The Mathematics of Drafting

The physics of cycling is a complex subject, but I will keep it simple for clarity. The equations and constants referenced here are drawn from the work of Crouch et al., 2016.

At a steady cycling pace, you generate just enough power to counteract the forces acting against you. On a flat, windless road, the power required for a solo cyclist to maintain a cruising speed (v) is determined by:

Two key coefficients vary with speed:

• Rolling Resistance (RR): This refers to the interaction between the tires and the road surface. Factors like mass, tire width, pressure, wheel size, and road conditions influence this coefficient.
• Air Resistance (AR): This is the interaction between the cyclist and the surrounding air, influenced by the cyclist’s size, shape, and the air they are moving through.

The power required to maintain a specific speed is illustrated here:

Cycling at 20 MPH demands five times the energy compared to 10 MPH. For casual cyclists, RR might be the primary force slowing them down, influenced by weight, tire size, and terrain. However, at common cycling speeds, AR becomes the chief factor, overshadowing weight and biomechanics, making reducing air resistance the optimal strategy for conserving energy.

If you’ve ever wondered about the unconventional attire of road cyclists (tight-fitting clothing, for instance) or the aggressive design of road bike frames, these choices are often aimed at minimizing air resistance.

AR explains why cyclists frequently ride in pacelines: positioning directly behind another rider can decrease air resistance by as much as 50%.

By drafting, you can achieve higher speeds with less energy expenditure.

Does Drafting Really Work?

In data we trust, all else is anecdotal.

Here’s a Strava trace demonstrating the effectiveness of riding in a paceline led by a strong tandem pair. Afterward, I rode home alone on the same mostly flat route.

The paceline averaged about 3 MPH faster than I could manage solo, with my heart rate notably lower during the group ride.

Measurements aside, the experience was undeniable: while drafting, I frequently found myself in my highest gear and often had to ease off the pedals to avoid overtaking the lead cyclist. My speed was dictated solely by the front rider. Conversely, my ride home was a struggle against the wind on a still day.

Why Is Drafting Less Critical in Running?

Next time you’re on a bike, pedal as hard as you can until you reach your highest gear. Maintain that effort for about a minute, then stop pedaling and see how far you coast. You might travel a quarter mile or more without pedaling. With favorable downhill sections, I’ve coasted over ten miles without pedaling at all.

Now try this while running. Push yourself to the limit, then come to a halt. How far did you coast?

Drafting is less significant in running for two main reasons:

• We don’t achieve high enough speeds.
• Humans can’t roll like a bike does.

The math can be intricate, but here’s a simplified equation for the power required for sustained running:

Here, Running Resistance replaces Rolling Resistance. While often referred to as the Energy Cost of Running (ECoR), I prefer to use Running Resistance to draw parallels with the RR in cycling. The Running Resistance coefficient is considerably higher than that of biking because humans cannot maintain momentum like a wheel can.

Although AR remains similar, the elevated ECoR means we can't accelerate sufficiently for AR to become a dominant force (except in strong winds).

Drafting does play a role in elite running, often utilized during record-setting events with the assistance of pace vehicles or teams to conserve energy and reduce air resistance.

For average runners, pacing another can provide more psychological benefits than aerodynamic advantages. When trailing a steady runner, we often find ourselves in a consistent rhythm that helps conserve energy.

Runners draft to shave off seconds, while cyclists draft to complete the ride.

Challenges of Pacing

Riding in a paceline enhances speed, and the concept is straightforward: ride within a bike length of another cyclist, match their pace, and let them lead.

However, I generally recommend riding solo for several reasons:

1. It Should Be a Workout!

   When cycling, the goal is often to exercise. Drafting might feel like cheating if your aim is to burn calories.

   Personally, I find drafting to be somewhat monotonous. My speed is still dictated by the front rider, which often leaves me coasting without feeling challenged.

2. Drafting Involves High Risk for Minimal Reward

   Justifying the risks for a mere 20% speed increase can be difficult.

   In a paceline, the cyclist ahead can obstruct your view. You may miss potholes, loose gravel, or other hazards in your path.

You have less than a quarter of a second to react before potentially colliding with the cyclist ahead. At 25 MPH, even if you manage to react quickly and brake, you risk locking up and being thrown over the handlebars.

If the lead cyclist swerves or stops suddenly without warning, an accident is likely. The same can happen with those behind you.

Responsible drafters do their best to enhance safety. Always ensure the cyclist in front is aware of your presence and comfortable with you drafting. Be prepared to communicate effectively, as pacelines rely heavily on trust and communication.

When I draft, I tend to develop a tunnel vision, fixating on the wheel of the cyclist ahead. This can lead to increased mental strain even though the physical exertion may decrease.

If you’re leading a paceline, always allow adequate time for hand signals and verbal cues. Discuss these signals and commands with the group before commencing the ride.

If you're interested in drafting regularly, consult local cycling groups. Some offer training sessions focused on safe paceline riding.

To Draft or Not to Draft?

While drafting can make high-speed, long-distance rides feel effortless, it does come with its share of risks. If, like me, you use biking as a means to enhance overall fitness, you might find drafting unnecessary and counterproductive.

For long rides, the primary benefit of joining a paceline is energy conservation. During organized rides, I often find myself forming impromptu pacelines with nearby cyclists. If you choose to draft, consider practicing in a group with experienced riders before your major ride, and always communicate your intent to the cyclist in front of you.

While it may be a low expectation, riding in a paceline can be one of the coolest experiences on a road bike. Engage in it if it helps you complete a ride more comfortably; otherwise, enjoy the benefits of riding solo for a more vigorous workout.

Interested in cycling and need guidance? Explore my other articles on introductory cycling: