Tire Pressure Demystified
Don’t be shy to mess with your PSI!
As a Mountain Bike Coach in Hood River, OR, I start all my mountain bike clinics with an “ABC” bike check. “A” stands for air. Specifically, how much air is in your tires. Ideally, I don’t want the bike check to take longer than 5 min, so I say, “Squeeze your tires. They should feel like an orange, or somewhere between a rock and a 5 day old birthday balloon.” I then quickly move onto B for brakes, and hope someone doesn’t ask, “Well how do you know how much tire pressure to have?” It’s not a bad question. Understanding tire pressure is actually really important to an improved ride experience. I only try to avoid the question because it opens up a rabbit whole that we can spend all morning on. I want to take the time here, to go down that rabbit hole, and help empower you to understand and start experimenting with your own tire pressure.
Your tires are the first point of contact with the ground, thus they are the first form of suspension on your bike. Riding with an appropriate amount of air in your tires makes a significant difference in your overall ride quality, and just may be the reason you feel like you’re struggling to improve. Tire pressure is especially important for kids or anyone who may be riding a hard tail or a fully rigid bike. But getting a grip on your PSI will, literally improve your grip which will help you to feel more confident and have more fun on the trail!
Terms:
PSI = Pounds per square inch. The common unit of measure for tire pressure in the US.
A few myths:
Firmer does NOT equal faster. This is a long held belief that if you pump your tires way up you’ll be faster. This is true if you are riding on a super smooth surface, like tarmac, but once you hit a trail with any sort of abnormalities to its surface, you want your tire to conform to the surface, not bounce off. If a tire is super firm (high PSI), when it hits rocks or roots it’s going to bounce off those objects. The fastest line is always the straightest line, and when there is more give in your tire they are better able to conform to objects in the trail such as roots or rocks. When a tire is able to rollover objects rather than bounce off them, you experience a smoother and more efficient ride. If you know you have a road climb, or a smooth gravel road, this is where pumping up your PSI for the climb, then taking air out at the top can work to your advantage.
Please ignore the recommended PSI reading on the sidewall :) Much like expiration dates on food, tire manufacturers are required to put a number to which the tire can be safely inflated. This number is on the higher end of safety and has nothing to do with ride quality or experience. So please ignore the number written on the tire and read the rest of the article!
The “It depends” factors.
The following are 4 factors to consider when deciding what PSI to run:
Tire width
Tire casing
Rider weight and ride style.
Ride conditions
There are others such as rim width and ride style, but for the folks who are typically taking my clinics, these 4 factors are a really good place to start. Let’s dive in!
1. Tire Width. The wider the tire, the lower PSI you can run. You want to run a lower PSI with a wide tire to get the benefit of that width. Too much PSI in a wide tire and you reduce the contact patch of the rubber on the ground! What’s wide and what’s narrow? These numbers have changed pretty considerably over the years. In my opinion, anything wider than 2.4 is “wide” and 2.3 or smaller is narrow. What does that mean exactly? If you and I are about the same weight, and we are both on 29” wheels but your tires are 2.3 and mine are 2.5 it means I can get away (and will benefit more) from running a lower tire pressure than you. If you have a wide tire, such as a 2.5, and you pump it way up say past 30PSI, you negate the benefit of that wide tire because less of it will be in contact with the ground. Think: rubber on ground = improved traction, as long as it’s not too soft.
2. Tire Casing: The generality here is the lighter weight tires require more PSI and heavier weight tires need less PSI. A dainty road, gravel, or even XC tire is going to need more PSI relative to a beefcake of a downhill or more aggressive trail or enduro tire. A road tire you might run anywhere above 40 PSI, where as a fat bike tire could be as low as 7 PSI! Tires will also come with a “TPI” score. TPI = Threads Per Inch. Much like sheets, the higher the TPI number, the more supple the tire. High TPI score = lighter weight = higher PSI. Low TPI = heavier duty tire, tends to have a more aggressive tread and can get away with a lower PSI.
Take total ride weight into consideration. This includes a stuffed pack!
3. Rider weight and style. Rider weight is NOT your morning weigh in weight. It’s your body weight plus clothes and pack. The general rule is a heavier rider will need to have more PSI than a lighter rider but your pack weight should be taken into consideration when loading a pack of 20ish pounds or more. Rider style also comes into play and I like to separate groups into Gazelles and Gorillas. Gazelles can can often get away with less PSI than Gorillas. Are you a hard charger who likes to go as fast as possible regardless of what’s in your way? If yes, you’re in the gorilla camp. If you ride as if the rocks are eggshells and the roots will cry out in pain if you hit them too hard you’re more of a gazelle. I like to think of myself as a Gazelle, but the reality is I like to go fast and I’ve broken 3 carbon rims by smashing them into rocks, likely a result of running too low of tire pressure. Moral of the animal story? Gazelles can get away with much lower psi than gorillas. The vast majority of adults I coach, especially women, are Gazelles.
Trail Conditions
Smoother and firmer trails = higher PSI. For example, go ahead and pump up those tires for a road or smooth gravel road climb, but take air out if you have a chunky climb, bumpy climb!
Wet and Roots = lower PSI. When you go to visit your buddy’s outside Seattle or Bellingham you generally lower your PSI due to all the roots! The exception here is if it’s too muddy you may find that a slightly higher PSI helps the tires cut through the mud. Although, if it’s that muddy you may be better off sipping your favorite hot beverage and working on a puzzle than slopping around in the mud. But the general rule is if you’ve got lots of roots, lower that PSI!
Sharp Rocks = higher PSI. Rocks such as shale or flint can cut through your sidewall if your psi is too low, thus pumping up those tires can help keep you rolling when you’re out in really rocky terrain. I was racing the Downieville classic one year and sliced my sidewall. It took me about 15 minutes of messing around with a Cliffbar wrapper, braking 2 tire levers, and wrangling my tire off the rim to get a tube to into my tire so I could safely finish the race. Slightly higher PSI may have helped my cause. Sometimes we just get unlucky with rocks.
Ultimately, the two best tools are a tire pressure gauge and time. Many floor pumps have a gauge attached, but they are often inaccurate. If you’re looking for a good starting point for your own PSI, check out SRAM’s Tire Pressure Guide. Then get yourself a tire pressure gauge and start to experiment. If you ask 5 different riders of similar weights what PSI they’re running you’ll get 5 different answers as much of tire pressure is personal preference. Personally, I am 130lb, I ride 29” wheels with rims that measure 30mm internally. My tires are 2.5 front and rear and I typically will run between 18-21 PSI in the rear and 17-19 PSI in the front.
Don’t be afraid to play with different air pressures! Remember the generalities: If you have too little air in your tires, they will feel squirmy which equals less control. If you have too much air in your tires they will feel bouncy and also out of control. Finding that sweet spot will be part trial and error and part patience, but is absolutely worth it, and one of the easiest things to adjust on your bike. Your optimal PSI will change based on trail type, tire choice and conditions as well as your ride style. As you can see, there are a lot of considerations when it comes to tire pressure! Thus, you now understand why at the start of a clinic I stick with the saying, “Your tires should feel like an orange, or somewhere between a rock and a 5 day old birthday balloon:)” But in reality, understanding PSI is really important and now you are much more informed. I hope you find this helpful and feel empowered to start experimenting with your own tire pressure and find that sweet spot! If you’re looking for a skills clinic, check out these Hood River and Portland offerings!