TIRE PRESSURE: MATERIAL AND SHAPE MATTER

In 2006, Krista Rust left her job as an Electrical Engineer for Motorola to race her bike, and she's been traveling, riding, and helping people keep their bikes rolling ever since. Krista is an ambassador for the benefits of tubeless and has spent years helping riders make the switch from inner tubes. Originally an XC racer and now on the enduro scene, Krista lives a life on the road traveling and racing around the world. We've asked her to weigh in on some of the most common questions people ask about tubeless performance. In Part 1 and Part 2 of our series of articles about tire pressure, Krista taught us about too low and too high tire pressure. In this Part 3, she explains how some other key tire properties affect our choice of tire pressure and influence our riding experience.

Tire Compound

While tire pressure certainly plays an important role in your perception of how a tire rides, another important factor is the specific compound(s) out of which tires are made. The lower the durometer number, the softer (stickier) the rubber. Typically mountain bike tires for the avid rider are in the range of 40-62 durometer. If a tire’s rubber compound is on the harder side, the bike will lose traction and slip out more easily on most surfaces, but they will last longer than a softer rubber tire. In my experience, “faster rolling,” harder tire compounds can actually slow riders down because they either expend effort to avoid sliding out or simply apply more brakes. Riders can be more easily sketched out on the harder tires and thus less confident in their handling. Krista considers what kind of compounds a tire is made of when selecting what to put on her bike. Harder tires will ride differently than softer tires. (Photo credit: Bill Freeman) If you are riding a tire made of a harder compound, you can lower the tire pressure to give the tire a bigger contact patch, which means it will then have more grip and feel less likely to slide out. While a low durometer rubber will provide an amazing grip, it will wear out much more quickly. As a starting point, I suggest buying dual or multiple compound tires. These have a harder base layer and a softer top layer. It means you’ll stick well and be less likely to puncture the casing. Also, the harder durometer on the lower part of the knob helps keep the knobs from folding over or ripping off. My recommendation is to ride with increased confidence and buy tires more often.

Tire Casing Thickness

Stiffer sidewalls better support a tire run at lower pressures, and thicker casings better protect from slices and punctures. That said, two-layer casings, basically downhill tires with Kevlar beads, are my favorite choice for riding rough terrain at a high rate of speed. Such tires allow for running a lower pressure in that they provide a supportive platform that doesn’t collapse while also offering an increased resistance to punctures and tire snakebites. Some weight-obsessed riders think that they want lightweight tires, but after losing speed in every corner and tiptoeing through rock gardens, it turns out that they would actually be faster, have more fun and be more confident by simply riding a heavier, stickier tire. Of course, for cross country riding, two-ply casings are overkill, but the same thought process applies. And I say this as someone who used to run 440-gram, 26” tires.Side Knobs

Confidence comes from a good tire setup, which means you are able to go faster, lay the bike over and actually engage your side knobs vs. riding in the transition area of the tire. If you do not lay the bike over enough to use the side knobs, buy a tire with a good transition knob until you develop this skill. Avoid tires with a void transition area as there are no knobs to ride on; these tires are for those who really utilize the side knob, and the missing transition knobs help the side knobs engage better. Being able to use and trust your side knobs means that you’ll ride that much more confidently.

Tread Pattern

Tread pattern is another important variable; tires with bigger and more widely spaced knobs, tires with smaller more tightly spaced knobs and semi-slick tires with good side knobs will all ride differently. Properly matching not only tire pressure, but also tread selection with terrain and ride style can make all the difference in your riding experience. For example, if pedaling efficiency is important and big knobs are not required for traction or braking, you could go with a semi-slick rear tire.

Tire Profile

Last but not least, you might want to pick different tire profiles for different wheels - for example, a round one on the front and a square one on the rear, so that the side knobs engage at the same time when leaning in a flat corner.