When Cycling Becomes a Drag: The Aerodynamics of Cycling
Cycling, like all other modes of transport, is not immune from the effects of drag. But there are a few things cyclists can do to reduce aerodynamic drag and save precious time, and energy, while in the saddle.
How does aerodynamics affect cyclists?
Apart from gravity, and perhaps an odd aggressive dog here and there, air resistance is the main issue cyclists face when traveling at speed — at least on a flat road, at low altitude, and with little wind. Even on a perfectly calm day, it is impossible for a cyclist to avoid the effects of aerodynamics — especially when traveling fast.
In fact, above speeds of around 10 mph (16 km/h), air resistance (drag) is the dominant force a cyclist needs to overcome. When speeds reach in excess of 30 mph (48.2 km/h), somewhere in the region of 90% of a cyclist's muscle power is used to blow through this invisible force.
Doubling your speed from say 20 to 40 mph (32 to 64 km/h) actually increases drag considerably more than you might expect — by around 8 times.
But even at lower speeds, air resistance consumes a fair amount of the energy a cyclist uses to keep moving. At around 10 mph (16 km/h) somewhere in the region of 50% of their power is used to overcome air resistance.
This phenomenon is called aerodynamic drag, it is a very interesting subject in its own right. Aerodynamics, in case you are not aware, is the study of properties of moving air and its interactions with solids moving through it.
By understanding the principles of aerodynamics, cyclists, and bicycle designers can help to make cycling that little bit more fun.
Cyclists tend to face two main types of drag when on the saddle. These are air pressure drag and direct friction, or skin friction drag.
The former is the result of a cyclist, and their bicycle, slamming into air particles as they travel forward. These particles become compressed towards the front and then become more spaced out as they flow over and around the cyclist.
This creates a pressure differential between the front and back of the cyclist, which imparts a drag force, or coefficient. This can be reduced by using aerodynamical shapes which minimizes the difference in pressure. This allows air to move more smoothly over, and around, cyclists at speed.
The clothes cyclists wear, and their very skin, traps a layer of still air close to the body at all times. When on the move, the air they push through is moving relatively free and fast in comparison.
Any transition between these two bodies of air imparts another kind of friction that also creates a drag coefficient — direct, or skin, friction. Texturized suits that act much like the dimples on a golf ball can be utilized to counteract this. Rough surfaces like these increase the skin friction effect and make the air more turbulent near its surface.
This enables air particle transfer between the two layers to be more efficient which allows the flow of air to remain attached longer over the surface, thereby reducing drag pressure overall.
How is aerodynamic drag calculated?
To get a fuller appreciation of the topic, it might be useful to briefly explain how aerodynamic drag is calculated.
Drag is calculated using the following equation:
FD = Drag force
= The density of air
A = Frontal area of the cyclist on a bike
CD = Drag coefficient
V = Velocity
The first component, FD, represents the force of drag that a cyclist experiences when moving themselves and their bike through a body of air at speed. To calculate this, we need to know a few things like the density of air (which decreases with altitude), the area of the bike and cyclist presented to the air, the drag coefficient, and, of course, the speed of travel.
As the density of air decreases at higher elevations, cyclists will experience less drag at the top of a mountain when compared to sea level. With regards to the area of the bike and cyclist, the smaller this is, the smaller the drag force — for obvious reasons.
The drag coefficient will depend on a variety of factors like the speed and surface roughness of the bike and cyclist and needs to be measured directly.
As you can see from the equation, aerodynamic drag increases as the square of the velocity. This means that if you double your speed, you will need four times the energy to overcome the drag.
Expressed another way, for every increment of extra power you apply to the pedals, your increase in speed becomes progressively smaller.
For this reason, finding ways to reduce drag is as important for cyclists as it is for aircraft or race cars.
What are some things cyclists do to reduce friction?
As you can see, drag can significantly influence the amount of energy consumed, and time on the saddle for any cyclist. While this might not be a problem for those cycling for fun, for professional or racing cyclists, every second and every calorie matters.
For this reason, there are a few things cyclists can do to help reduce the impact of aerodynamic drag.
1. Get yourself an aerodynamic bike
One of the best ways to reduce drag when cycling is to grab yourself a well designed, air-cutting bicycle. Bikes account for somewhere in the region of 30-40% of a cyclist's total drag.
Specially designed "aero" bikes can provide a cyclist with considerable drag reduction. For example, Cannondale's SystemSix has been found to save an amazing 2 minutes in time over a 40K time trial compared to a non-aerodynamically adapted bicycle.
Low profile wheels, using brake discs instead of pads, and specially designed handlebars, all provide modest, yet important, reductions in drag.
2. Aerodynamic cycling helmets also help
Another way cyclists can reduce air drag is by using aerodynamically designed helmets. These are streamlined, backward-pointing helmets that you've probably seen in competitive cycling before.
Aero helmets are excellent at reducing drag but are often not the most comfortable of things to wear.
They also tend to act like a large sail when a cyclist looks down due to the "spoiler" at the rear of the helmet. More modern aero helmets, like the Giro Vanquish and the Specialized S-Works Evade II, are excellent choices for those cyclists looking to reduce drag to a bare minimum.
3. Cyclists can even wear aerodynamically designed clothing
Yet another way cyclists can reduce drag is through specially designed clothing. Full-body skinsuits, akin to those worn by track-running sprinters, can help save cyclists a lot of effort, and time, when cycling at speed.
In some circumstances, this kind of clothing can reduce drag by as much as 10%. Some modern improvements include something called vortex generators, which are a series of tiny wings in strategic places like the shoulders, upper arms, and elbows, as well as thighs.
These tiny wings help mix the air as it flows over a cyclist's body and keeps it closer to the body to reduce drag. Other adaptations include aligning the seams of such clothing with the airflow, as well as, ensuring a skin-tight fit and smooth surface texture.
4. The wheels of a bike can also be designed to reduce drag
Well designed aerodynamically designed wheels are another way that cyclists can reduce air resistance. However, the savings made are only as good as the choice of tires too.
Some research has found that the best choice is 60 mm to 90 mm rim depth.
5. Cycling posture is one of the best ways to reduce drag on a bicycle
The cyclist, taking up a fairly large area relative to the bicycle, is the major reason for aerodynamic drag. By some estimates as much as 60 or 70% of it.
By lowering your posture while cycling, drag can be drastically reduced, as the amount of frontal area presented to oncoming air is reduced. However, there is a trade-off with this strategy -- there is such a thing as too low.
Some studies have shown that compared to the classic upright cycling position (back straight with your hands on the handlebars), putting a cyclist's hands lower on the handlebars of the bicycle and pushing their body close to the bike can make significant savings in drag.
However, there is a "sweet spot" for drag reduction that involves having your hands on the hods, with your arms bent and forearms parallel to the ground. This posture creates around a 13-14% reduction in drag compared to the classic riding posture.
However, lowering the body like this does have an effect on the cyclist's physical performance too. Breathing and power production can be reduced to such a degree that the savings in drag can be canceled out.
Finding the right position is, therefore, something of a trade-off.
And that, bike-fans is a wrap. We hope your know have a basic appreciation for how aerodynamics, and drag, affect cyclists, and how to overcome it.