Michael Hanslip Coaching

If you want to go faster, you have to pedal harder

The importance of air pressure 1

Pneumatic tyres were a critical development in the expansion of bike use in the late 19th century. Solid tyres just aren't very nice. Once Dunlop's air-filled tyres were put on bicycles, everyone was better off. Better traction, lower rolling resistance, better handling.

And all these years later, tyres filled with air are still mission-critical on all bikes.

Mountain bike tyres rely on conforming to the surface (where lower pressure is better) and forcing sharp edges into the ground (where higher pressures are better) to enable navigation of unpaved terrain. From MTB commercial origins in the 80s until the late 2000s, we were forced to use a rubber tube to contain that air. The tubes can be heavy - thinner is better from a riding standpoint. But thin tubes are easy to cut. Tubed MTB tyres were always prone to pinch flats necessitating more air than optimal for ultimate traction and handling.
And thanks to Keith Bontrager, we got skinny rims for most of those tubed years too. Keith re-rolled some road rims to 26" diameters, achieving something that the rim companies weren't doing: strong and light double-walled rims. The problem was that road rims were very skinny (around 13 mm inside width) and gave a light globe shape to a 2"+ knobby tyre. That, too, necessitated more air pressure.
Almost all mountain bikes above the very cheapest are now tubeless. To the point that high-end MTB no longer come with tubes inside and leave the tubeless step to the consumer or shop. Now they come with some sealant and valve stems and no tubes.
With tubes and large tyres on relatively skinny 26" rims, I had to run as much as 40 psi to prevent pinch flats. There is a lot of traction loss with that much air pressure in the tyre. In the case of some rim/tyre combinations, I had the tyre rotate on the rim under braking with (not much) lower air pressures - which ultimately rips the valve stem off the tube if it progresses far enough.
My current quiver of mountain bikes all run 30 mm inside width rims. That width supports the sidewall of the wide tyre much better than one half that width can. I run CushCore foam inserts in all of the wheels too. CushCore acts like a fork volume spacer by decreasing the space for air in the tyre (by roughly half). When a bump is hit, the pressure in the tyre increases more quickly due to the lower volume present. Less "travel" is used in the tyre for any given bump than would be the case without the foam. When a big enough impact occurs to bottom out the tyre on the rim the foam intervenes and cushions the impact. Impacts have to be much larger to cause rim damage. On 29" wide wheels with foam inserts I (@100 kg) can get away with around 20 psi - around half of what I used to use with tubes.

On the road side it is only very recently that mainstream tyres and wheels have gone tubeless. And it is by no means (yet, at least) a universal changeover the way it has been in mountain. High pressures and a desire for lightness push more strongly towards tubes. The biggest lever moving us to tubeless tyres has been the realisation that wider tyres are all of: faster, more comfortable, better handling and can be similarly aerodynamic if the wheel is designed for the bigger rubber.
My first new bike in Canberra came with 20 mm wide tyres on it. That wasn't a universal size, but it was very common. Not long after that, 23 mm tyres set in as the main size for a racing oriented bike. For a period of months, until I purchased a floor pump with a gauge, I inadvertently ran upwards of 160 psi in those skinny tyres. I had a frame pump that advertised "up to 150psi" and most of these claims are over-statements of the easy reality, so I pumped up the tyres as firmly as I could with that pump. I assumed it was around 130 or 140 psi. The gauge revealed that it was more like 180 to 200 psi! Good pump. Too much air.
My current race bike doesn't sport tubeless-friendly rims. I run latex tubes and light 25 mm tyres - which is actually considerably lighter than the tubeless alternative. Some 60 grams for a tube plus 200 grams for a tyre and nothing else; adds up to 260 g. I can run these at 100 psi for a solid but comfortable feel.
My commuting bike has tubeless-ready rims. I have used both tubes and sealant over the time I've had the wheels. I could run similar 260 gram tyre and tube options as on the race bike. In tubeless, the similar racy tyre is around 300 grams and the valve stem only weighs a few more grams plus 50 ml of sealant adds up to 330 grams or so. Currently I am running a less racy option (meant to be quite puncture resistant and long wearing) that is only 350 grams. So far they are great.
Sealant does its magic when pushed through a small hole to congeal and seal the hole. Often prior to the rider knowing it ever happened. With 25 psi, there isn't much force behind the sealant and it works effectively. With 125 psi, sealant often sprays under pressure (it has happened to me twice before). Sometimes everywhere (bike, rider, pavement, etc). Currently on 28 mm tyres I only require 75 psi tubeless and it feels fine.

The implied issue here is that as tyre pressures get lower and lower, a single psi difference becomes a higher and higher percentage of the total pressure, and therefore more important. An expensive pump might have an accurate gauge installed, but mechanical gauges are subject to being bumped out of accuracy even if they were good to begin with. An associated problem is scale. Road bike tyres require around 100 psi. The gauge typically goes to 150 psi or higher. That leaves very little resolution for distinguishing between 16 and 18 psi for a MTB tyre (those 2 pressures ride completely differently, by the way). You can get a MTB specific pump with a 50 psi gauge, which improves things a lot (but then you'll require a second pump for the road bike if you have one of those too).
I've just purchased my second digital tyre gauge. The first one recently stopped working after almost 15 years. I can only hope the new one lasts as long. This coincides with the gauge on my main floor pump deciding to become crazy in its readings. Instead of being around 3 psi out at 20 psi (consistently, so I could compensate) it is now something like +15 psi at indicated 20. With no markings below 10, the range I need to in when be filling up MTB tyres is useless to me. It seemed better to buy the gauge than to get a new pump - the pumping part works perfectly still.
The pump explains the difficulties in my household with off-road riding of late. An extra 15 psi in a tyre, when it is only meant to have around 20 psi inside in the first place, means it doesn't ride like you'd expect. Bouncing off of things was common, where normally it would roll over the top. I tried a portable mechanical gauge in the interim, but it really only reads to the nearest 5 psi and even with that I found out using the digital gauge that it was reading low; my road bike tyres had around 5 psi too much inside.

Your tyres are your connection to the terrain, be that the smooth boards of an indoor velodrome, the bare rockslabs in Squamish or anything in between. It really is the first adjustment after bike fit to get correct on your own bike(s) before adjusting anything else. Not the suspension sag or anything else related to suspension should be seen as more important. Without having the tyres right first, the suspension cannot work as it is meant to. On a road bike, go too low and it won't go around corners confidently and safely. Get it too high and you'll be prone to extra punctures, you might bounce off of road bumps and you might damage a rim if you go too high. It also rides poorly with too much air on any surface.
Accurate and consistent air pressure in the tyres is the start of a good experience with any bike.