Under Pressure: A Quick Guide To MTB Tire Pressure
In a sport where ‘marginal gains’ have professional athletes weighing their oats before breakfast, trimming bar tape short, and measuring every aspect of their bike, kit, and gear, we know how big the small details really are. One of the most overlooked and, at times, mysterious aspects of mountain biking, however, is something you should be doing before every ride. Your tire pressure can have as big of an impact on your ride as what you ride, so making sure you get it right is worth thinking it over.
The ideal tire pressure balances ride quality, traction, and rolling resistance; the ride is smooth, offers the ideal amount of rubber to the dirt, but not so much that it offers added resistance in the form of rolling resistance. Too high, and your ride quality suffers from tires bouncing off trail objects, which beats you up just as much as it slows you down. A few psi too much and you can also lose traction in loose gravel or in the sand. Too little air pressure, however, and you risk burping air, riding the tire off the rim, added drag, and even wearing out your tires prematurely.
Nowadays, nearly every rider will have tubeless wheels and tires. Tubeless offers much better insurance against punctures and pinch flats, plus a better feel on the dirt. If you do still have tubes, you’ll need to run a little more pressure that someone that has a tubeless setup in order to prevent those nasty pinch flats. ‘Snakebites’ occur when the ground and rim pinch the tube, usually from running too little air or taking a big impact. When in doubt, add 3psi for tubes.
How do you get it right? First, look at two things that will be relatively fixed; your weight, and your tires.
There are a few factors to consider when getting your mountain bike ready to roll. First, and probably the most important, is your weight. Lighter riders can run lower tire pressures, as they ‘squish’ the much less than a heavier rider. The best way to highlight this is to pull out a fat bike. With 4psi in both tires, a Clydesdale rider will have those tubeless tires just about flat to the ground, so much so they may even look like there is no air in there at all! Now, put a 100-pound kid on the bike, and those tires will barely compress at all. The effect is the same in your 29er tires, just less noticeable.
Of course, whatever your weight, it’s not balanced equally. Most of your body weight rests on the rear tire, which is why most riders run 1-2psi more in the rear to compensate. That’s why you’ll usually hear people refer to their tire pressure with two numbers, with the lower number always the front wheel.
Your tire’s volume also is a huge factor in how much pressure they need to perform optimally. Those who rode “back in the day” on 1.9” tires would run over 30psi, while today’s wider (and ever wider) tire and rim combos are designed to be run at lower and lower pressures. Your 2.25” tire will be just a little bit higher than your 3” ‘plus’ tire.
Pathway vs VST
Where you ride can change your ideal tire pressure, too. Many riders run slightly more tire pressure on the Vasa 25km as compared to a more technical and tight trail like the Vasa Singletrack. With higher speeds and few sharp turns, the advantage of lower psi (traction) matters less than the advantage of higher psi (less rolling resistance) on the 25km. But even here, we’re talking a 1-3psi difference, at most. Other terrain factors include rocks, sand, loam, or if the trail is muddy.
Finally, it’s your style that can often play a big role in your tire pressure. If you ride smooth and stay seated, corner conservatively, and rely on a steady and long power output, you may find more tire pressure to feel more efficient; riders who ride aggressively and need perfect traction for cornering and sharp accelerations may really need lower tire pressure to get the most out of their fitness.
Want to find out the perfect tire pressure? There’s no real formula, and the best way to figure it out is to get out there and ride! Start on the higher end of what you’d normal air up to. For a 160 pound rider, that might be as much as 28psi in a 2.25” 29er tire. From there, go ride, and when you feel sliding or bouncing, try to take a little air out at a time. You can use a digital gauge to take out a specific amount (think 1 psi increments), or just keep lowering pressure until it feels good. Then, check that pressure when you get home and try it again the next time you go. Some riders even make notes about their tire pressure, especially if they just picked up new tires or new wheels. They’ll track what psi they ran at a certain course, note conditions, and add it in the description of their ride on Strava or TrainingPeaks.
As a good general rule of thumb, a 150-pound cyclist will run between 21-23psi in a 2.25" 29er tire. If you weigh less, you may be able to go lower; weigh more, and you'll probably be closer to 25 as a good place to start!