Bicycle Aerodynamics and Cycling Performance

Overcoming Aerodynamic Drag

To improve cycling performance (increase cycling speed), the cyclist needs to optimize bicycle aerodynamics by mitigating the two primary components of aerodynamic drag: pressure drag and skin friction drag.

Pressure drag is the force created as the surface area of the cyclist comes into contact with air particles at increasing rates of speed. Skin friction drag is the force created as air particles closest to the surface collide with rough surfaces, which then collide with other layers of air further away from the surface.

Pressure drag is the primary competing aerodynamic force to be addressed by the athlete seeking to improve cycling performance. In fact, the bike rider accounts for 65% to 80% of aerodynamic drag directly. To overcome and reduce the forces of aerodynamic drag, a cyclist must find ways to reduce the frontal surface area coming into contact with air. Rider position on the bike, bicycle geometry and other aerodynamic equipment are the primary tools used to overcome aerodynamic drag.

Aerodynamic Position on the Bike:

Each of the three amateur cyclists featured in the photographs above (courtesy of Don and Dana McEwan) is seeking to improve their aerodynamic position on the bike in order to go faster. To varying degrees, each cyclist in taking advantage of the top five ways to reduce aerodynamic pressure drag:

-       Aero bars lower the riders’ position on the bike (all riders)

-       Aero helmets reduce aerodynamic drag by reducing the low-pressure zone directly behind the head (all riders)

-       Lowered head position reduces frontal area exposure (each rider to a varying degree is lowering their head position with rider 3 having the best position of the three)

-       Aero bicycles, also referred to as time trial bikes or tri bikes, further improve aerodynamic position (riders 2 and 3)

-       Hiding cables, bottles and brake components inside or behind the frame to maintain a low pressure zones for the rider (rider 3)

Reduce Frontal Area Exposure to Improve Aerodynamics

Reducing frontal area exposure is the primary concern of a cyclist seeking to improve time trial performance. A crouched position, akin to that of a skier tucked and going downhill, allows the cyclist to cut through the air and go significantly faster than if they were riding upright.

Let’s take a closer look at each cyclist featured above to see how well each is doing in reducing frontal area exposure in order to go faster.

Rider 1 has improved his position relative to riding upright on a road bike. He has attached aero bars to his road bike frame, and he is taking advantage of an aero helmet. His performance, however, is considerably compromised by his position on the bike. In part, this is due to the fact that his road geometry simply will not move his body forward and down enough to create an optimal time trial position. His head remains relatively upright and serves as a large surface area for the oncoming wind to drag. His water bottle, water bottle cage and cables are also creating a drag force. The result is a relatively large frontal area exposure and a significantly compromised cycling performance. Gloves, hairy legs and the apparent lack of shoe covers and a skin suit also slow his performance by creating skin friction drag.

 
Rider 2 is riding a time trial bike to improve his overall aero position on the bike. His back is relatively flat, and he is in a tucked position with a lowered head relative to Rider 1. Aero bars and an aero helmet reduce frontal area drag. The skin suit, shoe covers (not visible here) and cleanly shaved legs reduce skin surface drag. Even though this position is significantly better than Rider 1, there are still considerable opportunities to reduce drag and improve cycling performance. Rider 2 should concentrate on lowering his head by “turtling” the head whenever possible. This can be accomplished by looking down with some frequency during the race, which has been shown to reduce drag. This rider could also reduce drag by moving his hands up on the shifters and mitigating the “air scoop” effect created by his arms. Hiding the wires, removing the water bottle cage (not shown) and ditching the gloves will also help reduce aerodynamic drag.

 

Rider 3 has the best aero position of all of the amateur cyclists featured. Aero helmet, aero bars and time trail bike are the essential components of a faster position. But note how this rider, a cycling coach, has significantly mitigated his frontal area exposure by rounding his shoulders and lowering his head to reduce the total surface area exposed to the wind. His hands are also forward on the shifters and rotated upwards at a slight angle to cut through the wind, rather than scoop the wind as with Rider 2. This rider has also taken all the necessary steps to reduce other elements of drag. He is wearing a skin suit, removed his water bottles, taken off his gloves and shaved his legs to optimize his position and reduce both pressure drag and skin drag to go faster.

Reduce Drag, Improve Cycling Performance

Levi Leipheimer and Dave Zabriskie (above) are two of the world’s best time trialists. Look at each rider’s position: they have each gone to great extremes to mitigate frontal area exposure. With good reason: the aerodynamic benefits of reducing frontal surface area exposure – especially at faster speeds – are enormous.

 

This chart clearly indicates the advantages that an aerodynamic cyclist has in time trial competitions. The aerodynamic rider goes 18% faster than non-aerodynamic rider at the same force of 40 Newtons. Rolling resistance is constant while aero drag increases geometrically with higher speed. In fact, a 5% reduction in frontal surface area exposure results in the functional equivalent of 17 watts of power.

Next up: Top Ways to Reduce Aerodynamic Drag.

 

Comments

[Chris Eaves]

I've always wondered about the hairy legs thing. I get the road rash reasons for doing it, but do you know of any wind tunnel tests that have tried to quantify the drag reduction (if any).

[Ted]

Chris, reductions in aerodynamic drag from shaving the legs are a function of reducing skin pressure drag - a similar rationale to wearing a skin suit. Studies I have read suggest about a 5 second improvement over a 40K time trial resulting from clean legs. My source for this statement: http://www.sheldonbrown.com/rinard/aero/aerodynamics.htm

[jonathan kent]

at which speed beeing aerodinamic start to be important?
Thanks
Jonathan Kent