Drafting behind race motos provides a bigger advantage than first thought

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It’s stage 19 of the 2018 Tour de France and Primoz Roglic has just broken clear of his GC rivals on the day’s final descent. The Slovenian had tried to get away on the last climb, the Col d’Aubisque, but it’s only on the descent to Laruns that he manages to open a gap.

He wins the stage some 19 seconds clear of his main rivals but his success is soon soured by an accusation from Tom Dumoulin, who finished in the chase group.

“He was flying downhill,” said Dumoulin. “Eventually I got dropped on a straight part just because he was on his tube and full in the slipstream of the motorbike. I was sprinting to his wheel and I couldn’t get any closer … It’s ridiculous really.”*

Roglic isn’t the first rider to be accused of riding in a moto’s slipstream and he certainly won’t be the last. Watch just about any bike race and you’ll see riders duck in behind a moto whenever they can, snatching precious energy savings wherever possible.

Now, new research out of The Netherlands and Belgium has revealed that following a motorbike confers a greater advantage than was previously thought. Enough for the research team to call for administrators to do more to “ensure perfectly fair races.”

This latest research comes from a collaboration between Eindhoven University of Technology and KU Leuven researchers, and software company ANSYS — the same team that confirmed the efficacy of Caleb Ewan’s super-low sprint position and showed that Chris Froome’s supertuck at the 2016 Tour wasn’t as aero as it might have looked.

Led by Professor Bert Blocken of Eindhoven University, the researchers used two methods — wind-tunnel testing and computational fluid dynamics (CFD) — to test how much of an advantage a rider will get when following a motorbike. According to the researchers, both methods returned the same, somewhat surprising result.

“The aerodynamic advantages are larger than we expected,” said Dr Blocken. “We first considered the reference situation of cycling on level road with no strong crosswind, head or tail wind. In this situation, a cyclist riding at 2.5m behind a motorcycle has up to 48% less air resistance.”

They tested a handful of other following distances too. At 10m, the reduction in air resistance is 23%, at 30m it’s still 12%, and even at 50m the rider still experiences a wind resistance reduction of 7%.

By using a mathematical model from a seminal research paper on cycling aerodynamics, Blocken and his colleagues were able to convert their air resistance figures into time savings. They suggest that, for a rider in a time trial at 54km/h, the following time savings can be achieved:

– 12.7 seconds per kilometre when drafting at 2.5m
– 5.4 sec/km at 10m
– 2.6 sec/km at 30m

While few riders have the opportunity (or audacity) to follow a moto close enough for an extended period of time, the savings are significant enough than even a brief stint in the slipstream can be significant. As Blocken writes “In races where sometimes seconds or centimeters decide who wins and who loses the race, these differences are very large and can be decisive.”

A CFD analysis of air speed around a moto and a following cyclist. The regions of lower air speed behind the moto mean reduced wind resistance for riders following.

The fact that following a moto confers an aerodynamic advantage isn’t exactly unexpected. But this study is valuable because it quantifies the magnitude of that advantage and how it varies according to following distance.

To some, these findings are reason enough to limit where motos can and can’t go during races. That’s certainly the view of Fred Grappe, performance director at Team Groupama-FDJ and researcher at the Sports University of Besancon.

“As the performance in cycling is significantly linked to these aerodynamic effects, it is necessary to determine a ‘protected area’ around the cyclist, inside which no motorcycle should penetrate more than a few seconds,” he said. “Scientific studies such as this one are able to determine with accuracy this specific ‘protected area’ which belongs alone to the cyclist.

“Knowing the importance of one second in a ranking, we can no longer ignore the importance of such a rule.”

This latest research is yet to appear in a peer-reviewed journal but Blocken told CyclingTips that he and his colleagues are currently preparing for publication.

Will these preliminary findings spark any action from the UCI or race organisers? Time will tell. Either way, stay posted to CyclingTips for a more in-depth look at this research once it’s been peer-reviewed and published.

*Roglic neither confirmed nor denied drafting behind the moto, saying “I can’t comment on things on which I don’t have influence over. I don’t see where the problem is.”

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