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Bicycle Tires and Gases

Dateline: 05/15/00

By Alan Bruzel

Can filling one's tires with gases other than air provide a competitive advantage for bicycle racers? This article will not address the legal, ethical, or mechanical ramifications of such a maneuver, but will simply provide an amusing instance of the applicability of the Ideal Gas Law. The following demonstrates how different gases pumped into a bicycle tire result in different weights for that tire.

The bicycle tire under consideration– to be specific, a 26-inch diameter tire with a one-inch wide inner diameter – has a volume of about 62 cubic inches (about one liter).

Let's assume we will pump various gases into this tire at a pressure of 65 pounds per square inch (4.4 atmospheres or 448 kPa) and at a temperature of 300 kelvin (80^oF or 27^oC). What gas, under these conditions of volume, temperature, and pressure, would contribute least to the final weight of the tire?

When you think about weight, you should think about density. The bicyclist would have to move less weight if he chose the least dense (that is, the lightest) gas. There is a modification of the Ideal Gas Law equation, PV = nRT, which gives mass density: d=PM/RT. If we know the pressure (P), molecular weight (M), and temperature (T) of a gas (and we use the proper molar gas constant, R), we can calculate density (d). Substituting the appropriate values into d=PM/RT allows us to construct the following table comparing the masses of different gases in our bicycle tire. (For the molar gas constant, use 0.08206 L atm/mol K when working with atmospheres, and 8314 L Pa/mol K when working with pascals.)

Masses of various gases at 65 psi and 300 K
in a one liter volume bicycle tire:

The "molecular weight" of air is calculated from its constituents: 79 mole % nitrogen, 20 mole % oxygen, and 1 mole % argon. (The value 28.9 is the sum of 28.01 X 79% from N₂, plus 32.00 X 20% from O₂, plus 39.95 X 1% from Ar.)

By using hydrogen instead of air, the bicyclist sheds 4.8 grams from each tire. Unlike the other gases on the list, however, hydrogen is flammable. Therefore helium, with a savings of 4.4 grams per tire when compared with air, is recommended for the faint of heart.

Filling your opponent's tires with radon would add 34.5 grams to each tire (as compared to air-filled tires), but because of its scarcity (not to mention its radioactivity), quantity sources of radon are hard to find. This leads the ruthless (but resourceful and chemically astute) competitor to sulfur hexafluoride, an available industrial gas actually used by some manufacturers during fabrication to fill tires to give them extra impermeability.

What the Web Has to Say about:
Bicycle Tires and Gases

Bicycling
Home page of About.com's Bicycling Guide.

Consumption of SF6 and Substitution Possibilities
Uses of sulfur hexafluoride. From the Danish Environmental Protection Agency.

Gas Laws
Notes and interactive tools. From the Shodor Education Foundation and the University of North Carolina at Chapel Hill.

Ideal Gases
Examination of the Ideal Gas Law and some examples. From P.J. Brucat, University of Florida.

Mountain Biking
Home page of About.com's Mountain Biking Guide.

Nature of Gases
Collection of lecture notes dealing with the gas laws. From James A. Plambeck, Virtual University North.

Radon Progeny
An article from this Web site describing radon, its parents, and its descendants.

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