How Fast Do You Need to Ride for Aerodynamics to Matter?

It shouldn’t be a controversial statement to say that when you’re riding fast, aerodynamics matters. Still, many riders will constantly dismiss aero claims from bike brands as being “marketing BS.” Such riders will largely point out that most aero testing for modern road bikes is done at 28 mph, occasionally faster, which leads many riders to feel that aerodynamics isn’t all that relevant to their riding and therefore can be dismissed.

Over the last five years, though, that argument has gotten harder to make. Pro racing has made that pretty obvious. Teams are no longer saving aero bikes for flat sprint stages and time trials; now they’re regularly choosing them for climbing stages too. That shift didn’t happen by accident. It happened because teams and manufacturers have learned, over and over again, that aerodynamic drag still accounts for a large share of the resistance a rider must overcome, even at speeds most people would not consider especially fast.

I’ve been playing around with a few cycling simulation tools lately, and one of the more interesting ones is wattsthedifference.app. It has a graph that effectively visualizes the forces acting on a rider and how they relate to each other over the course of a ride. Instead of just spitting out a speed or power number, it breaks resistance into categories like drivetrain losses, rolling resistance, gravity, acceleration, and aerodynamic drag, then shows how the percentages of each shift depending on the terrain and the rider’s speed.

Using that software, I set up a simple test: a rider on their own, riding on the hoods, holding a constant power on a mostly flat 24.9-mile course with 217 feet of total elevation gain. The goal was to find the point where aerodynamic drag stopped being the main thing slowing the rider down, in other words, the speed where aero dropped below 50 percent of the total resisting force.

The answer was lower than you might expect.

At 24 mph, the graph is almost comical in how completely aero dominates. The blue band representing aerodynamic drag takes up the vast majority of the chart almost the entire way, with only brief interruptions from small rises in the road. At that speed, air resistance is plainly the main problem.

At 18 mph, the picture changes a little, but not much. At this speed, the air is still taking the biggest cut of your effort.

At 15 mph, things get more interesting, because this is around the point where many riders would stop thinking of speed in “aero” terms. But the graph suggests otherwise. Aerodynamic drag still appears to account for roughly 60 percent of the total resistance for much of the course. That means aero isn’t just relevant at 15 mph; it’s the biggest single factor slowing the rider down.

Even at 12 mph, aero is still a major part of the equation. It no longer dominates in the same overwhelming way, but it still accounts for a very large share of the total resisting force.

To be clear, this is not a claim that every rider should run out and buy an aero bike. First, there are some obvious exceptions. On steep hills, gravity can become the dominant force, especially as speeds fall. On gravel or rough roads, rolling resistance increases, pushing the aero crossover point higher. And in a headwind, aerodynamics matters at even lower ground speeds, because what really counts is your speed relative to the air.

Second, there are also a few caveats worth mentioning about this data, mainly that this was a simulation, not a wind-tunnel or a field test, and it reflects a single-rider setup on one mostly flat paved course ridden at steady power. Change the rider’s position, body size, clothing, tires, road surface, wind conditions, or pacing, and the exact point where aero drops below half of the total resisting force will move around.

Still, I’ve spent a few months now playing around with this particular simulation and plugging in my own data, and it’s been able to predict segment times for me down to a few seconds.

So while this shouldn’t be read as a universal speed/aero threshold that applies to every ride for every rider, what it does show is that on normal paved roads, aerodynamic drag is a major part of what’s slowing you down. Things are obviously messier and more complicated in the real world, but I would argue that, if you’re a rider that cares about performance, aerodynamics should be something you’re thinking about, even if you’re not riding at professional speeds.