When you're bombing down a particularly difficult track, the last thing you want is a tire slipping out on you. The right tire pressure is essential for a mountain bike's traction and control, and changing it to suit the conditions on the trail gives you the ability to better handle your mountain bike. With a portable pump, you can even adjust tire pressure on the fly for a better riding experience. Mountain bike tires are designed specifically for biting into loose and unstable terrain. The rubber lugs on the outer tire act like teeth, biting into sand and dirt and providing traction. This traction increases at lower pressure, since more of the tire is in contact with the ground.
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Tearing Up The Trail
Mountain bike tires come two varieties. Clincher tires use a tough outer tire and a flexible inner tube to make replacing a flat easier, while tubeless tires use an integrated inner tube that use a sealant to self-repair most punctures. Tubeless tires can be run at lower PSI than clincher tires, since the inner tube of a clincher tire is prone to pinching between the rim and outer tire at low pressure. The tubeless tire is much more resistant to flats overall, but repairing a flat tubeless tire can be difficult or impossible on the trail. Different sized tires have different ideal tire pressures, but generally, wider tires perform better at lower pressures than narrower tires.
The contact patch of the tire is the main principle that determines the traction of a tire at different pressures. The contact patch describes every part of the tire in contact with the ground. A tire at high pressure will resist deformation from your body weight or the weight of the bike, making the contact patch relatively small. At lower pressure, the tire can spread out over more ground, putting more lugs in contact with the terrain and increasing traction.
Finding Your Balance
Determine what the proper tire pressure for your mountain bike is. Over time, get to know how your tire behaves under stress and feel the difference between tire pressures to find a sweet spot. With a properly inflated tire, a mountain bike with a rider should distort slightly, flattening out along the ground and bowing out at the sides. A tire that deforms under pressure can properly absorb the bumps of smaller obstacles like roots and rocks, giving you better control and a higher rolling speed. However, particularly rough terrain with sharp, chunky rocks can damage your tire's sidewall or your rim at low pressures, so you'll need to keep the pressure high enough to maintain a cushion between your bike and the terrain.
Staying Cool Under Pressure
If you're looking to get the most out of your tire, you should adjust the pressure frequently to find the right balance between speed and traction depending on the terrain you expect to ride on. Finding the right pressure is a learned skill; you'll be able to feel what works best after a few rides. On smooth trails with few obstacles, a low pressure will give you the speed and traction to move fast. Loose sand or gravel can make high tire pressure a liability, since your traction will decrease. Most mountain bikers prefer a lower tire pressure, even below manufacturer minimums, since a lower tire pressure will give the bike a softer feel that is ideal for control and handling.