The science behind lead-out trains
Top sprinters rely on their lead-out trains or a specific lead-out technique. Learn if this is that just about strategy, or is there science behind it?
Lead-outs are one of the most well studied racing techniques in cycling, perhaps because they are so exciting to watch as they play out strategically, mere metres from the finish line. While a solid lead-out train is a strategic team skill, there is a lot of aerodynamic theory behind the technique.
The general stats of aero advantage are pretty well understood by most cyclists who ride in a group. Overall, aerodynamic resistance — or drag — is about 90 per cent of the overall force working against a rider. Drafting helps reduce this by up to 40 per cent, which is more than significant, as many of us have experienced. We understand this advantage to go to the cyclist behind the leader, so when we want to recover or rest in the group we don’t go to the front.
The general idea behind a lead-out train follows this logic: The sprinter is at the back, getting full advantage behind his or her team, and can therefore reach top speeds while remaining relatively fresh for the final few metres. But the true aero advantage is a bit more complex. First of all, as Blocken et al have found, the leading rider in a peloton or lead-out train also receives aero advantage of up to 3.1 per cent. That means aerodynamic advantage in a group doesn’t just go one way: the rider on front is also receiving an aero advantage. Take that logic a step farther: if you get aero advantage from someone riding in front of you and behind you, that means that the best position to be is not at the very end of the train — where the star sprinter usually is — but in the second-last position.
Let’s look at that a bit closer: airflow over a rider is smoothed by a rider being directly in front of him or her. That’s what we understand as "drafting." But the airflow corresponding to the front rider is also smoothed by the presence of the second rider. This is because an area of turbulent negative pressure behind the front cyclist interacts with the "overpressure" in front of the second cyclist. (You can read more about aerodynamics and airflow here). This causes the negative pressure, or trailing pressure, behind the front cyclist to decrease.
So why is the sprinter at the back of the train if that’s not the position for maximum advantage? That brings us back to strategy. Because the advantage received from a rider behind you is so low compared to the advantage from a rider in front (a possible 3.1 per cent compared to a possible 30 per cent), during a chaotic final sprint a team is going to be most concerned with sheltering their star sprinter from the front, and use all available team members to do so. The team is usually working as hard as they can to get their sprinter to the front, to keep the pace high to discourage other teams’ attacks, as the worst situation tactically could be running out of fresh riders to offer shelter to the sprinter. The team will not sacrifice a rider to stay behind the sprinter. In this way, the tactics and reality of a race situation win out over the science.
Of course, another team's rider may be sitting on a star sprinter's wheel, attempting to get carried through to the finish line. That could offer the sprinter both the rider-in-front and rider-in-back aerodynamic advantage - but perhaps not a tactical one.
Team time trials, however, are a different story. Because there is more control over the playing field, aero advantage can be strategically planned out, taking into account both the advantage from in front and behind each rider.