Think of the wheel as an inclined plane or a ramp.
The advantage of the mid-sized format is that it is supposed to deliver the benefits of a 29-inch wheel while making it easier to eke out clearance for rear suspension travel and for the action of the front derailleur.
With so much confusion and misunderstanding about the two wheel sizes, certain powers in the mountain bike universe have decided to make things even more confusing by offering a third wheel size, halfway between the 26er and the 29er wheel, the 27.5-inch wheel (also referred to by bike geeks as 650b). 26-inch wheeled mountain bikes than any other technical feature, and that includes suspension and drivetrain questions. The MBA wrecking crew fields more questions about 29- vs. Riders who speak negatively of the larger wheels have not ridden one recently or prefer more than four-inches of rear wheel travel. The final proof that 29ers have arrived may be their growing acceptance by our European counterparts, who avoided 29ers like the bubonic plague. Niner and Fisher Signature Bikes from Trek are only available with 29-inch wheels. There are even companies that only produce 29er bikes.
They enjoy plenty of wheel and suspension choices, and their geometry has been dialed to take full advantage of the 29-inch format. Today, 29ers are no longer an ember, but a forest fire. There were, however, enough riders who rode in desert (sandy), winter (snow) or wide-open (fire road) conditions to keep the glowing ember of the 29er alive. Most riders went right back to their 26ers after a demo ride on one of these early creations. Early adopters of 29er technology got bikes with heavy wheels, heavier steering response, hook-and-ladder-long wheelbases, mediocre fork performance and tires that were too big and heavy. Accepted frame geometry, intended to work around 26-inch wheels, needed modifications to take advantage of the larger hoops. The early 29ers had very limited support from tire companies and even less support from suspension suppliers. The best-known advocate for breaking the 26er mold was Gary Fisher, who championed a larger-diameter wheel that would deliver more contact with the trail, be less affected by the trail surface, lower the bottom-bracket height in comparison to the wheels’ axles, and, once up to speed, roll like a runaway freight train. Then came a group of frame designers who questioned the superiority of 26-inch wheels. Mountain bikes were originally designed around 26-inch diameter wheels, and these wheels had a long, unchallenged reign as mountain biking’s recognized “standard.” This meant that tire, rim, suspension and frame design and technology all started and ended with 26-inch wheels. We are talking about the wheel-diameter war. Not since dual-suspension mountain bikes debuted has there been so much controversy and confusion surrounding a mountain bike technology.