What is the perfect tire pressure?
Lets start by stating that there is no such thing as the 'perfect' tire pressure - it depends on many factors, some related to the equipment you use, some related to the environment you ride in, etc. This means that tire pressure is dynamic, and will change based on circumstances.
This is a question I get asked frequently, and it recently came up again from a lovely women I helped with her bike fit several months ago. She said she has been getting different information from different people, which is the kind of thing that frustrated me to no end. So, I thought I would share my answer to her, as I know it will be helpful to many other women (and men, too)!
You would think that getting your tire pressure right is simple and straight forward. I mean, all you need to do is pump your tires to the recommended range on the sidewall and call it a day, right? Well, not so fast.
First, lets consider what you put your tires through when you ride your bike, OK? Your tires are designed to grip the road so you have the ability to steer your bike properly and move forward. On a road bike in particular, they also provide some protection/cushioning from bumps on the road. You want your tires to be tough so you do not get constant flats, but at the same time, you want them to be light and thin enough so they roll well. All these things are affected by pressure, as well as the design and construction of the tire itself. That's a lot to ask of your tires, isn't it?! The pressure is on (Pun intended)!
When you ride your bike, your tires compress down. If they compress too much, they will make the bike harder to control, increase rolling resistance and putting you at risk of getting a flat. On the other hand, if they do not compress enough, you will likely find the ride really harsh, and the grip on the road will be lower because there is less tire in contact with the ground. When I first got into cycling/triathlon (way back in the early 90's), everyone rode 18mm tires on their road bikes, inflated to the max. I also remember, in the early 2000's, changing my tires the day before European championships to 23mm tires, because it was supposed to rain the next day during my race... I didn't know as much about bikes then as I do now, but turns out my instincts were right!
So, how do you figure out what the right tire pressure is, for your individual circumstances?! Lets break that into smaller, bite size sections.
Know your equipment
Lets start with the basics, without getting into tire material, tread, weight etc for the time being. Do you know what size tires you ride? Are you using 23mm tires? 25mm? 28mm? This should be your starting point before deciding what tire pressure to use, because the width of the tire matters for tire pressure (wider tire = lower psi). Of course, there is a range and regardless if you use lower pressure or higher pressure, it should be within the range stated on your tire's side wall.
You likely notices that bikes now days typically come with wider tires compared with several years ago. Wider tires generally run on lower pressure. Combined with their larger air volume, it means they absorb bumps and holes better, increasing comfort. It is important to note that wider tires do not suffer increase the likelihood of flats, increased tire wear & tear and don’t have a higher rolling resistance.
How much your tires compress when you ride depends on your weight, along side the type of riding you do, etc. When we mentions weight, you need to take into account both your weight as the rider AND the weight or your bike (and load of your bike, if you have panniers for a touring bike, etc), not to mention that the pressure on the front tire and the back tire is not equal (and can vary even more depending on your bike fit). While I wouldn't necessarily take into account that last point (because of bike handling when you ride), the take home message here is: If you weigh less, you do not need to pump your tires to the max and you can run a lower pressure, depending on your tire width. For reference, here is a handy tire pressure/weight reference table from schwalbe that can be used as a starting point.
If you were riding on a super smooth surface, higher pressure might be better. But in the 'real world', when does that actually happen?! If you run a tire at a reduced pressure, it will absorb shocks from bumps better instead of affecting the entire bike and ride. There will be less energy send upwards with each bump you hit on the road and there will be less energy expended to keep the bike rolling. Not to mention that your ride will potentially feel more comfortable! The take home message is: Smooth roads = higher pressure. Bumpy roads = lower pressure.
If you lower the tire pressure, the tire's contact with the ground will increase. This changes the grip level, so if you are riding in wet vs dry conditions, a bit lower pressure than usual is a good idea (~[7psi) . You also need to take into account the air temperature... It is hot outside? Is it cold? What is the road surface like if it is a very hot day? Although air temperature changes are typically not significant enough to cause issues, it is something to be aware of if you ride in extreme conditions.
Know your ride
Adjust based on conditions, circumstances, the bike you are riding and your goals. Your tires do not need be pumped to the same PSI every time you head out to ride... Remember to consider the surfaces on your planned riding route, the weather conditions and the kind of ride you plan on doing (easy spin, commuting, racing, cyclocross, MTB, road riding, TT, etc) and adjust based on these factors. Of course, using a lower pressure might increase your ride comfort, but it can also increase the risk of getting a flat, so you need to experiment a bit and find the right mix of tire pressure and flat resistance, along side performance. Depending on your riding goals, you might want to explore various tire options for durability, weight and tread to suit your riding style, goals, etc!
Remember that it is a balancing act... Lets take a page out of Goldilocks and summarize - Not too low, not too high, juuuuust right based on individualization and specificity! And when in doubt... Simply ask someone you trust - I get asked about tire pressure guidance probably once a day on average!
Do you have specific questions? Ask in the comments!