The tire is the interface between the bike and the dirt. The tire has to roll, turn, provide traction, stop the bike, and add suspension. As little as a 5 pounds per square inch (psi) variance in tire pressure can make a big difference on how a tire will perform these duties. The following is a guide to help you determine the correct air pressure for your mountain bike tires.
Most mountain bikers know to read the tire sidewall to figure out the tire’s recommended tire pressure, but there are many who don’t know how to select a pressure for maximum tire performance. Most off-road tires have a pressure range specified with the minimum pressure often as much as 30 psi less than the maximum. This is quite a range to select from, and as a result, many mountain bike tires are overinflated.
An inflated, unweighted knobby tire has a round profile. You want to adjust your tire pressure so that when you put your body weight on the bike, the tire flattens out a bit where the tire contacts the dirt. If a tire is overinflated, the surface area of tire in contact the ground in minimized and traction is reduced. This effect is amplified further when turning, a moment when you want maximum traction. Low rolling resistance may be the goal when riding on the asphalt, but traction is our biggest battle when riding a mountain bike. It’s also important to remember that the more knobbies you have in contact with the dirt, the better your braking will be.
When we think of mountain bike suspension, we instantly think of the expensive and elaborate hydraulic telescoping devices that are today’s forks and rear frame triangles. But good suspension begins at the tires. A properly inflated tire will better absorb and conform to trail irregularities, and help keep your wheels from deflecting off rocks and roots.
How Low Can you Go?
Here are some guidelines for getting the most traction and suspension from your tires:
- The minimum pressure recommendation on the sidewall is a good starting point for most mountain bike tires. Ride, test, and adjust to suit your tastes and requirements.
- In some cases, you can actually run less pressure than the minimum listed on the sidewall, and get better performance. This is especially true for front tires. I run my 2.5″ wide front tire at 22-25 psi, even though the recommended minimum is 35.
- When experimenting with pressures below the recommended minimum, start with small increments of about 2 psi, and work your way down.
- Keep an eye on how your tire interacts with the rim when running low pressure. Too low, and your tire may slip along the rim when braking, leading to a broken valve stem. You don’t want the tire so soft and squirmy that it rolls off the rim either. Keep enough air in the tires to prevent pinch flats! Again, start off with small psi changes, and see how your tires react.
- Rear tires generally need more pressure than front tires. Not only is there more weight over a rear tire, we tend to wheelie over trail junk and let our rear wheel slam into the obstacle. I run my rear tire 5-7 psi greater than the front.
- The fatter and/or taller the tire, the lower the pressure you can run. Although it comes with a weight penalty, you can make your ride pretty plush or float over sand with aired-down 2.2 – 2.5″ tires. You may even consider running a big aggressive front tire, but keep your rear tire fast and light.
- When inflating your tires after a tire swap or flat repair, you may have to overinflate your tire first to seat the bead of the tire into the rim. Once the bead is seated, you can let air out and recheck the pressure.
- The little round nut on a presta valve stem is helpful for keeping the stem in place during inflation, but take it off when complete and put it in your tool kit. If you’re running big aired-down tires, the valve stem may tear off if it can’t flex with the tube. I went through a few tubes before I figured this out!
Other factors that can influence tire pressure settings
Now that I’ve talked you into experimenting with lower tire pressures, keep the following in mind:
- Body Weight – Heavier riders require more tire pressure.
- Terrain – Rocky trails may require greater tire pressure to keep the tube from pinch-flatting, or to prevent damage to your rims.
- Tire Size – Skinny race tires need more pressure; wide and tall All-Mountain tires can run with much less
- Speed – The faster the impact, the more tube and rim protection you need. Consider how fast you’ll be riding and the conditions of the trail.
- Tubeless Tires – Tubeless tire systems can run lower pressure without fear of pinch flats, but you still need to be wary of rim damage.
Get out there and experiment with your tire pressure! In the end you should have a tire that provides better traction and suspension, without sacrificing much rolling resistance. If you already own a pump, it could be a FREE performance upgrade.
Did I miss anything? What additional recommendations do you have?