29ers or two-niners are mountain bikes that are built to use 700c or ISO 622 mm wheels. Most mountain bicycles use ISO 559 mm wheels which are commonly called 26" wheels. The ISO 622 mm wheel is typically also used for road-racing, trekking, cyclo-cross, touring and hybrid bicycles. In some countries, mainly in Continental Europe, ISO 622 mm wheels are commonly called 28" wheels or "28 Incher".
The term 29er (or two-niner) is a little misleading. 29er rims have a diameter of approximately 24.5" (622 mm) and the average 29" mountain bike tire has an outside diameter of about 28.5" (724 mm), though tires exist with diameters of over 29.15" (740mm). The typical 26" rim has a diameter of 22.0" (559 mm) and an outside tire diameter of about 26.2" (665 mm).
In the early 1980s, the size of the wheels for the emerging mountain bikes was undecided. English off-road cycling pioneer Geoff Apps contacted Gary Fisher and Charlie Kelly with news of 700c snow tires from Finland, that Geoff had built a bike around. In December 2006, Gary Fisher, speaking about the growing popularity of 29ers, gives his perspective: “We got some tyres from Geoff Apps really early on and we [Fisher and Kelly] said ‘Holy Toledo!’” But the poor supply situation of the larger diameter tyres meant the fledging MTB industry stuck with the smaller wheel size." 
Various people claim to or are attributed to be the originator of the term "29er", the most plausible being Wes Williams of Crested Butte, CO. The US division of Bianchi Bicycles offered a line of 29" wheeled off-road bikes beginning in 1991 called the Project bikes. Their 1992 product catalog raved about the advantages of the larger wheels and showed three different bikes, the Project 3, 5 and 7. The original company Klein produced a small quantity of a 29" wheeled version of their successful "Attitude" MTB racer, and named it the Adept. It failed to find a market and was discontinued. In the mid 1990’s, Diamondback Bicycles made their Overdrive bike, and Specialized made their Crossroads bike with 700c wheels, but were actually hybrid bicycles with frame and fork clearance for larger tires. The Project and Overdrive bikes were not a success for many reasons, primarily a lack of proper off-road tires and suspension forks competitive with the 26" offerings of the time.
A key product release, the first true 29" tire, was produced by an early supporter of the 29" movement Wilderness Trail Bikes. The company introduced the first true 29" tire, the Nanoraptor, in 1999. At about the same time, White Brothers produced the first commercially available 29" suspension forks. Before then suspension forks used were forks designed for trekking bikes or hybrids. For many years 29" frames and bikes were usually only available from small little-known manufactures like Niner Bikes. Surly Bikes introduced their 29" frameset, the Karate Monkey, in 2002. Gary Fisher Bicycles, a division of Trek Bicycles, became the first of the major manufactures to offer a line of 29" bikes. Their lines never sold well until the introduction of single-speed 29" bike the Rig, in 2004. Today most bicycle manufacturers in the US market offer at least one 29" bicycle or frame. Even companies that openly dismissed 29" as a bad idea or passing trend, Specialized and Turner, are bringing 29" wheels to market.
A tire with a tread width of less than 2.0" (50 mm) is considered a Cyclocross tire by 29" enthusiasts, even though in cyclocross, any tire wider than 1.5" (38mm) is not a cyclocross tire. Although they are both used offroad and typically use a 622mm rim, cyclocross bikes and 29" wheeled MTBs differ in every other possible way, from bikes basic handling geometry, to construction methods, durability and intended lifespan. Bikes exist that blur the distinction by combining attributes of both, however.
A debate over the advantages and disadvantages is currently raging in the mountain bike community. Those who believe the 29" wheel to be inferior often mention added weight, perceived sluggishness in handling, and problems with fit (specifically, front wheel/toe overlap and high standover height). 29" enthusiasts respond with comments about reduced rolling resistance, perceived increased stability without sacrificing quick handling, and an enhanced ability to roll over obstacles.
- Larger wheels roll over obstacles more easily due to decrease in approach angle
- 29” wheels are less prone to sinking in soft material such as sand and mud
- The longer contact patch increases cornering and straight line traction
- The larger wheels tend to raise the allowable height of the bottom bracket, cranks and chain wheels improving ground clearance
- 29" bikes tend to offer taller riders a more "natural" frame geometry
Most of these claims have yet to be objectively investigated. Small scale, unpublished studies (including one done by Pepperdine University, reportedly at the request of Gary Fisher) exist but both proponents and detractors of 29" wheels are generally unimpressed with their lack of scientific rigor. Long debates over how to conduct a "fair" test of the efficiency of 29" vs 26" mountain bikes have raged online, but no serious efforts have been made to conduct a large-scale, scientific study.
- Increased wheel weight and rotating mass - the spokes, rim, and tire are all larger.
- Longer spokes and rim result in a more laterally flexible wheel.
- Longer spokes and increased angle between hub flange and rim result in a weaker wheel.
- Many types of tires, rims and forks do not come in 29"-compatible versions, though the expanding popularity of the size is reducing this problem.
- Smaller riders (i.e. less than 5'5" tall) may not be able to find a 29" bike with a geometry suitable for them. Numerous examples exist of custom bikes built for very small riders with 29" wheels, but in many cases smaller riders face significant geometry tradeoffs, especially with regard to toe overlap, handlebar height, and standover.
96 or 69 VariationsEdit
One variation is to have a 29" front wheel and a 26" rear wheel (commonly called a "96er"). Using the smaller rear wheel allows shorter and quicker handling frames, more options for rear suspension designs and lighter bicycle weight. Another variation is to have a 26" front wheel with a 29" rear wheel (commonly called a "69er"). Depending on the manufacturer, these names can be, although rarely, reversed. For example, Trek introduced a "69er" in 2007 with a 29" front wheel and a 26" rear wheel. Carver Bicycles has a "96er" with a 29" front wheel and a 26" rear wheel.
- article on bicycle tire and rim sizes from Sheldon Brown's website
- Twenty Nine Inches, A 29er enthusiast site
- 29pollici.com an italian site entirely committed to the 29 inch wheels
- A 29er informational site
- Test data comparing 26" vs 29" bicycle wheels
- Cycling News Article comparing 26" and 29" performance
- Big Wheels or Small Wheels?, Lennard Zinn's July, 2005 VeloNews article
- Trigonometric basis for increased wheel strength at reduced spoke angles (Archived 2009-10-22)es:Bicicleta 29