Why Tyre Pressures Matter – And Why You Need A Pressure Gauge

Modern mountain bikes come with a dizzying amount of adjustability. To get your riding position nice and comfy, you’ve got things like bar height, saddle tilt, saddle height and brake lever reach to dial in. For those with a hardtail or full suspension bike, there are spring rates, damper settings and lockout levers to contend with. If that’s still not enough for you, well, then you can look at bar width, stem length, grip diameter, gear ratios, disc rotor diameter, saddle shape, saddle width, crank length, cleat position, pedal setup and…

Even with all of these at our disposal, tyre pressure remains one of the most commonly overlooked adjustments on a mountain bike. And yes, I’m looking at you – the ones who squeeze their tyres to ‘check’ air pressure. Heathens!

Fortunately for all you tyre prodders and rubber squeezers, tyre pressure is one of the simplest adjustments you can make on your bike. And it can also have the biggest impact on ride quality too.

Run too much pressure and your tyres can lack grip and flexibility, leading to a harsh and bouncy feel on the trail. Put too little pressure in your tyres though, and not only do you drastically increase the risk of pinch-flatting, you also lose significant stability through the turns. With insufficient tyre pressure, your bike will tend to feel sluggish and vague.

Spend the time experimenting to get those pressures right however, and you can achieve that magical balance of rolling efficiency, support, traction and comfort. You know, when the angels sing and all that.

Hold up – What Kind Of Sorcery Makes These Things Work?

Inside the vast majority of pressure gauges you’ll find a mechanism called the Bourdon Tube. This is the most common mechanism used for measuring air pressure, and it was first invented in 1849 by a French engineer and watchmaker by the name of Eugéne Bourdon. It’s a very clever, yet simple mechanical device that comprises of a sealed brass tube that is curved into a C-shape. Air pressure enters the gauge, then into the brass tube, which distorts and begins to straighten (or ‘erect’ if you will) as air pressure increases. Attached to the brass tube is a linkage. As the brass tube straightens, it pulls on this linkage, and then drives the needle on the dial to indicate a change in pressure.

Here’s a video to show how it works, accompanied by some terrific commentary;

Though the thickness and diameter of the brass tube changes depending on the range of the gauge, the mechanism itself is basically the same whether it’s being used for a bicycle tyre pressure gauge, or a 6000 psi industrial gauge. All Bourdon Tube mechanisms need to be calibrated for their respective pressure ranges, which is why we had a few different pressure ratings across the six gauges I tested recently. As for the digital gauges, most of these still use a Bourdon Tube inside, but simply with a digital readout instead of a traditional needle.

The Lezyne Digital Check Drive is a notable exception, which (like a digital shock pump) uses a Piezoelectric Sensor – a type of electrical diaphragm that deforms under pressure. This results in an electrical charge that is then calibrated and used to measure pressure to a particular value and range. The piezoelectric effect was also discovered by a French scientist, this time a bit later in 1880, but its use in pressure-sensing applications wasn’t made apparent until nearly a century later. This piezoelectric sensor is more compact than a Bourdon Tube, hence why the Lezyne gauge is much slimmer than the other gauges on test.

Ok, So What’s The Right Pressure Then?

It’s no secret at Singletrack Towers that I’m particularly anal when it comes to bike setup, and tyre pressure is something I spend considerable time on getting right. But while I’m regularly asked what is the right tyre pressure to run, unfortunately (even if your mate Bob from the pub says otherwise) there is no golden rule to tyre pressure.

This is because there are so many variables to the equation. Rider weight is one of the key variables, though thankfully this one is pretty easy to understand – the heavier you are, the more pressure your tyres require. Riding style also comes into play. Jumpy and aggressive riders will need higher pressures to keep the tyre from getting mushed when landing heavily into sharp rocks, whereas smooth riders can potentially get away with lower pressures. Likewise, the terrain you frequent will place different demands on your tyres compared to riders in other parts of the world. An EWS racer smashing double-black trails in Whistler is probably going to need more tyre pressure than an intermediate rider on a blue run at a Welsh trail centre.

On top of this, the size of tyre and rim you own will change the parameters. In general, the bigger the tyre, the less pressure you’ll need. To give an extreme example, this is why a 26×4.0in fat bike tyre will run comfortably with 8 psi, while a 700x23c road bike tyre needs closer to 100 psi to work as intended. Wider rims, and how they affect the shape and volume of the tyre will also affect tyre pressure. It’s often possible to run lower pressures when fitting the same tyre onto a wider rim, since the broader rim bed tends to minimise casing wobble.

