Airing up a bicycle tire is not rocket science, but it does require more than simply pumping up a flat tire to full air capacity. Overinflated tires give a rough ride, but underinflated tires slow you down. The key is to find that happy medium of tire pressures that will deliver both comfort and speed.
Fifteen Percent Drop
According to Frank Berto, bicycle expert and editor of "Bicycle Quarterly," the optimum tire pressure gives a 15 percent drop, so that when the bicycle is bearing its full load, the tires sink by 15 percent from their original height. Calculating the pressure that will give a 15 percent drop is not easy. However, if you know the load on each of your tires, you can refer to Berto's chart (see Resources), which plots the air pressure for 15 percent drop for tires of varying widths. To find the load on each tire, place one tire on a scale and the other on a support of equal height to the scale so the bike is level. Put all equipment you plan to carry onto the bicycle and then mount the bike. You may need another person to hold the bike steady for you and read the scale. Turn the bike around and repeat this process to find the weight load on the other tire. Because the seat is usually toward the back of the bicycle, the load on the rear tire is typically about 10 percent greater than the load on the front tire.
Air Pressures for Different Situations
Narrower tires require higher air pressure to achieve 15 percent drop, and wider tires need less pressure. For rough surfaces, you will want a lower tire pressure that will act as a shock absorber for bumps in the road. For smooth surfaces, you can use higher tire pressures that will give more speed. Heavier persons will also require higher pressures for their tires to achieve 15 percent drop.
Overinflation
As tire pressure increases, the tire's contact patch with the road becomes smaller, reducing the friction, or rolling resistance, between the tire and the road. However, modern tires are so well designed that the reduced friction and increased speed gained by putting more air into the tire is negligible after a certain point. Unless you are on a very smooth surface, the ride becomes rougher as pressure increases, because at high pressure the tires transmit every bump in the road through the bicycle frame to the rider. This causes the bicycle to bounce, which is especially dangerous in turns and can cause the bicycle to wipe out. Decreased rolling resistance also means less traction, which leads to slipping and wipe-outs in turns. Additionally, overinflated tires are at increased risk of being punctured due to failure of the tire to conform to external objects when at high pressure. At high pressures you also get much faster flats that can leave you stranded. High inflation pressures require stronger and heavier rims, adding weight to the bicycle and making cycling more difficult. Stronger and stiffer carbon composite wheels are used by bikers who require higher pressures for performing stunts.
Underinflation
Underinflated tires also pose risks for the cyclist. With low tire pressure, the tire flattens against the road, creating a larger contact patch and giving more rolling resistance. This slows the bicycle and makes it more difficult to pedal. Tires are also more likely to come out of their rims, especially if you have wide tires on narrow rims. At very low pressures you can get "pinch flats," where the tire is pinched between the rims and the road surface, causing the rim to cut through the tire. For optimum tire wear and performance, it is imperative to avoid the two extremes of tire pressure.
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