When you start plugging in numbers, you see that even a couple of millimeters’ extra width yields surprising differences in tire volume. The formula for volume of a cylinder is V = π r 2 h (where h is the height of the cylinder, which would be the same as the circumference of the wheel). Unroll an inner tube and it’s basically a cylinder. But a 28mm tire at 90psi is a bouncy mess, so you have to adjust air pressure downward. If you’re swapping from a traditional 23mm road clincher to 25- or 28mm tires, or from a 2.1-inch mountain bike tire to a meatier 2.3, you’re increasing tire volume significantly. Whether road or mountain, plus-size tires are all the rage right now. You should adjust it according to tire volume Rims can cool down significantly in even a few seconds when brakes aren’t applied. To keep tire pressure manageable, don’t ride the brakes. Ever feel like your bike is jarringly harsh on descents? Hot tires might be why. At that temp, a clincher originally inflated at 70 degrees to 90psi is actually closer to 115 psi. Sustained rim braking during long descents can boost the temperature inside the tube past 300 degrees. It's not just environmental temperatures that matter. Doesn’t sound like much, huh? But Poertner says even a 3- to 5psi drop can make a larger impact on a rider’s perceived vertical stiffness than the difference between, say, Specialized’s aero (but stiff) Venge and its all-around, more comfy Tarmac. For roadies, that means roughly 2psi for every 10 degrees up or down from whatever temperature it was when you inflated your tire. RELATED: The Mechanics of Perfectly Inflated Bike TiresĪ good rule of thumb is that a 10-degree change in Fahrenheit will increase pressure roughly two percent, says Silca’s Josh Poertner, who probably thinks about tire pressure more than anyone. Here are five things you need to know about tire pressure. But it’s also pretty simple: Because it’s free, you can experiment with it almost endlessly. Tire pressure is surprisingly complex, in that it’s affected by everything from your rim width to how much you brake on descents. Given how much cyclists think about things like upgrades, it’s amazing how easily we miss one simple and free way to improve how our bikes ride: getting the right tire pressure.
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