This is the first of what will be an occasional column in which we’ll try to use very simple science to try to explain not just how people win bike races, but why. If you’ve got questions about bicycling science, let us know in the comments section below, and we’ll try to tackle them in future editions.
A few days after Astana’s Michael Valgren’s late breakaway win at Omloop Het Nieuwsblad, a Danish cycling journalist friend of mine sent me a link to a video of the end of the race.
The video shows Valgren’s move and an overlay of his real-time power data in the final kilometers of the race. And watching it got me wondering, why do attacks like this one actually work?
7 W/KG (497 WATTS) for 2 minutes and 10 seconds as Valgren attacked in Omloop Het Nieuwsblad. Wow????Eurosport?Strava
Posted by CyclingHub on Tuesday, 27 February 2018
I’ve been writing about bike racing for a decade, but most of the time I work as a physicist. So I’m interested in understanding this from a scientific perspective. What I was after was a quantifiable explanation of why it worked. Was there physical logic underlying Valgren’s move? Was Valgren just lucky, or is there something more happening that makes a move like this a savvy strategy?
It turns out Valgren definitely had physics working for him. In spite of attacking a breakaway stacked with some of the best classics riders in cycling — Greg Van Avermaet, Sep Vanmarcke, Zdenek Stybar, Philippe Gilbert, and Jasper Stuyven, among others — as soon as Valgren had earned a significant gap, his victory was all but assured. Here’s why.
At relatively high speeds, the vast majority of the work a cyclist does to maintain a constant speed is pumped into fighting aerodynamic drag, or wind resistance. Anybody who rides a bike knows this intuitively: the faster you go, the harder you have to work.
In fact, the power it takes to overcome drag actually increases in proportion to the velocity cubed. You double your speed, the power you have to put into overcoming wind resistance actually goes up by a factor of eight!
This is an easy experiment to do yourself if you have a power meter. Go find a straight, level road on a calm day and ride at a steady 10 miles per hour. (It takes so little power to do this you might find you can’t even maintain a steady speed.) Now double your speed to 20 miles per hour and watch the power. You might not see it increase precisely by a factor of eight, but it will be much more than doubled.
It turns out this effect works enormously to the attacking rider’s advantage in a situation like we saw at the end of Omloop.
In the video we see Valgren attack and his power spikes to well over 1,000 watts as he opens a gap, but he quickly dials it down to about 500 watts, which he holds, more or less steadily, for the roughly two minutes it takes him to finish the race. Holding 500 watts for two minutes is a big effort, but it’s not a superhuman effort by pro cycling standards. It’s likely any rider in the front group could throw down the same power, so surely with just a little extra effort any of them should have been able to chase him down, right?
We can use just a little math to show why this would have been much harder than it sounds.
One rider who gave it a go was EF Education First-Drapac’s Sep Vanmarcke. Vanmarcke initiates the first serious response to Valgren’s move with about 1.5 km to go, when the gap was around 10 seconds. We know Valgren covered the last 2km of the race in about two minutes, so he must have been going about 60kph. That means he’d cover the last 1.5 km in about 90 seconds. Starting from 10 seconds behind, a chaser like Vanmarcke would have to do it in 80 seconds in order to reel Valgren in, which works out to a speed of about 67.5 kph.
Here’s where the power to overcome air resistance comes in. The air resistance equation is actually pretty simple. First you multiply together a bunch of numbers that take into account the air density, the rider’s profile in the wind, and a drag coefficient that represents how easily air can flow around the rider. We don’t know what any of these were were, but for the sake of this discussion, it doesn’t really matter. We can guess they were more or less the same for everybody in the race, so for a quick estimate we can just ignore them, they cancel out of the equation. All things being equal, then, all that matters is how fast you go.
We know that the power to overcome drag increases as the velocity cubed, and we know it took Valgren about 500 watts to sustain a pace of about 60 kph, so we can estimate proportionally it’d take more than 700 watts to chase him down. This is more than 40% more power than Valgren — just to go a measly 7 kph faster. It’s also a number that is a whole lot harder to sustain for a minute and a half, especially after almost five hours of hard racing.
We’ve swept many details under the rug here — rolling resistance, friction in the drivetrain, any headwind or tailwind, differences in the riders’ profiles and aerodynamics — but these are small corrections compared to that hulking 40% difference in the power needed to sustain an attack and the power needed to chase down that attack from behind. And of course there’s also tactics and psychology, but most of those favor the attacker, too. Who wants to turn themselves inside out to chase down an attacker, only to lose the race in the final meters to the guy who grabbed your wheel? If an attacker can sow confusion in the group for even a handful of seconds, it can easily be enough to end the race.
The bottom line is this: At high speeds, small differences in how fast a rider travels rack up big differences in power, and quickly. If you’re in the breakaway at the end of a race, and you see an attack like this — even by a rider you might not consider to be a big threat — cover it right away. Every second he or she stretches that gap makes it more and more unlikely you will be able to close it. Likewise, if you find yourself in Valgren’s position in the closing kilometers of a race, a move like this turns out to be a surprisingly good gamble. Go for it! You’ve got physics on your side.
About the author
Dan Seaton has been photographing and writing about cycling for the better part of a decade. He has been a contributor to VeloNews, Grit.cx, and Cyclingtips. He also has a Ph.D. in physics and, when he’s not writing about bikes, is a solar physicist at the University of Colorado. He lives with his family in Boulder, Colorado.