How Do They Go So Fast? The Technology Behind the Tour de France.

Tight formation. The secret to superhuman speed in the Tour de France is the peloton, the dense group of riders in a bike race. (Image by Eric Michelat from Pixabay.)

The most efficient form of transportation ever invented is the bicycle. A bike makes moving a human from point A to point B almost effortless. But the most celebrated bike event, the Tour de France, succeeds in making cycling into one of the most grueling events of any organized sport. Professional cyclists ride 2,100 miles around France, clockwise one year, counterclockwise the next, each year crossing the Pyrenees and the Alps. The oldest of Europe’s three Grand Tours, it has only paused once since 1903, which was due to World War II.

The current pandemic reduced the number of fans to the few: the brave, the foolish, and the masked. They stood outside in their quaint little towns for a fleeting glimpse, just a flash of colors as the peloton (the big group of riders) went by.

The cyclists average over 25 mph¹ during the entire 20 plus days of the Tour, from plummeting down mountain roads faster than you drive on the highway to a relative crawl up steep mountainsides but, even, then they ride quicker than the average person on flat ground. While die-hard cyclists attribute this blazing speed to modern training and nutrition as well as their legs, lungs, and bodies with less body fat percentage than fashion models, the real answers lie in fitness and physics. In physics, aerodynamics plays the most pivotal part – although not in the way you might think.

Aerodynamics

The aerodynamic force (F), or drag force, against a moving object, is given by the formula below. It is the largest force a racing cyclist, or any cyclist going over 10 miles per hour, has to overcome. Since aerodynamic force is a function of the square of air velocity, at the speeds encountered in bike races, aerodynamic drag becomes 80% of all force facing the cyclist.²

F = ½ Cd A ρ v2

Where:

F is the aerodynamic force

Cd is the coefficient of drag

A is the projected frontal area

ρ is the density of air

v is the velocity of air

Bradley Wiggin leading the 2012 Tour de France on a time trial bike, keeping his back flat as a tabletop to minimize the frontal projected area and achieve the minimum aerodynamic drag. Note the absurdly streamlined shapes of the cycle and the rider’s extreme position minimizing projected frontal area, resulting in a lower coefficient of drag. (Picture by Denismenchov from Wikipedia.)

Racing bike manufacturers have outdone themselves in streamlining the bicycle itself, and the time trial version of the road bike is proof: a machine so specialized for straight, flat roads that riders have to switch bikes if the road is different. The time trial bike pitches the rider’s torso forward, making the back flat as a tabletop while their neck and eyeballs strain to look ahead. The position is difficult to hold for more than a few agonizing breaths for the untrained or overweight, but pros may have to hold the position for hours during a time trial. Another key feature is the wheel rims, which are deep and tapered. Blade-like spokes that can cut fingers off replace the usual spokes with round sections that thrash the wind. To get the absolute minimum drag, time trial and track bikes forego spokes altogether and use aerodynamically slippery plastic membrane.

The Power of the Peloton

While most focus on individual winners, professional road racing is a team effort. Sports fans flipping channels for ball sports will not understand that what appears to be a group of riders in static formation effortlessly cruising down the road is, in reality, flying down the road in a dynamic swarm buzzing with tactics. A few riders in the group riding their hearts out, the rest riding along with considerably less effort. Nor will they understand the overall winner of the Tour or any stage race will not be the lead on any given day of the race. Despite his fastest overall time over the multi-stage race, viewers are most likely to see the eventual overall winner behind teammates, and often, behind other teams. Tour officials give the overall leader (called the general classification or GC) a yellow jersey, the maillot jaune, so they – and fans – can spot the leader.

The cyclists will eat up most of the two thousand plus miles of the Tour in the tight formation of the peloton. The peloton’s reason for existence is to cheat the wind – and it does so with amazing success. Shaped like a spear, the cyclist at the tip bears the full brunt of the aerodynamic force, fully “in the wind,” hitting the static air head-on. The air passes over and around him and the riders on the edges, states Dr. Bert Blocken in what may be the definitive CFD study of a peloton. Engineering.com has featured his work cycling aerodynamics previously. The riders behind the leading edge of the peloton are not just sheltered from the wind, they may even feel like they are being swept along. The riders on the very back feel only six percent of the aerodynamic force they would feel at the same speed if riding solo will sheepishly admit to “getting a free ride.”

The minimal drag force at the very back of the peloton might seem to be the best place to ride as well as the least dangerous. A team leader could get caught in a crash or miss a breakaway, which is when a rider turns on the afterburners to try to get some time or go for a stage win. The downside of riding so close together, wheel to wheel, is that if a rider goes down, which happens from just grazing the wheel followed, it will cause a cascade of tumbling bikes and bodies as each rider mows down the rider behind them. Therefore, the team leaders – those the team counts on to win the overall race -- rides in pockets closer to the front of the peloton, several riders back from the leading tip.

What keeps the peloton moving at a superhuman speed without wearing down the leading edge riders? They take turns being in the wind. Riders in a support role for their leader (called domestiques) will ride at maximum effort on the forward edge, then fall back behind other riders to get relief until it is their turn to go back to the front.

Top view of a group of riders in a Tour de France simulation. The leading rider at the tip of the peloton deals with the most wind, almost as if he was riding alone. The rider towards the back (green cellsin the top frame) will feel as little as six percent of the aerodynamic drag, like a rider riding alone at the same speed. (Picture courtesy of Science Direct.)

