“How much air should I put in my tires?” is a common question we get from new bike owners (and those bringing their bikes out of the garage for the summer). The solution is more intricate than simply looking at the sidewall, as it turns out – let’s have a look!
Bike tires were originally constructed of solid rubber, which made them heavy, rigid, and resulted in an unpleasant, bumpy ride. When pneumatic tires (air-filled tires) were invented, they addressed many of those issues while introducing a new one: flat tires. It’s evident why air-filled tires weigh less than solid ones (air weighs nearly nothing), but a less obvious advantage of pneumatic tires is shock absorption.
Consider the air in your tires to be a lengthy cylinder that wraps all the way around the wheel. As the tire compresses (when you hit an uneven surface or just shift your weight on the bike), the air in your tube compresses and the extra pressure that results is spread around the rim, “pushing back” on the tire until equilibrium is established and the tire stops compressing. The air in your tires functions essentially like a spring, compressing under load and springing back when unweighted. Because there is no air to function as a spring and, as a consequence, no internal pressure to hold the tire off the rim, a flat tire contracts completely.
Understanding that your tires play a significant influence in your bike’s suspension can assist you in determining the appropriate pressure for your requirements. Too much air pressure prevents your tires from compressing, resulting in a rough ride, a lot of unpleasant road noise, and less grip as the wheels “skip and hop” over bumps instead of deforming to hold to the road. It’s the same as employing a too powerful spring for the task — if it can’t compress, it may as well be solid.
If there isn’t enough air in the tube, the tire will collapse all the way to the rim over big bumps (since there isn’t enough pressure to hold them apart), resulting in pinch-flats and some very painful saddle experiences. It’s the same as employing a “too soft” spring for the task – it’ll compress too readily and not provide enough push-back to be effective.
How about the rolling resistance?
Rolling resistance is the boogeyman of bike performance; you hear it discussed in whispers, it’s superstitiously worried about by racers and commuters alike, and it’s always threatening to take vital seconds (and, even more valuable, effort) from your rides. So, what exactly is it? In summary, when your tires compress and spring back, as mentioned above, energy is used, which means that some energy is not transported from the wheels to the road – it is consumed by the “suspension.”
For a long time, narrower tires (smaller contact patch) and higher pressures were thought to be the greatest strategy to combat rolling resistance – but that’s beginning to change. New research shows that when tire pressures go too high, rolling resistance starts to rise (as “skipping and hopping” over bumps diminishes traction and loses energy in its own manner), and that bigger tires aren’t always a bad thing. The idea is to find the correct pressure for what you’re doing rather than just pumping it all the way to the top.
So, what is the proper amount of pressure?
We know we don’t want the tires to be too firm (which would result in a rough ride, poor grip, and higher rolling resistance) or too soft (which would result in pinch flats and increased rolling resistance) – but where is the Goldilocks zone?
It’s not printed on the sidewall, at least. If you look at the outside of your tires, you’ll see a range of suggested pressures (for example, “Inflate to 55-80 PSI”), and many riders will just pump until they reach the top limit. But you’re not like other cyclists, right? It’s likely overkill on the bike for the same reason you don’t get your automobile tires pushed to the maximum permissible pressure. What you should strive for is 15% “tire drop” on each wheel.
Tire drop is a measurement of how much a tire compresses under load; it is calculated by comparing the ride height of a weighted and unweighted wheel and the tire’s subsequent compression. It might be difficult to measure your own tire drop, but thankfully we have this helpful chart!
Please keep in mind that they are the recommended pressures based on weight for each wheel, not for both wheels together. Because most bikes carry greater weight over the back (due to rider position, geometry, common storage/mounting choices, etc…), you’ll probably require extra pressure in your rear to compensate for the higher weight. Similarly, if you’re running to the coffee shop, you’ll be OK at a lesser pressure than if you’re filling up 100lbs of camping gear and headed out on a trip due to the extra weight.
If you want to determine the weight on each wheel on your own, balance your bike with one wheel on a scale and the other on a block, then jump on while a companion holds you from toppling over. Then turn the bike around and weigh the opposite wheel to discover how much weight each one is liable for. A decent general rule of thumb is that a fixed gear bike will have a 40 percent / 60 percent ratio on the front and rear, while a city bike would have a 35 percent / 65 percent split. Once you’ve determined how much you and your bike weigh combined, as well as how the weight is divided, reference the chart and start pumping!
As a 150-pound rider on a 25-pound bike, I’m hauling roughly 70 pounds on my front wheel and 105 pounds on my rear, so I keep my 28c tires filled to 85 PSI in the back and 65 PSI in the front. It’s a little higher than the suggested chart pressures, but it’s what I’ve found to feel the greatest and offer enough support to go through the less-than-ideal streets of Los Angeles. When I pack up the bike for a day out (locks, food, water, and fun things), I raise the pressure to compensate for the extra weight. Similarly, in rainy circumstances or on loose ground, I’ll reduce the pressure slightly to improve traction (more tire deformation = more rubber on the road = greater grip).
In a nutshell, it all boils down to you! The chart above is a good place to start (far better than the tire sidewall), but only your own personal experience, experimentation, and assessment can really lock it down for you. Make some adjustments! If you’ve been running pressures that are too high, you’ll probably appreciate the smoother ride of a lower PSI. Similarly, if you’ve been running your pressures too low, adding additional air will make you feel like you’re flying over smooth pavement rather than trudging through sand.
Try it out, figure out what works for you, and document it (so you can take out the guesswork for next time). Let us know in the comments what pressures you use, and have fun on the journey!