29ers or two-niners are mountain bikes and hybrid bikes that are built to use 700c or 622 mm ISO (inside rim diameter) wheels, commonly called 29" wheels. Most mountain bikes use ISO 559 mm wheels, 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".
29er rims have a diameter of approximately 622 millimetres (24.5 in) and the average 29" mountain bike tire is (in ISO notation) 59-622 - corresponding to an outside diameter of about 29.15 inches (740 mm). The typical 26" MTB tire has a rim diameter of 559 millimetres (22.0 in) and an outside tire diameter of about 26 inches (660 mm).
In the early 1980s, the size of the wheels for the emerging mountain bikes was undecided. So when English off-road cycling pioneer Geoff Apps contacted Gary Fisher and Charlie Kelly with news that he had built a prototype off-road bicycle that used large-diameter 28 x 1¾ (ISO 47-622), 47mm wide Nokian Hakkapeliitta snow tires from Finland, they were intrigued. In a letter to the December 2006 issue of Bike Biz magazine, Gary Fisher, speaking about the growing popularity of 29ers, gives his perspective: "We got some tires from Geoff Apps really early on and we [Fisher and Kelly] said ‘Holy Toledo!'" But the poor supply situation of the larger diameter tires meant the fledgling MTB industry stuck with the smaller wheel size."  The first Geoff Apps-designed 700C wheeled off-road bicycle was made in 1981.
However the bicycle marketed as the Aventura by Apps' own company, Cleland Cycles Ltd, between 1982 and 1984 used the more readily available Nokian Hakkapeliitta 26 × 2 (ISO 54-584). The 650B size, also known as 27.5", reappeared in 2007 as a compromise between the 26 inch and 29 inch sizes. The 650B size is called 27.5". 
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-1990s, Diamondback Bicycles made their Overdrive bike, and Specialized made their Crossroads bike with 700C wheels, but they 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 in (51 mm) in width is sometimes considered a cyclocross tire by 29" enthusiasts, even though in cyclocross any tire wider than 1.5 in (38 mm) is not a cyclocross tire. Although they are both intended for off road use, and typically use a 622 mm rim, cyclocross bikes and 29" wheeled MTBs differ in their basic handling and geometry, construction methods, durability, and intended lifespan. Bikes exist that blur the distinction by combining attributes of both, however. One example of this is a Monstercross bike, often using a standard mountain bike frame intended for use with 26" wheels with 700c wheels and 700 x 38c-45c tires, disc or cantilever brakes, MTB or cross gearing, and the drop bars of a cyclocross bike.
The advantages and disadvantages of 29ers are often debated in the mountain bike community. Disadvantages of the 29" wheel include added weight, perceived sluggishness in handling, and problems with fit (specifically, front wheel/toe overlap and high standover height). Advantages include reduced rolling resistance, perceived increased stability while retaining quick handling, and an enhanced ability to roll over obstacles.
One item that is often raised is tire contact patch size and shape. All else being equal, such as tire width, rim width, inflation pressure and rider weight, the contact patch of a 29" wheel has the same area and is slightly longer (~5%) than that of a 26" wheel.
- Larger wheels roll over obstacles more easily. The ability of a wheel to roll over obstacles is proportional to its size. A 29" wheel, which is about 10% larger than a 26" wheel, can roll over 10% larger obstacles.
- The larger diameter wheels have more angular momentum so they lose less speed to obstacles and rough sections but the same effect can be achieved with larger tires.
- 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 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 (larger tire, tube, rim, and spokes) makes the wheels slower to respond to acceleration and braking,  and adds to unsprung mass.
- Longer spokes and decreased angle between hub flange and rim result in a more laterally flexible wheel (ceteris paribus).
- Less responsive handling because of longer wheelbase and heavier front wheel.
- Smaller riders (i.e., less than 5'5" (=165 cm) tall) may face significant geometry tradeoffs, possibly resulting in toe overlap, high handlebar height, and less standover clearance.
96 or 69 variations
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" though Trek introduced a "69er" in 2007 with a 29" front wheel and a 26" rear wheel).
