Mountain biking is an entirely different experience from taking a casual bike ride on a smooth road. When riding on a road, you'll want to limit the surface area of the part of your tires that touch the ground. This helps ensure a smooth ride.
But in mountain biking, you'll want more surface area rather than less. The increase in surface area creates a better grip on the terrain -- an important element when you're tackling a steep climb or navigating along an uneven path. That means you'll need to use less air in your tires. You can monitor this by measuring the air pressure within each tire, expressed in pounds-per-square-inch (psi).
Tire manufacturers mark the psi range recommended for their tires. There is no standard psi range for all mountain bike tires -- it varies by manufacturer and tire model. If you inflate your tires toward the upper limits of the psi range, riding on uneven ground could be an uncomfortable and bumpy experience. A rider of average weight should aim for the lower limit, particularly for the front tire. The ideal psi will vary based on several factors including the weight of the rider, the terrain and the psi range recommended by the manufacturer.
Imagine you've purchased a new set of mountain bike tires that recommend a range of at least 35 and no more than 60 psi. Begin by inflating your tires to the middle of the range, adjusting the inflation depending on your weight. If you're a heavier rider, you may want to start off at around 50 psi for each tire. Lighter riders can go lower and start off at 40 psi. Some mountain bikers will inflate their tires significantly below manufacturers' recommendations. Bikers who do this should be careful. An under-inflated tire can experience a pinch flat, which happens when a tire is caught in a crevice or crack and tears as the wheel continues to turn.
If you find that you're having trouble with your bike gripping the terrain, you can try letting out air in 3- to 5-psi increments. Test your bike's performance after each adjustment to find the level that works best with the terrain you're in. Keep an eye on how your tire looks when you're on the bike, too. If the tire rolls under the rim as you ride, you need to add air to it. And if you experience a pinch flat, your tire is definitely under inflated.
Also, you don't have to set both the front and back tires to the same psi. The psi on your front tire can be lower than that of the back tire because the back tire supports more of the rider's weight. Just remember to start by inflating your tires to the middle of the recommended range and adjust the inflation according to how much comfort and control you experience as you ride. Never over-inflate and be careful when testing lower inflation rates. Now hit that mountain!
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