Bicycle tires often bear printed or embossed tire pressure specifications on the sidewall. Road bikes may indicate a narrow range, such as 95 to 105 pounds per square inch, or psi. Mountain bike tires may indicate a much wider range, such as 29 to 58 psi, notes bicycle product consultant Andrew Caxton of the Cool Biking Zone website. The proper mountain-bike tire pressure within this range depends on your weight and other factors.
Benefits
Proper tire pressure is essential to a good ride on a mountain bike, notes Pete Prebus on MountainBikeRiding.com. Proper tire pressure enables better traction when you're rolling over rocks and roots, he notes. You'll also enjoy a smoother ride, especially if you set your pressure on the low side. You'll avoid inner-tube pinch flats, which happen if the pressure is too low and the tire hits a sharp object. This can cause the tire and rim to pinch and puncture the tube.
Settings
If you have a normal 2-inch-wide mountain bike tire and weigh 100 to 150 lbs., Prebus recommends starting with 30 to 35 psi. Try 35 to 40 psi if you weigh 150 to 175 lbs., 40 to 45 psi if you weigh 175 to 200 lbs. and 45 to 50 psi if you weigh 200 to 225 lbs. Test your tires in action to see what suits your riding style. If you have a narrower tire -- 1.8 inches wide -- you'll need slightly higher pressure; if you have a wider tire -- 2.2 inches or higher -- you'll need lower pressure, the Bike198 website advises.
Function
Pump the tire to 45 psi with a floor pump containing a built-in gauge and try the bike out on your normal riding surface, be it a gravel path or a rocky trail. Lower the pressure 5 psi by pressing the center stem of the tire valve until it releases air and measuring the result with a tire gauge. Repeat the test ride, Bike198 advises. You should notice a softer ride and more "grip" of the trail. Repeat the process until you find the lowest pressure that avoids pinch flats, which you'll notice when you're riding over a rock or root and feel the rim bottom out on the obstacle and pinch the inner tube.
Expert Insight
Use your hand to press the tire to estimate its pressure so you are less reliant on a tire gauge, which may not always be readily at hand, Caxton suggests. If it barely deforms under your touch, you won't have the deformation of the casing you want for proper traction. You want to roll well, with a good "contact patch" of tire hitting the trail, instead of having the tire jump off every deformation. Loosen the nut on a Presta valve and press its stem, or directly press the valve stem of a Schrader valve, to release air and achieve a slightly softer tire.
Warning
Avoid setting your tire pressure too high, which can contribute to crashes. A highly pressurized mountain bike tire doesn't grip the ground well, Prebus writes. The rubber becomes hard and unyielding, which can lead to a wipeout on corners, for example. Prebus recommends asking buddies on group rides what pressure they are using to arrive at the what may be the best setting for a particular terrain or application.
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