Then there’s the construction of the tyre itself. Tyres with more robust casings and sidewalls can be run with lower pressures compared to lighter tyres that are built with much thinner casings. Many tyre manufacturers offer the same tyre tread and size in multiple versions. For example, Maxxis offers a 2.3in wide High Roller II tyre with EXO sidewalls and a lightweight 120tpi casing, as well as a Double Defence version that looks identical, but has a double-layered 120tpi casing. The latter is heavier and much more robust, and allows you to run lower pressures with less risk of pinch-flatting.

Another factor is whether you’re running tubes or tubeless, which will also affect your ideal pressure range. And as is becoming increasingly common, riders who use tubeless inserts – such as Procore, Huck Norris, or CushCore – will be able to run lower pressures due to the protective insert.

So, as you can probably gather, there are a whole lot of factors that influence the ideal tyre pressure.

I Don’t Care – Give Me Some Numbers!

To give you an idea of how different sized tyres can require different pressures, here are some of the pressure ranges that I typically run. Bear in mind that I weigh about 70-72kg with gear, have my tyres setup tubeless 99% of the time, and ride on the less smashy side of things;

• 29×2.2in XC tyres: 23-26 psi
• 27.5×2.3in trail tyres: 21-23 psi
• 29×2.3in trail tyres: 18-20 psi
• 29×2.6in plus tyres: 15-17 psi
• 27.5×2.8in plus tyres: 14-16 psi

Now, factoring in everything I mentioned earlier, you’ll need to take these pressures with a pinch of salt – they are merely there as an example, rather than a guide. Your riding style, weight, and trail conditions will see your ideal tyre pressures vary from mine.

To begin with, I’d recommend starting off with a higher tyre pressure, and ride a section of trail you know well – one that has a good variety of surfaces and obstacles along it. As a general rule of thumb, I’ll run 2-3 psi more in my rear tyre than the front. Ride it a second time, but this time with 2 psi less in both tyres. Ride it a third time, with another 2 psi less, and monitor how the bike feels and reacts on the trail. The idea is to slowly work your way down to a lower pressure that will enhance traction and comfort, but not to go so low that the tyre feels floppy and vague. If that starts to happen, bump the pressure back up 1-2 psi and repeat the section of trail.

Don’t leave it there though. It’s important to check your tyre pressure regularly – particularly if you’re running tubeless tyres with a sealant inside, which can lose pressure more quickly than a tubed setup. To the ire of my riding peers, I’ll literally check tyre pressure before every ride, but that’s probably going over the top for most. Once a week or fortnight is fine to make sure you haven’t lost significant pressure since the last ride.

If you’re heading to a different trail where you know the rocks will be particularly pointy, or perhaps a bike park with high-impact jumps and berms, try bumping up the pressure a little over your usual setting and keep an eye on how the tyres feel through the ride. Note down the pressures in your phone, and use them as a point of reference for future rides. That way you’ll have your tyres feeling right from the get-go, and hopefully you won’t be that person in the group who ends up with a pinch flat on the very first descent.

How Important Is Accuracy?

You might assume the answer to that question is directly proportional to how fussy a rider you are. However, any rider – fussy or un-fussy, punter or pro level – can benefit from measuring and recording their tyre pressure. Having worked in bike shops for over a decade, I’ve seen my fair share of high-end full suspension bikes in the workshop that have had their tyres pumped up to kingdom come. And it doesn’t matter how much XTR or World Cup you have on your bike – if your main contact point with the trail is set up poorly, expect the ride to reflect that.

Because mountain bike tyres these days are quite a bit wider than they were in the past, small changes in pressure result in a more significant change in terms of a percentage value. By that I mean, the difference between 20 psi and 22 psi in a 29er tyre is much more noticeable compared to the difference between 30 psi and 32 psi in a 26in tyre. A couple of psi difference in a modern, high volume tyre can work out to be a 10-15% change in pressure. And that’s very much noticeable.

OK, OK, I Get It! What Should I Buy?

So what is available on the market, and how do they perform? All the nerdery attention to detail above is important, but is irrelevant if the gauge itself isn’t user friendly so you don’t end up actually using it. Since I’m sure you’re now convinced that you absolutely definitely need a tyre pressure gauge, hop on over here to check out the results of our group test and decide which one will be making its way into your kit bag.

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