Cyclist Are the Biggest Drag

While the biggest force against the racing man/machine combo is aerodynamic drag, a substantial part of it is not the machine but the man. The frontal area of the rider is many times the size of the minimalist geometry of the road bike or the ultra-streamlined time trial bike. Ducking the wind with an aggressive posture, keeping arms forward and close together during a time trial, or wearing a smooth, aerodynamic helmet and skintight clothing helps the ride, but only to an extent. It can only partially overcome the size and shape of the rider. Pro teams make use of air tunnels and CFD to reduce drag as much as possible, even pulling smooth shoe covers over aerodynamically dirty shoe buckles and straps. However, it is hard to overcome the horrible Cd of the human body, still mostly in the position inherited from the “safety bicycle” of the late 1800s.

On a Roll

A comparison of the energy cost of various forms of transportation shows that the bicycle is most energy-efficient. (Picture courtesy of Exploratorium.)

For completeness, let us consider other forces working against the speed-obsessed cyclist, such as rolling resistance.

Converting human energy into rolling motion may be the most efficient form of locomotion ever invented. A bicycle is five times more efficient than walking in energy use because walking involves increases in potential energy as it uses energy to raise your center of gravity with each step, and on the way down, recover some energy as the muscles absorb the impact. But, rolling keeps your center of gravity from bobbing up and down.

Rolling resistance of a bike tire results from the force required to deform the shape of the tire by the road. Some of the energy is recovered when the tire returns to its circular shape. The difference is due to hysteresis and dissipated mostly as heat. (Picture courtesy of Flo Cycling.)

Rolling resistance is the force that overcomes the resistance of the wheel surface to deform from round to flat is mostly reduced with a hard wheel on a smooth surface. The best example is a steel wheel on the steel rail of a train. How else could you pull a train with just a rope and your mouth? The high pressure in a Tour de France bike tire is 115 psi in the front and 125 psi in the back for a normal stage. It is 10 to 15 psi higher for time trials and 10 to 15 lower when the road is slippery, says one Tour de France bike mechanic.³ This reduces the rolling resistance but still keeps tires soft enough to keep it from losing energy from bouncing up from irregularities in the road. On a guaranteed smooth track in velodromes, racers use special tires pumped up to 300 psi.

Aerodynamic forces increase as a square of velocity, whereas rolling resistance stays constant, as do mechanical losses (not shown) from friction in the chain, gears, wheel hubs). At racing speeds, aerodynamic drag is by far the most formidable opponent of the cyclist. (Picture courtesy of University of Waikato.)

Losing Weight

Riders with road bikes are obsessive about the weight of their bikes, knowing a light cycle will be faster up hills. The most obsessive rider will weigh their bikes after component upgrades, grateful for every gram saved on bikes already chosen for their initially low weight. Cyclists are known to drill holes in their bikes (not recommended). Frames are super light constructions of thin wall tubing or paper-thin carbon fiber construction. If the frame is too light, it can lead to disaster (See How to Design the Lightest Possible Bike – And Still Sleep at Night). The thin walls can collapse under compression or local forces from impact during a collision. Even more, the clamp of a bike stand can also lead it to collapse during routine maintenance.

The UCI, which makes the rules for professional cyclists, has taken a stand against overly light bikes to protect cyclists against themselves. The Tour bike can’t weigh less than 6.8 kg or 15 lbs. The technology surrounding design and materials can cause the lightest of bikes to be underweight by several pounds. The issue has lead to the creation of newer technology. Teams used to add dead weight to bring the underweight bike up to spec but, recently, tech-savvy teams are adding data gathering/transmitting gear and communication equipment that lets the rider share all sorts of ride details: speed, cadence, power output, heart rate that transforms the rider and his machine to a stream of performance data. Some Tour riders have made their performance public – if only to prove to the world that what they make look easy most definitely is not.

One company, Decision Data, supplied journalists with live streams of video plus GPS and performance data to a roomful of journalists covering the race.

Tradition Over Technology

Aerodynamic innovation is far from welcome by the tradition-minded governing bodies of professional cycling who hold sway over all the bike races that matter. The first use of aero bars, now common on time trial bikes, was met with great trepidation when American Greg Lemond used them during the final stage of the 1989 Tour de France. Pouting, the organizers decided against other components that's only purpose was aerodynamics – thereby eliminating the one component that would make the biggest difference for a bike rider moving at race speed – the fairing.

A fairing would change everything. Time trials will be faster, taking more than an hour off the clock. While an aerodynamically optimized rider and bike has a Cd of 0.6, a recumbent bike with fairing has a phenomenal reduction down to 0.07.⁴ A recumbent position and a fairing allow this bike to reach a speed of almost 90 mph. Granted the rider would only reach the speed on a flat road, not on the Alps and Pyrenees, where the recumbent with it oversized gearing would be useless, and those tradition-minded race officials would deem such feats of technology a travesty of tradition.

Thus, the image of this bullet of a bike riding down the Champs-Élysées in triumph, the traditional finish of the Tour de France, remains a fever dream of the tech-addled brain of the engineer.

References

Aerodynamic Drag in Cycling Pelotons: New insights by CFD Simulation and Wind Tunnel Testing, Dr. Bert Blocken, et. Al, Journal of Wind Engineering and Industrial Aerodynamics, 2018

¹Achieved by Chris Froome in the 2017 Tour de France

²Update of Cycling Aerodynamics, Science4Performance, May 20, 2017

³14 Fun Tour de France Facts You Can Use to Impress Your Friends, Molly Hurford, Bicycling, Jul 24, 2020

⁴Update of Cycling Aerodynamics, Science4Performance, May 20, 2017