can have a massive effect on the effort needed to keep up.
"Suppose that you are riding in the belly of the peloton and the peloton is cruising at around 54km/h, then for you it feels like you are cycling at 17km/h," Prof Blocken says. "That is a very low speed most amateurs can attain, but this doesn't mean that you and I would be able to ride along with a professional cycling peloton.
"We could maybe do that for a few kilometres, when the peloton was tightly packed and on a flat and straight road. But as soon as we go through bends, you get this spike in air resistance and you need the skills, training and power of a professional cyclist to be able to catch up."
Often in a race a rider or small group of riders escapes from the peloton in what's called a breakaway - and they sometimes win the race.
"What we want to indicate with this research is how amazing it is that breakaway attempts sometimes are successful," says Prof Blocken, "because you have this group of 121 riders chasing you that have much less air resistance than you have, and if such an escape is successful, it really means there has been a tremendous effort. That deserves a lot of respect."