For almost all of the last two decades, there was no gray area when it came to wheel diameter sizes for mountain bikes: 26-inch wheels reigned. Over the last several years, however, two other sizes have rolled to the forefront and changed the “standard” for dirt lovers. Bikes with 29-inch and 27.5-inch wheels are now in the mix, which hasn’t made it any easier for consumers.

The 29-inch wheels (or 29ers) have been a part of the mountain bike landscape since at least 1999, when prominent bicycle frame-builder Gary Fisher introduced one of the first. The 29-inch wheel provides more traction due to a bigger contact patch with the ground; it offers greater rolling inertia; and it can plow through technical objects with more ease (not necessarily finesse; think monster truck).

Designers began to wonder how to capitalize on the benefits of the 29er but still keep the good qualities of the 26-inch wheel, namely maneuverability and handling. The 29ers of just a few years ago all had longer wheelbases, making them stable at high speeds and good at tracking in a straight line but scary to maneuver in tight corners and down steep rocky terrain.

This led to the 27.5-inch wheel, or the “middle size,” which balances out the pros and cons of both wheel sizes. This wheel size has surfaced as the happy medium for riders seeking out rockier terrain and twisty trails. You’ll get the rolling momentum of a larger wheel in a smaller, more nimble bike.

The 29er may be the preferred choice where neither suspension nor the agility of the bike are critical, such as dirt roads, pavement or smooth trails, while the handling and suspension offerings of the smaller-wheeled bikes shine on technical, rocky trails.

One other wheel size in today’s mix is also 26 inches in diameter, but it is 4.6 inches wide. This is the wheel of choice for fat bikes. Often considered a snow or sand bike because the large, low-pressure tires “float” on soft surfaces, a fat bike isn’t limited to those conditions. Some mountain bikers use their fat bikes on all types of trails year-round.

Don’t discredit physique when making a purchase. One could make a blanket statement and say that rider height should dictate wheel size, and some manufacturers do. Shorter riders may feel like they’re driving boats when they’re on 29ers, and taller riders may greatly prefer them.

The best thing to do is visit the experts at your local bike shop, tell them your riding goals, and test-ride as many different bikes as you can.

Trina Ortega is managing editor of Mountain Flyer magazine.