Low air pressure in your bicycle tires can cause flats, and damage to your tires and rims. It will also cause you to slow down and pedal harder because more of the tire is exposed to the road surface. The air inside your tires is under a great deal of pressure and tends to seep out every day whether you ride or not. So it's important to check your tires before each ride to ensure the correct amount of pressure.
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Different bikes require different amounts of air. A mountain bike generally requires less pressure than a road bike because they need to absorb the shock of uneven terrain and provide more traction. Road bikes require higher pressure because they are designed to go faster and smoother on even surfaces. Each bike is different, and the amount of air needed varies by make and model, so don't base the amount you need from another bike. It is important to know what your specific one requires.
Know the recommended pressure for your bike. Look on the sidewall of your bike tire and find the recommended PSI (pounds per square inch). This will vary based on the bike and the tire size. Sometimes this is given in a range such as 35-65 psi on a mountain bike. Or it might give a maximum such as 120 psi on a road bike.
When inflating road bikes, get as close to the maximum limit as possible. For mountain bikes, you typically want a lower psi so your tires will absorb the shock of rough terrain such as dirt, tree roots and rocks. The amount of air needed also varies by the weight of the rider. For the same performance on the same tires, a heavier rider needs a higher psi, while a lighter rider needs a lower psi. Likewise, because your rear tire carries more weight than your front tire, you almost always need a higher psi there--generally 10 percent, according to Sheldon Brown of Adventure Cyclist magazine.
Once you determine how much air your tires need, use an updated pump to inflate them. Most pumps tell you the exact psi as you are pumping so you don't need to guess. Once the tires are pumped, feel them with your hands to make sure they are hard enough.
It is important to check your tire pressure before each ride, and make adjustments according to the terrain and road conditions you are going to have. Be sure to monitor the best air pressure for your bike too. If you get a lot of flats, you might be over- or under-inflating.