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Clydesdales/Athenas (200+ lb / 91+ kg) Looking to lose that spare tire? Ideal weight 200+? Frustrated being a large cyclist in a sport geared for the ultra-light? Learn about the bikes and parts that can take the abuse of a heavier cyclist, how to keep your body going while losing the weight, and get support from others who've been successful.

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Old 08-08-07, 09:31 AM   #1
AeroJoe
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Tire Pressures

A few tire pressure basics to remember- be careful comparing tire pressures with different size tires! The energy inside a tire is determined by its Pressure times its Volume (P*V). Therefore, for two different tires of the same wheel size, say a 700x23mm compared to a 700x35mm, (P23mm)*(V23mm)=(P35mm)*(V35mm). The volume is proportional to the cross-sectional area of the tire, and so also to the square of the radius of the tire cross section. That way, if we know the ratio of the tire radii squared, we can get an equivalent tire pressure (from an energy standpoint) for two different tires. For the example above, if we have a 35mm tire pressurized to 85 psi, the equivalent 23mm tire would have to have (35mm/2)**2/(23mm/2)**2 *85psi= 197 psi!!! (Conversely, a 23mm tire at 120psi is equivalent to a 35mm tire at 52 psi). The tire pressure is inversely proportional to its volume, so that the larger the volume of the tire, the less pressure is needed to get the equivalent amount of energy. ‘Nuff of the physics lesson- bottom line? Us big guys (and gals) are probably better off with a larger tire at maximum pressure to carry the extra weight, but small enough to get good rolling resistance (which is another physics lesson for another day!).
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Old 08-08-07, 09:54 AM   #2
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Is it just me, or does BF seem to have a greater proportion of Math and science geeks than the average internet forum? I'm not saying this is a bad thing, by any means......it makes me feel right at home! I'm just askin' is all!

OK, Hijack over and great post, AJ!
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Old 08-08-07, 06:12 PM   #3
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I think you forgot to factor in absolute pressure.
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Old 08-08-07, 06:42 PM   #4
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Originally Posted by Bill Kapaun View Post
I think you forgot to factor in absolute pressure.
Don't forget that you yawn to equalize he air pressure behind your ear drums. This slightly changes the air pressure outside your ear drums so you must let a little air out of your tires.
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Old 08-08-07, 07:03 PM   #5
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I just pump to the maximum pressure rating on the sidewall and ride. I paid for these tires, and someone else was paid to come up with the rating.
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