Monday, May 21, 2012

Calculate Aerodynamic Drag: Part II


Every bicyclist has to overcome wind resistance. Most recreational bicycles in which the rider sits up have very poor aerodynamics. While newer bicycles are being designed with better aerodynamics in mind, the human body is simply not well designed to slice through the air. Bicycle racers are aware of the problem of wind resistance and over the years have developed techniques for reducing it. Bicycle designers and inventors have experimented in developing alternative bicycle designs and HPVs (human- powered vehicles) with an emphasis on better aerodynamic performance.

Charley "Mile-a-Minute" Murphy was an early cycling racer. His "mile-a-minute" feat was accomplished in 1899. At that time he traveled faster than the fastest automobile. Notice the large windscreen on the train in front of him which greatly reduced wind resistance.

Wind Resistance

Every cyclist who has ever pedaled into a stiff headwind knows about wind resistance. It's exhausting! In order to move forward, the cyclist must push through the mass of air in front of her. This takes energy. Aerodynmaic efficiency--a streamlined shape that cuts through the air more smoothly--enables a cyclist to travel much faster, with less effort. But the faster the cyclist goes, the more wind resistance he experiences, and the more energy he must exert to overcome it. When racing cyclists aim to reach high speeds, they focus not only on greater power, which has its human limitations, but also on greater aerodynamic efficiency.

Aerodynamic drag consists of two forces: air pressure drag and direct friction (also known as surface friction or skin friction). A blunt, irregular object disturbs the air flowing around it, forcing the air to separate from the object's surface. Low pressure regions from behind the object result in a pressure drag against the object. With high pressure in the front, and low pressure behind, the cyclist is literally being pulled backwards. Streamlined designs help the air close more smoothly around these bodies and reduce pressure drag. Direct friction occurs when wind comes into contact with the outer surface of the rider and the bicycle. Racing cyclists often wear "skinsuits" in order to reduce direct friction. Direction friction is less of a factor than air pressure drag.

On a flat road, aerodynamic drag is by far the greatest barrier to a cyclist's speed, accounting for 70 to 90 percent of the resistance felt when pedaling. The only greater obstacle is climbing up a hill: the effort needed to pedal a bike uphill against the force of gravity far outweighs the effect of wind resistance.

Calculate the Aerodynamic Drag and Propulsive Power of a Bicyclist

Fill in the information in the boxes.

Velocity is your velocity (mi/hr) as read on a speedometer.
+ (plus) is forward
- (minus) is backward.

Wind velocity (mi/hr) is - (minus) if it is a tailwind, + (plus) if it is a headwind (relative to the ground).

Weight is in pounds.

Grade is the angle of the slope. 0 is flat, 90 is a vertical wall.

Click on the "Calculate" button.

Notice the drag force and power required to keep you moving at a constant velocity.

Your velocityWind velocityYour weightGrade





The relative velocity is


The drag is


The power required to maintain a constant
velocity is





Calories per minute

This calculation requires a JavaScript-capable browser.

Notes on the calculator:
Please be aware that we've made some assumptions in order to simplify this calculation. For instance, this calculator does not take into account the body position (or size) of the rider in regard to wind resistance. In addition, other factors, such as the coefficient of friction are fixed. Also, if you put in "unrealistic" figures you will get unrealistic results. Finally, please be aware that the "Calories per minute" figure is assuming that the human body is 100 percent efficient--this is not the case (20 percent efficiency is closer). For a more accurate figure try multiplying the "Calories per minute" by a factor of five.

Reducing resistance

Frame builders and designers have been working on creating more aerodynamically efficient designs. Some recent designs have concentrated on shifting from round tubes to oval or tear-shaped tubes. There is a delicate balancing act between maintaining a good strength-to-weight ratio while improving aerodynamic efficiency. Improvements to wheels have made perhaps the biggest impact. A standard spoked wheel has been described as an "egg beater," creating many small eddies as the tire rotates--creating drag. Disc wheels, while generally heavier than their spoked counterparts, produce less wind drag and turbulence when they spin.

Aerodynamic Frame
This racing frame uses tear-shaped tubes
to reduce drag.

While improvements to frames and components have improved aerodynamic performance, the cyclist is the largest obstacle to dramatic improvement. The human body is not very streamlined. Body positioning is important; road cyclists use "drop bars" to allow themselves to reduce their frontal area, which helps reduce the amount of resistance they must overcome. Reducing the frontal area helps riders increase their speed and their efficiency over time. In addition to positioning, small details like clothing can also make a big difference in reducing "skin friction." Tight-fitting synthetic clothing is worn by almost every professional rider, both road and mountain. Many recreational riders are also wearing bicycle clothes for the improvement in aerodynamics as well as comfort.

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