The topic of tire pressure inflated while chatting with some new riders the other day, and one of them mentioned running 27psi — when they remember to check. This particular athlete fell in love with trails over the summer as a new pandemic hobby, and she now rides a couple times per week. She also weighs 125lb, and that piqued my attention as to why she chose to run such high pressure.
“Someone at the bike shop told me to,” she said. While bike shops are often the best source of information for most things MTB-related, this was clearly a misspoke notion.
My friend asked if that number seemed accurate and how she could determine a more precise pressure for her weight and riding style. There are countless ways to find the ideal pressure, and the result is inarguably subjective. A starting point that’s always worked well for me is to use a formula I found on the Stan’s NoTubes website when they first started making tubeless sealant.
That formula is (rider weight in pounds divided by 7) – 1 for the front tire and the same formula +2 for the rear tire. In the case of my friend, that’s 125 LBS / 7 = 17.9psi. So she would want to run roughly 17psi under the bars and 20 beneath the saddle. This formula has always worked well for me, with small tweaks to accommodate terrain, temperatures, and other important variables.
Following the above computation, the best way to determine your optimal tire pressure is to go ride. You may hit the trail and find that the front tire is too low for your body position and the dirt surface. If you’re rolling your version of fast down a favorite descent and you hear the rim strike more than once against rocks and roots you may want to add some air. Conversely, if the rim never bottoms out at max speed and you’re having trouble maintaining traction, you may want to lose 1-2psi and reassess. If you have appropriate tread for the trail surface and your riding style, the right pressure will do much of the work to help you achieve maximum grip and go faster.
Certain trails and weather conditions call for different tire pressures. A lot of folks like to drop their tire pressure slightly to achieve more grip on muddy trails and raise it for fast rocky tracks or jump sessions. The difference between those condition-specific pressures is often 1-3 PSI, and it’s important to know how much air is inside the rubber so you can adjust it in the right direction. Loads of riders check their pressure with the palm of their hand, using feel to determine how much air is inside. I tried this for several years, and when I actually checked the pressure with a gauge it varied by 3-5psi despite feeling the same as the prior pump. That discrepancy was too wide for me, and I now use a digital gauge to dial in the numbers before every ride. It adds 1-2 seconds to the ride-prep process, affords me some mental security on the rocks, and provides the ability to more accurately test tires and wheels.
Speaking of variables, there are a lot of things that affect tire pressure, and many more that are affected by the amount of wind inside that C-shaped rubber. Tire pressure changes with temperature and elevation, and you may need to adjust the numbers depending on where and when you’re riding. The pressure amount will affect the way the tire deforms over obstacles, in turns, and through compressions, how it grips, overall rolling resistance, bump absorption, puncture and rim protection, and the size of the traction patch, among other factors. Tire pressure is one of the most important pieces of pre-ride bike setup, and it’s worth playing around with different numbers to find what feels best for you.