ALL ABOUT TIRE PRESSURE

It’s more important than you think

Of all the tuning and adjustment options you have on a bike, none is more important than tire pressure. Yet, it’s the single thing we see most riders taking for granted. We can’t even count the times we’ve witnessed riders with a 10k-plus bike give their tires a quick squeeze, shrug their shoulders, say “good enough” and ride away. Even a 1/2 psi one way or another can be felt by ultra-sensitive riders. Tire pressure has a dramatic effect on traction, flat prevention, ride quality and, above all, safety.

John Boyd Dunlap invented the modern pneumatic tire in 1887 when he upgraded his son’s solid rubber tricycle tires with a homemade, air-filled tire. Although technology has improved the overall design, an air-filled tire still offers the best combination of traction and ride quality—if you can keep it inflated of course. The key to making your mountain bike tire work optimally is finding and maintaining your ideal tire pressure. As simple as it sounds, there are so many variables that you will not find a tire-pressure guide or chart on any of the tire manufacturers’ websites. You might find some charts for road tires, but that’s because variables such as riding surface and general-use parameters are fewer.

“The ideal tire pressure is unique to each rider,” says Jason Richmond, lead design engineer for Bontrager tires. “It depends on factors such as rider weight, terrain, tire construction, rider aggressiveness, etc. Typically, for mountain biking, the best tire pressure is the lowest pressure that you can run without your tire bottoming out or squirming under load. This will provide you with the largest footprint, best bump compliance and fastest speed.”

Carbon rims are tough, but bottoming out the rim with too low of a tire pressure can break even the best of them.

 

TOO MUCH OF A GOOD THING

Get your tire pressure wrong and quite a few things can happen. The first thing you’ll notice is a jarring ride. The tire can’t conform to irregularities on the trail, so instead of being absorbed by the tire, bumps are transferred to the rider. Related to that ride quality is traction. Not only is the tire bouncing off the ground, the tire’s footprint or contact point with the ground is smaller. It’s commonly thought that over-inflating your tire will make it roll faster, but that all depends on the surface you’re riding on. Tinker Juarez used to be known for his rock-hard tires. If it’s smooth as glass, then yes, higher pressure often rolls faster, but, as you know, mountain bike trail surfaces are anything but smooth. Lower pressure actually rolls faster in many situations. Are there benefits to having too much pressure? Sure, since your tire cannot bottom against the rim as easily, pinch-style flats are greatly reduced or eliminated.

You will notice small wet spots at the rim/bead interface when the pressure gets too low. If taken too far, a puncture will occur.

 

HITTING ROCK BOTTOM

Too-low tire pressure comes with a whole other set of problems. The first and most obvious issue is the rise of flats. If the pressure is too low, your tire will bottom out on the rim, causing what’s commonly referred to as a “snake bike” puncture. The secondary problem here is potential damage to your rim. In the best-case scenario, you dent your aluminum rim but can bend it back to a usable form. Worst case? You shatter or crack your expensive carbon wheel. Even worse than that? You crash, too.

Another problem with going too low is that the tire will squirm when it has too little support under hard cornering. The sidewall of the tire folds over, giving a vague feeling in the best case, or peels completely off the rim in the worst-case scenario. Somewhere in the middle is what’s referred to as “burping” your tubeless tire. This happens when the tire’s bead separates from the rim for a split second, allowing air to escape and resetting itself.

Fat bikes are incredibly sensitive to pressure changes so they must be checked often and with a low-pressure gauge.

 

GOLDILOCKS

Somewhere in between these extreme scenarios sits the perfect pressure that’s matched to all the variables. How do you find that magic number? “The only way to find your perfect tire pressure is to ride,” says Richmond. “As a starting point, air the tire up enough that when you sit on the bike the tire sinks into its travel far enough to reach the transition area. This is the region where the center knobs are fully in contact with the ground but the shoulder knobs are completely clear from the ground (see lower right image)”

At this point, Richmond says you have to ride to get a feel for pressure. If you complete your ride without the tire bottoming out or squirming under load, he suggests that you try dropping pressure. If you sense bottoming or squirming, add pressure. He says to keep repeating this process, making small 1–2-psi adjustments, until you find your perfect pressure.

Riders will feel changes differently. Richmond says beginners may take 5 psi to notice a difference, while World Cup riders can feel a difference of 0.5 to 1 psi. He adds that the lower your tire pressure, the more you will notice a pressure change. Fat bikes, for example, can have a dramatic difference in handling on hard surfaces with just a slight psi change. Road bikes, on the other hand, take 10–20 psi to really feel a change.

