There are many confusing things about bikes and mountain bikes, in general – you can get more info here about them – but tire pressure is just one of those that seem to be a total mystery to many. You can clearly see the recommended pressure on the tires (those little numbers beside PSI), but you have no idea what the ideal one for you is.
Chances are, you’ve never really bothered to find out (and you probably don’t even have a pump with a pressure gauge). If this sounds confusing, read on!
The (in)exact science of tire pressure
First of all, let’s get the basics sorted out. Tire pressure is measured in PSI, which means pound per square inch or, to be more exact, pound-force per square inch. In scientific terms, this is the pressure in a vacuum, or in this case, the tire, which is not exactly a vacuum.
Since this is measured in a vacuum and atmospheric pressure is not considered, the actual pressure of a tire, at sea level, will have the tire pressure plus the atmospheric pressure. For example, a tire that’s pumped up to 65 PSI will have 65 PSI + 14.7 PSI (atmospheric pressure at sea level), so a total of 79.7 PSI.
This all sounds boring, and the thing is that, well, actually, it doesn’t really matter. ‘Why’, you ask? Because, even if you understand what pressure is and know the tire manufacturer’s specified/recommended PSI, this value is not relevant unless you take other factors into consideration.
Variables such as tire volume, rider weight, terrain type, plus a few other details can drastically influence what the ideal pressure rating actually is.
There are quite a few things that need to be considered when trying to determine the right tire pressure. These include rider weight, tire volume, rim width, terrain, tire construction, and riding style.
While this might seem obvious to quite a few people out there, the heavier the rider is, the more pressure the tire will have to have. So if a small and light rider, weighing 150 pounds, would find that 35 PSI feels great in a 26×2.20” tire, a heavy rider weighing over 220 pounds would definitely not have the same opinion.
This one might also seem obvious, but the higher the volume of the tire, the more pressure you’ll need. So if you inflate your 26×2.20” tire to 35 PSI, it might seem like it’s rock-solid, but do the same thing on a 29×2.4” tire and you’ll begin to wonder whether you have a flat or not.
A wider rim is able to better support a tire with a lower air pressure, which is usually associated with increased performance off-road. The thinner the rims, the more problems you’ll have by running your tires on low pressure, and you could also risk damaging them if the pressure is too low.
The terrain is an important variable when it comes to tire pressure. Generally, you get a lower rolling resistance by having a higher tire pressure when riding on tarmac. The exact opposite is valid for mountain bikes: in most cases, a lower pressure is associated with lower rolling resistance.
This might sound weird, but on tarmac, the surface is smooth, so you want a rock-solid tire to roll really fast. On off-road trails, you get many bumps, roots, gravel, small/large rocks, and all sorts of other obstacles. An over-inflated tire will cause the wheels to bump off the obstacle and then land at a tiny distance after it, even if you don’t actually see it.
This causes you to lose momentum and energy as a lot of it that goes into the pedals is just wasted by all these bumps. If you deflate the tires a bit, you’ll see the rolling resistance actually decreasing, as the tires are able to somehow absorb the impact forces and kind of wrap around the obstacles, if we’re talking about the smaller ones.
However, too low a pressure and you’ll run an increased risk of pinch flats, rim damage, and worse, loss of control.
The higher the TPI rating, which stands for threads per inch, for the tire casing, the less pressure you’ll be able to have in the tire before you feel that it is affecting performance and even safety.
If you’re a casual rider and aren’t really pedaling downhill at crazy speeds, you’ll probably be able to have a lower pressure in your tires. However, if you’re the aggressive type and love slaying those trails or descending at maximum speeds, you’ll want to inflate those tires a bit more.
That’s especially true if you have tubeless tires. This is because, as opposed to normal tires with inner tubes, tubeless tires are supposed to sit tightly against the rim sidewalls, and if the pressure is too low, when taking a corner aggressively, some air pressure can be lost. This is commonly referred to as burping.
Do this for an entire course, and you run the risk of losing so much pressure that you’ll compromise handling and, besides lower performance, you might also be exposed to a higher risk of crashing.
Aren’t there any calculators for this?
Given all the variables presented above, it’s quite hard to calculate all these things, especially since riding style and feel are quite hard to measure. To quote a guy from Schwalbe regarding what the recommended tire pressure is, “it’s a lawyer thing”.
Manufacturers tend to be conservative when it comes to these recommendations, and those values don’t mean that your tire will burst when you inflate it to the maximum value. And they’re conservative because the manufacturers can’t possibly know all the combinations of variables mentioned above, and they aren’t planning on being dragged into countless lawsuits either.
The risks of riding under- and overinflated tires
Riding with too high or too low tire pressure is never a good thing, and there are quite a few disadvantages to each, which is why you really need to figure out what a good pressure would be to ride comfortably and safely.
If your tires are overinflated, and you’re riding on off-road terrain (which you should, by the way – it ain’t called a mountain bike for nothing), then the wheels will bounce off of every little obstacle. You’ll certainly feel all those bumps through your legs and arms and up to your head.
This will not only lead to a not-so-pleasurable ride, but you might also make some jumps higher than you’d want and run the risk of skidding off the trail and crashing.
This is because a higher tire pressure means a smaller contact surface with the ground and less grip. You also won’t pedal as efficiently as you’ll have lower traction and lose momentum.
Riding on under-inflated tires is not a good idea either. If a lower pressure would lead to better traction, too low a pressure would lead to increased rolling resistance, again (ever tried pedaling on a flat tire?). You’ll also risk skidding off the trail, and the spongy tire feeling isn’t that great.
Worse, you could get pinched flats, that is, having so spongy tires that hitting a rock would cause the tires to compress and have the rims directly hitting the surface. The flats are also called snake bites (because there are usually two punctures, for each rim sidewall contact). In severe cases, you can also dent and damage the rims, or even break them.
Still, if you had to choose, and you’re mostly riding off-road, it’s better to have a slightly lower pressure rather than higher.
So what’s the ideal tire pressure?
To find the ideal tire pressure, you’ll have to take into consideration all the factors mentioned above, and do some qualitative testing yourself. This means trying different tire pressures, and that implies different pressures for the front and rear tires as well, and seeing how they roll, how you feel, and how it affects your performance.
This way, you’ll be able to optimize the tire pressure and know when to deflate/inflate the tires for a specific terrain type.
And as a joke, next time you’re out with your mountain biking buddies, you can ask them what tire pressure they’re running in their tires. If most of them just have the reflex of reaching down to the tires and pinching them, saying “about this much”, try not to laugh.