Lately, there has been debate on the optimum size wheel for off roading. Some say one size for both is not even right! I find that there are alot of histrionics involved in many of the statements I've seen. I hope to present here, a somewhat opinionated yet measured response, marginally grounded in historical mythology. ( History being a somewhat moldable thing, depending upon the point of view the historian has!)
The "standard" off road size wheel has been the "26" inch sized wheel. It was adopted by the early pioneers of modern, California style off roading as a reasonably tough, cheap, and readily available choice to use. It didn't hurt that the "clunker", or cruiser style frames represented fun, youthfullness, and rugged exploration to these hardened roadie cyclists. It was as about as far away from the jaded race seen, and it's accompaning hoopla as you could get. No gears, fat tires, and heavy, somewhat durable bikes that could be ridden in serene off road settings, far away from the clamour of regular life. Then someone suggested compitition. Then the technology had to change, to keep up with the demands of compitition. Soon, the old cruiser frame was replaced by a purpose built frame. The single speed replaced by a wide range derailluer set up. Parts were sourced that had lighter, stronger, and higher performance than the old cast offs. And then......someone discovered that you could make money selling this stuff!
What's that got to do with wheels, you say? Well, check out that short history lesson again my friend, and you will notice that the wheel size was chosen not for technical superiority, but as a choice of convienience! The wheel size in question happened to work out okay from a standpoint of availability, price, and toughness. The early California pioneers of the MTB didn't have a research lab to see what would make the optimum wheel size for off road adventures. They made their choice out of necessity. However; if they had read their bicycle history books, they may have found a better way!
Years ago, the Safety bicycle was developed around two wheels of similar size, with a chain driven rear wheel. Much experimentation was done to arrive at a wheel size that was the most efficient for the rider, and the terrain of the day. This was usually unpaved, dirt roads! The tire size most regarded as being suitable for adult sized riders was approximately 28 inches in diameter. Keep in mind that the average size for an adult male at that time was well below six foot! Another interesting fact is that in order to accomodate the burgeoning youth bicycle market in the early part of the twentieth century, Arnold Schwinn Company introduced the "balloon tire" bike, which sported an approximate diameter of 26 inches. This was done to reduce the frame sizes so that any child might be able to ride the bike. The balloon tire was wider, and tubular to be run at pressures ranging from 18psi-22psi! This done in an effort to make them more durable, and withstand the rough treatment doled out by youths on not so smooth roads, and dirt paths.
Fast forward to today. Now, once again, the ability to run a more suitable, wide off road tire and wheel exists again with the "29 inch wheel" which is an approximation of the old "adult wheel size" mated to a "balloon" type, lugged tread tire. The tire mounts to the now standard 700c road bike sized rim, optimized for off road use. It's a design drawn from history. It's a wheel size that was proven to work best over other choices for rough, unpaved riding situations. It's a size that was designed for adults of the time. (Keeping in mind, the average adult male is over six foot today!) It's a better choice for trail riding than it's smaller diameter brethren. That's why 29 inch wheels are here to stay.