Two Bontrager tires with lines to reference the approximate loaded footprint width.

 

VARIABLES

Here are where things get complicated regarding the variables involved in mountain biking. Got a thin, lightweight tire? A few extra psi can help prevent punctures and give it support. Heavy-duty DH tires or ones with a reinforced casing can often tolerate lower pressures. Another huge factor is whether you’re running tubes or tubeless. Tubes require higher pressure since they are far more sensitive to pinch-style flats than tubeless tires.

Narrow tires also need more air than larger ones. Got narrow rims? Extra air is required to prevent tire squirm. Wider rims reduce the balloon shape of the tire and give the tire more support, allowing for a few psi lower in extreme cases. Be careful, though. Just because you can go lower doesn’t mean you should if rocks are part of the program. It’s not uncommon for wide rims to allow low-enough pressure to cause puncture and rim damage. Heavy-duty tires are often required to run low pressures that wide rims allow.

Heavier, more aggressive riders will naturally need more pressure, so will e-mountain bike riders. “Tire pressure is typically higher for an e-bike to account for the extra load and forces,” says Richmond. “This load is not only the weight of the bike, it is also the additional force from impacts at higher speed.”

This Pirelli cutaway shows their ProWALL sidewall reinforcement technology to increase puncture protection and traction at low pressures.

 

Perhaps the biggest variable of all is the trail itself. Soil composition, moisture content, and the number of rocks, roots and other terrain covered on a trail can all affect pressure needs. Different trails require different tire air pressure to get the most out of the tire. It’s also not uncommon to need a different pressure for changing conditions on the same trail. Extremely wet or dry conditions both have different requirements.

One more variable to be aware of is elevation. If you start at sea level and travel to 5000 feet for a ride, expect your tire to be 2–3 psi higher when you get there and vice versa. Richmond says that the elevation’s impact on tire pressure is relative to the atmospheric pressure at that elevation. “The atmosphere exerts a higher pressure on all objects at sea level than at higher elevations,” he says. “Since the amount of air inside the tire does not change (in a short amount of time), the tire pressure changes relative to the external air pressure.”

Temperature can have an effect, too, mostly in fat biking. “When you set pressure indoors and then go out into freezing temperatures, your tire pressure will drop. Plan accordingly to ensure you are not stuck with a flat tire in the snow,” says Richmond.

Vittoria Air-Liner is a high-quality polymer insert, which replaces a portion of the air volume in the tire.

 

IT’S LIKE CHEATING

Technology has stepped up recently in the form of tire inserts that allow you to run lower than usual pressure and get away with it. Some work better than others, but all are based on a similar concept. They offer a cushion when the tire bottoms out on the rim, add sidewall support to reduce squirm and, in some cases, reduce the volume of air, making the compression of the tire more progressive. Tire inserts are not just for gravity riding, either. It’s not uncommon for top cross-country racers to use them, too, and allow lighter riders to race with pressures in the mid-to-high teens!

We typically keep a pump in the car so we can check air pressure right before we head out on the trails.
Photo by Tucker Adams @tuckerscreative

 

BEST PRACTICES

Most riders do not do this, but everyone should be checking their air pressure right before every ride. Richmond says that the higher your level of performance, the more critical this is. Air has a tendency to escape all tire systems, even tubeless tires with sealant. This is because the molecules that make up air are smaller than the rubber molecules, so they slowly sneak out over time. How long depends on the thickness and condition of your tire. Checking your pressure often can also reveal an unknown leak before it’s a problem. And, if a tubeless tire is suddenly low, it’s probably time to check and replenish your sealant.

How you check your pressure is just as important as how often. For the average rider, a standard floor pump with a built-in gauge is fine, but only if the gauge is suitable for your pressure. Most floor pumps have gauges ranging from 0–150 psi, which is fine for a road bike but less than ideal for a mountain bike. A gauge has an accuracy of +/- 2 percent throughout, but that accuracy will be closest to +/- 0 percent in the middle. So, if you’re one of those sensitive riders trying to squeeze every ounce of performance out of your bicycle, a low-pressure gauge that puts your ideal pressure in the middle is best. You can find low-pressure gauges on some high-volume mountain bike-specific floor pumps and as stand-alone gauges. One word of warning: if you mistakenly max out your low-pressure gauge with, say, a road bike tire, it damages the gauge and it must be replaced. If you use a high-pressure floor pump or compressor (like we do), make sure you use a stand-alone gauge with a range appropriate for your pressure needs to really dial things in. And, do it every ride!

