Feeling the Pressure
A question we get a lot from new bike owners (and those pulling their bikes back out of the garage for Summer) is, “How much air should I put in my tires?”. Turns out, the answer’s more complicated than just peeking at the sidewall – let’s take a look!
Originally, bike tires were made of solid rubber and that made them heavy, stiff, and resulted in a harsh, bumpy ride. When pneumatic tires (ones filled with air) were developed, they solved a lot of those problems and only gave us one new one: flats. It’s easy to see why air-filled tires weigh less than solid ones (turns out air weighs almost nothing), but the less-obvious benefit of pneumatic tires is shock-absorption.
Think of the air in your tires as a long cylinder, wrapping all the way around the wheel. As the tire compresses (when you hit an uneven surface, or even just shift your weight on the bike), the air in your tube is compressed and the increased pressure that creates is dispersed around the rim, “pushing back” on the tire until equilibrium is reached and the tire stops compressing. Essentially, the air in your tires acts as a spring, compressing under load and springing back when unweighted. A flat tire compresses all the way because there’s no air to act as the spring and, as a result, no internal pressure to keep the tire off the rim.
Understanding that your tires play a big role in your bike’s suspension will help you dial in the right pressure for your needs. Too much air pressure and your tires won’t be able to compress, resulting in a bumpy ride, lots of harsh road noise, and reduced traction as the wheels will “skip and hop” over bumps, instead of deforming to stick to the street. It’s just like using a spring that’s too strong for the job – if it can’t compress, it might as well be solid.
Too little air in the tube and the tire will compress all the way to the rim over hard bumps (as there’s not enough pressure to keep them apart) and that can result in pinch-flats and some really uncomfortable saddle experiences. It’s just like using a spring that’s “too soft” for the job – it’ll compress too easily and not offer enough push-back to be effective.
What About Rolling Resistance?
Rolling resistance is like the boogey-man of bike performance, you hear it mentioned in hushed whispers, superstitiously fretted over by racers and commuters alike, and it's constantly threatening to steal valuable seconds (and, even more valuable, effort) from your rides. So, what is it? In short, when your tires compress and spring back as discussed above, that takes energy, which means some energy’s not being transferred from the wheels to the road – it’s being used up by the “suspension”.
It was long believed that the best way to beat rolling resistance was narrower tires (smaller contact patch) and higher pressures – but that’s starting to change. New data demonstrates that rolling resistance actually begins to increase once pressures get too high (as the “skipping and hopping” over bumps reduces traction and wastes energy in its own way) and that wider tires aren’t necessarily a bad thing. The key is to find the right pressure for what you’re running – not just pump it all the way to the max.
So What’s the Right Pressure?
We know we don’t want the tires too hard (harsh ride, worse traction, and increased rolling resistance) or too soft (at risk of pinch flats and increased rolling resistance) – so where’s the Goldilock’s zone?
Well, it’s not printed on the sidewall. If you look at the outside of your tires, you’ll see a range of recommended pressures (ex. “Inflate to 55-80 PSI”) and many riders will just pump away until they hit the upper limit. You’re not most riders though, right? For the same reason you don’t have your car tires pumped to their maximum allowable pressure, it’s likely overkill on the bike too. What you should be aiming for is 15% “tire drop” on each wheel.
Tire drop is a measure of how much the tire compresses under load, it's determined by comparing the ride-height between a weighted and unweighted wheel and the tire's subsequent compression. Actually measuring your own tire drop can be a pain, but luckily we’ve got this handy chart!
Note: Those are the recommended pressures by the weight on each wheel, not on both wheels together. Because most bikes carry more weight over the rear (due to rider position, geometry, popular storage/mounting options, etc…) you’ll likely need more pressure in your rear to counter the increased weight. Similarly, if you’re making a quick run to the coffee shop, you’ll be fine at a lower pressure than if you’re loading up 100lbs of camping gear and heading out for a tour because of the increased weight.
If you’re interested in determining the load on each wheel yourself, balance your bike with one wheel on a scale, the other on a block, and hop on while a friend keeps you from tipping over. Then flip the bike around, weigh the other wheel, and you’ll be able see exactly how much load each one is responsible for. A good general guess is that a fixed gear bike will have about a 40%/60% split on the front and rear, where a city bike will be closer to 35%/65%. Once you know what you and the bike weigh together, and how the weight is distributed, consult the chart and pump away!
For me, as a 150lbs rider on a 25lbs bike, I’m carrying about 70lbs on my front wheel, 105lbs on my rear, and so I keep my 28c tires pumped to 85 PSI in the rear and 65 PSI in the front. It’s a bit over the recommended chart pressures, but it’s what I’ve found to feel the best and provide enough support to survive the less-than-perfect streets of LA. When I load up the bike for a day out (locks, food, water, and fun stuff), I’ll up the pressure to make up for the increased load. Similarly, I’ll drop the pressure a bit in wet conditions or on loose surfaces for the increased traction (more tire deformation = more rubber on the road = more grip).
In short, it really comes down to you! The chart above is a great place to start (way better than the sidewall of the tire), but only your own personal experience, experiments, and evaluation will really lock it down for you. Try making some changes! If you’ve been running pressures that’re too high you’ll likely love the less harsh-ride of a lower PSI. Similarly, if you’ve been running your pressures too low, getting some more air in there will make you feel like you’re flying over smooth pavement as opposed to slogging through sand.
Try it out, find out what works for you, and write it down (so you can take out the guesswork for next time). Let us know what pressures you use in the comments, and enjoy the ride!