5 Things You Should Know About Tire Pressure
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. Tire pressure is surprisingly complex, in that it’s affected by everything from your rim width to how much you brake on descents. But it’s also pretty simple: Because it’s free, you can experiment with it almost endlessly. Here are five things you need to know about tire pressure.
RELATED: The Mechanics of Perfectly Inflated Bike Tires
It fluctuates as temperatures change
A 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. For roadies, that means roughly 2psi for every 10 degrees up or down from whatever temperature it was when you inflated your tire. 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.
It's not just environmental temperatures that matter. Sustained rim braking during long descents can boost the temperature inside the tube past 300 degrees. At that temp, a clincher originally inflated at 70 degrees to 90psi is actually closer to 115 psi. Ever feel like your bike is jarringly harsh on descents? Hot tires might be why. To keep tire pressure manageable, don’t ride the brakes. Rims can cool down significantly in even a few seconds when brakes aren’t applied.
You should adjust it according to tire volume
Whether road or mountain, plus-size tires are all the rage right now. 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. But a 28mm tire at 90psi is a bouncy mess, so you have to adjust air pressure downward.
Unroll an inner tube and it’s basically a cylinder. The formula for volume of a cylinder is V = π r2 h (where h is the height of the cylinder, which would be the same as the circumference of the wheel). When you start plugging in numbers, you see that even a couple of millimeters’ extra width yields surprising differences in tire volume.
Mountain bike tires are even crazier, especially when you start playing with plus sizes (a 27.5 plus system has essentially the same outer diameter as the nominally taller 29er, but with larger volume):
RELATED: A Plus-Size Tire Primer
These are starting points to play with, not absolutes. They’re a good general rule of thumb for how to think about tire pressure relative to size, but ultimately, what you end up with is going to be highly dependent on your riding style, your weight, and how you like your bike to ride.
Your pump gauge may not measure it correctly
Most floor pumps come with a gauge, but when we’ve checked various pumps against an independent gauge, they can be off by 10psi or more at 90psi. It’s particularly important when dealing with low pressures, as with wide mountain bike tires (or cyclocross racing tires), where even a few PSI difference can be outsize in terms of percentage of total pressure. If you want to ensure you’re getting an accurate pressure reading, an independent gauge is a good way to check your pump. Options range from an inexpensive Meiser gauge (about $15) to one of the super high-resolution, use-specific digital gauges from Kappius Components ($160). Gauge accuracy is most important if you’re using different pumps. If you only use one pump and it consistently reads low or high, that’s less of an issue as long as you know how far off it is.
You might be using too much
We’re constantly telling people to read the manufacturer’s recommendations, but a tire is one place where you want to take official advice with a grain of salt, or you’ll end up with vastly over-inflated tires. Tiremakers often print a pressure range on the tire sidewall, but it’s not designed for real-world use. In some instances, the pressure printed on the side is a max recommended pressure (say, 9 bar, or a teeth-rattling 130psi). Other times, the label will read “recommended” but the pressure has no bearing on real-world use (108psi on a 28mm Panaracer GravelKing, for example).
RELATED: Five Great Tires for Gravel Grinding
It doesn’t affect rolling resistance nearly as much as you fear
We’ve known for years that a wider tire actually rolls faster at a given pressure, due to less sidewall deflection. But there’s more to it. The rougher the surface, the more lower pressure helps. On unpaved roads or even chip-and-seal surfaces, which we generally loathe for their buzzy feel, a high-pressure tire will deflect off the surface, actually creating more rolling resistance than a lower-pressure tire that conforms to the imperfections.
Past that, even large differences in tire pressure in a given tire size have small impacts on rolling resistance—on the order of low single-digit watts, in terms of how much extra power you have to produce to overcome them. Relative to speed, rolling resistance increases linearly, as opposed to wind resistance, which is exponential. Long story short: In any tire, dropping a few PSI can yield big ride-quality dividends with almost zero impact on efficiency.
You Might Also Like
