As triathletes and time trialists we’re all familiar with the importance of aerodynamics. At least we’ve all heard that it’s important. However I still frequently meet folks who don’t really believe that it’s all that important in the real world (outside of wind tunnels) or that you have to be Fabian Cancellara for aerodynamics to matter.
I’m here to tell you it is REALLY important for everyone, and hopefully I can illustrate why. In these days of ubiquitous power meters, we’re all starting to understand pretty well how power relates to speed on a bicycle. You may well know your FTP and have a good idea of how fast you can go at FTP. You’ve probably also heard that most of your power output is used to punch your way through the air. All of us know how a headwind feels. So we probably all “get” that air is a major drag on speed. But how much of a drag, and can we really do much about it once we have a set of aerobars on our bike?
I found some nice tables which clearly illustrate the huge influence aerodynamics has on power output and speed in a book called “Bicycling Science” by David Wilson (2004). I referred to these in my previous article about TJ Tollakson’s new position. Aerodynamic drag is a function of frontal area (equivalent to a 2D projection of your profile from the front) and Cd (or drag coefficient). Cd is a function of the shape of the object, and how smoothly air flows around it. We want to minimize both factors to decrease the amount of power needed to sustain a particular speed.
So let’s look at how much power various setups need to go 22mph. In each case, rolling resistance and drivetrain losses will require about 50 watts additional to overcome – not much.
Power to overcome aerodynamic drag at 22mph:
- Rider on an upright 3spd: 345 W
- Rider in aero position w/ skinsuit: 176 W
- Rider on recumbent bike: 148 W
- Rider on a road bike with various fairings: 39 to 160 W
- Faired recumbent racers: <30 W
So we can see a huge difference here between the “Starbucks racer” and a regular aero position (nearly a 50% decrease in power), and then another huge jump down to the various faired designs. In each case the power required is a function of the Frontal Area x Cd. So reducing your frontal area is good (getting lower and narrower generally). It is pretty amazing to think that a faired racer requires <100 W total to go 22mph. Imagine what that would do for your bike splits!
Even more interesting (at least to propellor heads like me) is that ALL of the power reduction in the faired racers is due to reduced Cd, not reduced frontal area. In fact none of them have smaller frontal areas than a rider on aerobars. But where said rider might have a Cd of 0.9, the faired racers have Cd’s as low as .13, or 1/7th as much. So assuming similar frontal areas they require 1/7th the power at a given speed.
If we compare the upright Starbucks rig to your typical aerobar guy, some of the power savings are due to a reduced frontal area (frontal area reduced by 1/3) and the rest are due to a lower Cd (or better aerodynamic shape – Cd here is reduced by 1/4). Net net, it saves a whopping 169 W at 22mph. How much training would it require to add 169 W to your FTP?!
So here’s where I’m going with all this. If you could form yourself into a super duper streamlined aero position where you look like a bullet sitting on top of your bike, you might need only 100 W to go 22mph. All because of improved aerodynamics. Even if the position wasn’t optimal for power output, you might be much faster. And most of this gain would come from lowering Cd once you have your frontal area minimized. In other words, it would come from your SHAPE, and how well air flows around it.
Another table in Wilson’s book compares the Cd’s of different shapes with the same frontal areas. For example let’s look at various objects that all look like a disc of the same size from the front – i.e., they have the same frontal areas:
Cd’s of various objects:
- sphere: .47
- disc: 1.17
- open half-sphere (like a coffee cup going down the road rim-first): 1.42
- closed half sphere (flip the coffee cup bottom-first): .38
So the shape effect is huge: the “coffee cup” would require 3x the power needed to push a sphere down the road at the same speed, even though they have the same frontal area. Yet if you flip the coffee cup around, Cd becomes less than the sphere. And certainly more streamlined shapes can have Cd’s <0.1. That aero downtube on your snazzy carbon TT frame might have a Cd <0.1, whereas a round tube might have a Cd of 1.17. So all else equal, the round tube has 11x the drag of the aero tube (yes I am ignoring the fact that the downtube is at an angle to flow which affects the Cd). So my quick back of the envelope calcs say that the round downtube might require 7 watts at 22mph, whereas the aero downtube might require <1 Watt of your precious power.
For those of you whose eyes haven’t completely glazed over yet, here are the takeaways:
1: Aerodynamic drag is your enemy, and it packs a big punch.
2: Reducing frontal area is good, but so is improving SHAPE (reducing Cd)
3: Some of the more unusual aero positions (Obree, Superman, Mantis) probably all improve the rider’s shape and hence reduce Cd, maybe significantly.
4: The standard flat forearm aero position MAY more closely approximate the coffee cup shape than the sphere or a bullet.
5: Perhaps more attention should be paid to rider “shape” in addition to frontal area. There is only so much you can reduce frontal area and still be able to ride.
6: Your body aerodynamics are the SINGLE BIGGEST FACTOR determining your speed at a particular power output. Sure we all love aero frames and wheels and helmets, but your body drag far overshadows equipment drag.
7: All of this matters not if you can’t pedal the bike or stay in the position for the required distance!
8: What does matter is the fastest setup net net – the most speed per given power output that can be ridden comfortably and safely for the required distance.
Lots of $$ is being thrown at reducing the drag of frames, wheels, etc. But I still think there is more that can be done with rider positioning, even at the highest levels of competition, especially in regards to improving the rider’s aerodynamic shape. Certainly the increasing popularity and availability of wind tunnel testing addresses this to some extent. Perhaps even more valuable are carefully controlled open road or velodrome tests. If you have a power meter, you might just be able to run your own tests and figure out your optimal position.