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.

I actually used the Stan’s formula for years, running 30/32 psi until my wife showed me the error on my way. Now I run 18-20 up front and 20-22 on the rear tire. Magically, I‘ve gotten better getting up, over and through abrupt, rocky features.

This does vary by tire size also. Volume makes a big difference. A 2.1 tire vs a 2.6 tire are going to have a few psi difference in ideal pressure. My Salsa Rustler had 2.5 tires and I run 21 up front and 24 in the rear. I weigh 185 pounds. This works quite well for me. I also have 35mm internal width WTB rims so the tires are much more supported than say a 30mm internal width.

This formula might be an okay starting point, but there are a lot of other factors involved to make it this simple.

Also, if riding pump tracks or jumps, Sam Pilgrim says to run much higher pressure and you will have better smoother take offs because the tire won’t flex and cause you to go off track when pumping hard. He is right. It’s why I run 60 PSI on my DJ Bike.

imho, this is so general. on my XC bike at 105kg i run around 24 front, 26 psi rear on 2.25 tires. and play around that, which works for me really well. this formula doesn’t take in account bike’s weight, type of tires, what riding you do, rim width… which all plays major role in finding that perfect pressure. i found Srams tire calculator to be decent https://axs.sram.com/guides/tire/pressure

That’s a good one! Much more detailed form and you’re right, there are a lot of important factors that should be considered. Yet somehow I still get almost the exact same # from SRAM as the Stans formula, at least for my front tire (22.2psi from SRAM, 22.6psi from Stans) haha! At least the Stans formula is quick and easy.

I don’t have much experience with different tire width and rim widths. Do you use same pressures on different wheels(tires and different rim width) ? I’m curious especially what differences would be on 26/27.5/29 wheels. Hard to imagine they all would work same on the same pressure.

I have a 2020 Norco Sight and their Ride Aligned system puts me at almost 5 psi lower in the rear than the Stan’s formula. The Sram link that majstor.yoda shared was just about the same as Norco. They might be using the same system. Norco’s recommendations have worked out well for me with some experimenting.

Not disputing the formula – it is very close to the tire pressures I use. However, the URL in the story is from 2012 and no longer exists. Their FAQ doesn’t mention anything about MTB tire pressures and the only reference I could find was an article at https://www.notubes.com/stories/tire-pressure-how-to-find-the-perfect-tire-pressure

Again, the formula seems to work well and the pressures given are in the range I ride for my weight. But like the article says those pressures are starting points and you should play around with them to find what feels right on the trails.

Right, that reference has since been removed from the Stan’s website; we had to really dig to find the linked FB post from 2012 just to confirm! It’s not clear why Stan’s no longer promotes the formula. To me it does seem a bit high for today’s modern tire and rim widths. Using the formula I should be running 23/26psi front/rear but I tend to prefer 15psi front and rear on my 2.4s / 30mm rims.

Perhaps it’s time for an update to the formula. I propose weight divided by 8, or maybe 8.5?

Surprising how well that formula still seems to work – gets me in a small ball park of where I usually try to run my pressures: 25/26 for the rear, 22/23 for the front, but depends on the terrain. Usually I’ll go softer if there aren’t many rocks involved on the ride I’m prepping for.

I guess this is good for [Beginner Tips]. As others have already mentioned, tire size and rim widths have changed a lot since then, and it makes a definite difference in the pressures we run (often lower than those recommendations).

Kinda need to get out in the real world with a bike and rider paired up. Factor in rider skill as well as every other item mentioned or not in the previous posts.

Formaulas are dandy for crash test dummies.

This is a great tip for beginners. Funny to hear how you had to dig to find this formula. I posed the question in a forum on this site a few years ago and got all the hot air about depends on weight and tires and wheels and conditions…etc. With experience I know better how to adjust all this but this formula helped me get in the ball park. As we all ride more we know what we like tire width and seat height and lever position but new riders need a starting or reference point and this formula gets them close instead of them running 60-65 psi because the side of the tire says it. Keep these simple rules and short articles for newer riders. Make it easy for them to learn the basics and over time they will learn all the other stuff with experience and further reading when they are ready.

Interesting you quibble over 1-2 psi yet the calculation doesn’t include the weight of the bike itself.

I run 26 up front and 26 in the rear. I’m 185 lbs and love to jump and hammer rock gardens. Seems to work for me

Actually the pressure inside your tire does not change with the pressure outside the tire. Unlike a balloon, a tire’s volume does not change as the pressure inside increases. Once the pressure exceeds a few psi, the tire is fully expanded and volume is constant, as will be pressure until more molecules are added or the temp changes.

Temp effects pressure based on the difference in degrees Kelvin For example, if you pump your tires to 25 psi at 60F you will have about 8% higher pressure at 100F or about 27psi at the heat of the day. The one time to worry would be roadies on black tarmac which can get considerably hotter than 100(especially if using rim brakes), but MTBers need not clutter their minds with such worry.

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