After several stages run along coastal roads where the threat of crosswinds is obvious, it was ironic that the stage where the wind finally tore the peloton asunder took place about as far away from the sea as it is possible to be. Let’s have a look at what happened and why.

Stage 13 between Tours and Saint-Amand-Montrond was one virtually everyone had chalked off as a dull transition stage, a necessary evil to get the race from one side of the country to the other in three days. Other than one early cat 4 climb, this was the perfect day for the sprinters to enjoy a day in the sun and for the GC contenders to get their picnics out and enjoy a (relatively) leisurely ride. Or so we thought.

Cycling, however, is nothing if not unpredictable. As we often say, it’s the riders who make the race, not the parcours. This stage averaged 47.2kph – low-40s is more typical for a flat profile – and with a consistent 20kph wind coming from the riders’ left practically the whole way. Two separate moves showcased what can happen when a team puts its mind to turning a calm procession into an episode of Wacky Races.

What happened?

The first of these occurred with barely 60km gone, as Omega Pharma-Quick Step accelerated and split the peloton into three echelons – diagonal formations stretched across the road – leaving Mark Cavendish’s key sprint rival Marcel Kittel out of the front group. It took the best part of an hour of full-on effort – aided by Belkin after Alejandro Valverde stopped with a broken wheel – before they finally snapped the elastic and established a large enough gap to stay away for good.

The second attack saw Saxo-Tinkoff take advantage of Chris Froome’s isolation and a general lull in proceedings to make a similar attack with 32km remaining. Cavendish barely made the split, later explaining:

Riding in echelons is similar to falling through ice. You’ve got five seconds to rectify it, to save yourself, or it’s over. I did more work to be in the front group than I did in the [final] sprint.

Froome was less fortunate. Without teammates to help him and with other riders around him either unwilling or unable to contribute, he opted not to risk burning himself out with a solo effort. By the time Sky and other teams had organised themselves, the gap was too great and the lead group’s persistence meant they were able to first maintain and then increase their advantage all the way to the finish. Froome ended up losing 69 seconds to several of his GC rivals.

Aero 101

In normal conditions where there is no significant crosswind, the peloton adopts a familiar arrow-head formation. This is the most aerodynamically efficient way for the bunch to ride. The men at the head of the bunch effectively act as a shield for those behind. By cutting through the air in front of him, the front rider creates turbulence which results in a low-pressure area (more colloquially, a ‘hole in the air’) immediately behind him. A rider following closely is not only shielded from the airflow but is sucked along by this low-pressure space – called ‘drafting’ – which means they can maintain the same speed with less effort, as much as 40% less.

If you watch riders in the peloton closely, you will often observe a difference in their cadence depending on their position in the bunch. The rider at the front may be turning the pedals at a normal tempo, but those in the middle of the pack may appear to be barely turning their legs over – aerodynamics in action. Equally, a rider who has dropped off the back will be pedalling furiously just to keep pace. It’s also why drafting behind cars or motorbikes is not permitted. Following another cyclist creates a significant aerodynamic benefit – this effect is multiplied when following a larger vehicle.

Equally in a team time trial the riders adopt a tight in-line formation, keeping as close to the man in front as possible to maximise the drafting effect. Each man takes a turn on the front – where they encounter the greatest air resistance – before peeling off and neatly slotting into the back of the line, where he can rest and enjoy the benefit of having others in front of him punching through the air.

Interestingly, although the front rider has the hardest job, having another rider behind him also creates a smaller but reciprocal aero advantage, meaning he can achieve a higher speed with the same power output than if he was riding on his own. Add all these factors up, and it’s no surprise that solo or small breakaways rarely succeed – there are fewer riders to share the workload, and less aerodynamic benefit to gain versus a large peloton.

The effect of crosswinds

The established method for riding in crosswinds combines the same principles used in both peloton and team time trial riding, only the optimum formation is different. When the wind is coming from, say, the left, the low-pressure sweet spot which normally sits directly behind each rider is instead located to the right and slightly behind them. Therefore the lead rider hugs the left-hand side of the road to act as the most effective wind-break, with the next man to his right with his front wheel slightly overlapping his teammate’s rear wheel – the optimum position depends on the exact wind direction – and so on to the back of a diagonal line.

Then, as in a team time trial, after a short high-effort stint the front rider peels off and the rest of the line shuffles across to allow him to slot into the rear-most position, and so the rotation continues. Everyone except the man currently in front receives the maximum possible aero benefit and uses the least possible energy.

Once the bunch has been split into two or more echelons, it is difficult and sometimes impossible to bridge the gap. The effect of a strong crosswind is similar to that of riding into a headwind – physically even a small increase in speed requires a disproportionate increase in effort and psychologically the frustration of being unable to close an apparently small gap quickly becomes disheartening. It’s difficult for a group to achieve – for even the most talented individual it is nigh on impossible to overcome the disadvantage of being one man with no aero gains versus a group enjoying mutual benefits. You just don’t see a single rider bridging across to a larger group in echelon conditions. Hence Cavendish’s comments about having only five seconds to react – any longer than that and the additional effort required to bridge the growing gap becomes too much for one man alone.

There are other secondary factors at play too. For instance, where crosswinds are present, everyone understands the importance of staying as near to the front as possible. This raises the speed of the peloton, as no one wants to be the one left hanging vulnerably off the back. Higher speeds dull everyone’s legs slightly, making it that little bit more difficult to respond in those critical second after a split occurs.

As a rider facing crosswinds, a moment’s inattention by oneself or even the riders immediately in front can have a disastrous effect, much more so than in normal conditions, as Valverde (who dropped ten minutes) and to a lesser extent Froome discovered to their cost. Crosswinds are one of the biggest racing threats a top rider has to deal with, but they can also provide exciting racing on the most innocuous of stages. And the sight of echelons in full flow must surely be one of the most beautiful sights in cycling. Unless you’re caught behind the front group, that is!