Why Do Cyclists Ride in a Pack? Drafting, Aerodynamics and the Tour de France
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A professional cyclist riding alone at 45 kilometres an hour is spending about 90 percent of their power on moving air out of the way, and the same rider tucked a wheel's length behind another is spending a third less for the same speed. That one fact explains almost everything about how bicycle races look: the bunch of 150 riders that rolls through the French countryside is not a crowd but an engine for saving energy, and the tactics of a three-week race are the tactics of who spends and who hides. The physics was measured in wind tunnels in the 1970s, and the riders had known it since the 1890s.
The wall of air
Air resistance rises with the square of speed, and the power to overcome it with the cube, so doubling speed takes eight times the power. At walking pace a cyclist barely notices the air; at 30 kilometres an hour it is the main resistance, and at 50 it is nearly everything, rolling friction and the bearings taking only a few percent. The drag depends on the rider's frontal area and shape, which is why racers crouch, wear skin suits and shave their legs, marginal gains worth a percent or two each, and it also depends on what is in front. A moving cyclist leaves a wake of turbulent, slower air behind, and a rider sitting in that wake meets less resistance; the saving is about 30 percent directly behind one rider, more in the middle of a bunch, where measurements in a wind tunnel with 121 model riders in 2018 found that riders in the belly of the peloton experience as little as 5 to 10 percent of the drag of riding alone.
The peloton
The main bunch in a road race, the peloton, is therefore a shelter, and its shape changes with the wind. In still air or a headwind it bunches wide and the leaders rotate, since the front is the only place where the work is done; in a crosswind it strings out into diagonal lines, echelons, each rider sheltering behind the one ahead and to windward, and a rider caught at the back of a full echelon, with no shelter, can lose minutes in kilometres. Teams ride together to control it:
- •Domestiques ride at the front to set the pace, fetch bottles and shelter their leader, who sits behind them saving energy for the moment that matters
- •A breakaway of a few riders escapes early and shares the work, hoping the bunch will not bother to chase, and is usually caught before the finish because a bunch working together is faster than any small group
- •The lead-out train, a line of teammates riding flat out in the last kilometres so that their sprinter, tucked behind the last of them, is delivered fresh to the final 200 metres
- •The team time trial, where nine riders rotate at the front every few seconds and the team's time is taken on the fourth or fifth across the line
Where it does not help
Drafting matters least where the race is decided. On a mountain climb at 20 kilometres an hour the air resistance shrinks and gravity takes over, so that the peloton breaks apart and the strongest climbers, who have the highest power for their weight, ride away on their own; the Tour de France is won in the Alps and the Pyrenees, and the flat stages that fill the first week are decided by sprinters because nobody else can escape the bunch. The individual time trial, ridden alone against the clock, is the other decider, and it is where aerodynamics is purest: the bikes have deep wheels, the riders lie along low handlebars, and the helmets are teardrops, all to cut the frontal area that in the peloton someone else's back had been cutting for them.
The numbers
A Tour rider produces around 250 watts averaged over a five-hour stage, spikes of over 1,500 in a sprint, and about 400 sustained for half an hour on a summit finish; a fit amateur manages perhaps 200 for an hour. Over three weeks and 3,500 kilometres the riders burn 5,000 to 8,000 calories a day and eat continuously on the bike. Shelter changes those figures by a third, which is the difference between finishing a stage and abandoning the race, and it is why a rider dropped from the bunch on a windy flat stage is usually out of the race by evening: alone, they cannot hold the speed the bunch holds together.
Elsewhere
The same physics runs through every sport in which bodies move through air or water. Speed skaters and runners tuck in behind a leader, which is why paced world-record attempts use a rotating team of pacemakers and why the sub-two-hour marathon of 2019 was run behind a phalanx of them in a formation designed in a wind tunnel; birds fly in a V so that each rides the upwash from the wingtip ahead, saving up to a fifth of their energy; racing cars slipstream on the straights; and lorries on a motorway follow each other more closely than is wise for the same reason. The peloton is only the most visible version, a hundred and fifty people who have agreed, without any of them saying so, to take turns in the wind.
The takeaway
Cyclists ride in a pack because at racing speed almost all their effort goes into overcoming air resistance, which rises with the square of speed, and a rider in another's wake saves about a third of that effort and one buried in the bunch far more. The peloton is a shared shelter whose shape follows the wind, teams use it to protect leaders and deliver sprinters, breakaways are caught because a bunch is faster than a few, and races are decided where drafting counts least, on climbs and in time trials.