Velo Tools are ideal for a rider who is keen on having accurate measurements.
Photo: Traece Craig @toc_photo

 

Efficient Velo Tools Bleedin’ Gauge

If you obsess over the details and want your tire’s air pressure spot-on every time, then the Bleedin’ gauge was made for you. To use this gauge, simply over-inflate your tire slightly. Slide the gauge over the open Presta valve (it’s not Schrader-compatible) and bleed the air down to your desired pressure via its side-mounted release valve. It’s easy and incredibly precise. The gauge is unaffected by temperature, and it’s protected from sealant contamination by a serviceable cotton filter. It’s easy to pick one that lands in the middle of the gauge range for maximum precision, since they are available with 0–15, 0–30, 0–60, 0–100 and 0–160 psi gauges or as a complete set.

Price: $93 per gauge

Contact: www.efficientvelo.com

The TyreWiz gives real-time tire pressure data and will pick up any fluctuations in pressure before it becomes a problem.

 

Quarq TyreWiz

The TyreWiz is basically a valve extender with a real-time tire-pressure sensor attached to it, and each sensor unit has ANT+, Bluetooth LE and NFC to connect to your phone or transmit to Garmin and Wahoo head units. This data is delivered live and has an accuracy of 0.1 psi. Each 10-gram TyreWiz is powered by a CR1632 battery with a claimed life of around 300 hours. The unit powers on automatically when it detects motion and will also turn off when idle for 10 minutes.

THE SETUP

We used the Quarq TyreWiz app first to set up each unit. The app can also be used as a handy starting point for tire pressure for everyone. There is an input for rider weight, bike weight, wheel diameter and tire width. The app will offer a suggested front and rear pressure, as well as a range that is saved for each unit. We used a Garmin Edge 1030 to display the pressures live. Unlike most sensors on the Garmin, all you have to do is download the TyreWiz data field at the Garmin IQ store. There is no sensor pairing; the data field will automatically populate when it is near a TyreWiz. If you don’t have a compatible computer or phone handy, there is an LED light to indicate the tire-pressure status as well. The LED will only flash when the wheel is not in motion. There are three display modes with flashing lights to detect when tire pressure is too low, too high or within the suggested range.

THE RIDE

At first glance, we felt that the TyreWiz was one of those products that is unnecessary and would only appeal to the self-indulgent cyclist. After we tried it, however, it was obvious how this could help optimize the ride quality and performance for any cyclist. Additionally, we now had a universal pressure gauge no matter what pump we used, even a mini-pump out on the trail. Although the suggested pressure was a bit under what we thought we should run, this seems to be the case with all the new pressure charts available. We went with the suggested psi and were surprised at the amount of tire-pressure fluctuation due to the ambient temperature change. We had set the pressure while our bike was in the shade and cooler than the average temperature during the ride. As we rode, the pressure rose, and we got to the point where it was well above the recommended range. We stopped and adjusted it back down for the riding conditions. This same practice could be very useful when out riding in changing conditions or terrain. We also had a few instances when we hit a rock and felt like the tire was losing air. A quick check of the pressure verified that we did indeed have a slow leak and allowed us to stop before it got dangerously low. On a few occasions with our new lower pressures, we were reassured that it was still good and just needed to get used to the added compliance.

THE VERDICT

The TyreWiz is one of those products that you don’t know you need/want until you try it. The knowledge and understanding of how much tire pressure affects one’s ride are astonishing. It is a safety device like those found on your car, and it is a performance device that allows one to specifically test and pinpoint the perfect balance of pressure and traction. Even if you don’t use the suggested pressures, they are a great starting point. It’s not without its faults, though, and one had us in a tough spot on the side of the road when we suffered a torn tire on our tubeless setup. Normally, that wouldn’t be a big deal—just pull the valve out and install a tube. But not so with the TyreWiz. The pressure sensor itself was easy to remove, but there is a small flange on the valve extender that the sensor rests on. It was too big to fit through the valve hole in the rim. We didn’t have the tool to remove the extension and instead had to force a few things.

Of course, priced at a touch over $100 per wheel, this is an expensive luxury. This is about how much this same technology costs on a car per wheel. While some see it as a good investment, it is a lot of money for something that many cyclists get by just pinching their tires before each ride. With the latest update from Garmin, this data is now available to analyze on their Connect site. The pressures are separated front and rear, and you can overlay ambient temperature, as well as a few other performance metrics that the Garmin Edge unit records. This offers riders and coaches insight on what tire pressures result in the best overall results, as well as how much temperature changes affect things.

Price: $203

Contact: www.sram.com

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