Tire Pressure: When It’s Too High
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 of our recently launched series of articles about tire pressure, Krista explained how too-low tire pressure can cause rim dings and tires to collapse in corners - something that produces a general feeling of squirminess and can undermine rider confidence. Proper tire pressure is based on rider style, weight, tire casing thickness, rim fit and tire profile, but what about going too far the other way? How high is too high?
Typically when you are riding and have the experience of bouncing off rocks or can’t hold your line in choppy terrain or rock gardens, it’s time to take a look at two possible culprits: your suspension setup and your tire pressure.
First, dial in your suspension to make sure you fork’s air pressure is not too high and its rebound is not too fast. However, also note that too low fork air pressure and too slow rebound can result in a similar feeling because you are sagged too far into the travel - not rebounding out - and thus operating in the firmer part of the travel (and to make matters worse, you’ll have more weight over the front end). If your suspension air pressure, rebound and compression settings can’t be dialed, then it is time to chat with a custom tuner. I use a the DSD dual air chamber after-market RUNT and have a custom softer compression tune on my 160mm RockShox Pike. I’ll soon be doing the same for my 140mm Pike.
Once your suspension is properly set up, then it’s time to check your tire pressure settings because you might be running too much pressure. Ideal tire pressure will vary among different tires, riders and terrain.
Tire Casing Matters
Tire choice affects ideal tire pressure. For example, a tire with a thicker casing has stiffer, more supportive sidewalls and thicker rubber overall. Their overall sidewall stiffness makes those tires harder to compress, and the thicker rubber is harder to puncture. Use of such tires may result in a tire pressure that is too high because both characteristics actually allow you to run a lower pressure vs. a lightweight tire of the same volume. With such tires, you get more protection against rim dings, and your ride feels feel a bit more damped and less bouncy for a given tire pressure.
Volume Matters, Too
Higher volume tires can typically be run at lower pressures than lower volume tires. Think, for example, about the pressures you use in your road tires vs. your cross country, downhill, plus or fat bike tires.
The Stickiness Factor
A stickier, lower durometer tire will connect better with a variety of surfaces. Depending on terrain, such a tire can be run at a higher pressure than the same tire with a longer lasting, harder compound to achieve the same grip. The harder tire will need a greater footprint (lower tire pressure) to achieve a similar coefficient of friction.
Profile: Round vs. Square
A tire of a given size with a square profile may have a larger footprint than a tire with a round profile. Thus, if a certain footprint is required – say for technical climbing (think of rock-crawling jeeps) – a tire with a rounder profile may have to be run at a relatively lower pressure than a square-profile tire to create the same footprint.
One thing I don’t have to think about is rim selection. All Stan’s rims have low, durable side walls which means that tires mounted to them end up shaped so that they have a higher volume and less chance of a rim strike. They also have a nice, tight bead socket. Tall rim side walls, on the other hand, typically allow the tire bead to move and produce a sloppy fit that contributes to burping a tire under load. To combat this, some people add layers of tape to build up their rims and create a tight bead socket, but this makes the tire harder to get on, and the bead socket is only as good as the tape/adhesive.
Stan’s rims are designed for a tight bead socket with a trough in the rim for easy tire installation whereas taller rims are also thinner and can take less of a hit before denting. Tall, thin (sharp) rim walls are also more prone to snake bite a tire.
Rider weight and style play an important role in determining ideal tire pressure. A lighter rider or a more mellow rider will feel like they are being bounced off rocks at a tire pressure which may be too low for a heavier or more aggressive rider.
Simply being aggressive or passive also changes your ideal pressure for any given rider. For example, I need to run a tad more pressure in a race run than I do on a mellow pre-ride.
Stay tuned for Part 3 of our series on tire pressure!
All photos by Kenny Wehn.