- "29er Mountain Bikes - All you ever wanted to know about 29 inch wheeled bicycles". ConsumerREVIEW.com. Retrieved 2012-09-08.
29ers are mountain bikes built around 700c sized wheels resulting in an outside tire diameter of about 29 inches
- Sheldon Brown (April 18, 2012). "Tire Sizing Systems". Retrieved 2012-09-08.
Unfortunately, evolution of tires and rims has made these measurements lose contact with reality. Here's how it works: Let's start with the 26 x 2.125 size that became popular on heavyweight "balloon tire" bikes in the late '30's and still remains common on "beach cruiser" bikes. This size tire is very close to 26 inches in actual diameter. Some riders, however were dissatisfied with these tires, and wanted something a bit lighter and faster. The industry responded by making "middleweight" tires, marked 26 x 1.75 to fit the same rims. Although they are still called "26 inch", these tires are actually 25 5/8", not 26". This same rim size was adopted by the early pioneers of west-coast "klunkers", and became the standard for mountain bikes. Due to the appetite of the market, you can get tires as narrow as 25 mm to fit these rims, so you wind up with a "26 inch" tire that is more like 24 7/8" in actual diameter!
- "The Beginnings of the Modern 29"er: A History". Guitar Ted Productions. Retrieved 2012-03-10.
- "A brief history of Cleland Cycles". clelandcycles.wordpress.com. Retrieved 2013-02-07.
- Gary Fisher on the history of mountain bike wheel size https://www.youtube.com/watch?v=8syt59gK65o
- Josh Patterson (2012-03-22). "Catching Up With: Kirk Pacenti". Dirt Rag. Retrieved 2013-07-05.
- www.peterwhitecycles.com Studded Bycycle Tyres - Retrieved 2017-02-24.
- Don Cook (August 15, 2007). "The Big Wheel Mountain Bike Store". Dirt Rag Magazine. Retrieved 2014-11-01.
The usage of the term 29er comes from the first production bike available to the masses from the Fisher bike company, called the Two Niner, which came out in 2001.
- Beth Carter (2012-09-10). "Building Sweet Bikes and a 'Business of the Future'". Wired Magazine. Retrieved 2012-03-10.
Niner staked its entire existence on the 29-inch wheel, a radical idea in 2005.
- "29er Mountain Bikes: Strengths and Weaknesses". Mountain Bike Buzz. Retrieved 2013-03-10.
- Bill McCready. "Wheel Size 26 vs 700C". Santana Cycles. Retrieved 2011-06-27.
Between 700c and 26" there is no difference in the AREA of the contact patch.
- Zinn, Lennard (Dec 2002). "Velonews: Technical Q&A with Lennard Zinn". Retrieved 2015-07-27.
- Lennard Zinn (January 28, 2011). "Calculating a wheel's moment of inertia". VeloNews. Retrieved 2017-11-12.
- Keith Scott (October 7, 2013). "Educating the Debate – Part I". NSMB. Retrieved 2017-11-12.
- Simon Collis (October 31, 2012). "29er bikes: what you need to know". Mountain Bike Rider. Retrieved 2017-11-12.
- Vernon Felton. "Mavic Goes Big, Three New 29er Wheelsets Available Soon". Bike Magazine. Retrieved 2013-03-10.
Mavic’s engineers, however, put a much finer point on it. Twenty-nine inch wheels, they contend, are 39 percent less stiff than a similar 26-inch model built exactly the same and will weigh 10 percent more than the 26’er.
- Vernon Felton (March 4, 2013). "War of the Wheel Sizes". Bike Magazine. Retrieved 2017-11-13.
The longer wheelbase adds stability but has the unfortunate effect of also reducing a bike's maneuverability. Compounding the cumbersome, long wheelbase problem was sluggish steering. The increased gyroscopic effect of the bigger front wheel requires that you apply more effort at the handlebar in order to make quick steering changes.
- Towle, Dan, "Why Use Smaller Wheels on Smaller Size Bicycles?", bicyclepaper.com.
- "Carver Bikes 96er". Retrieved 2007-01-08.
- Stevenson, John (2005-08-29). "Tech News: Travis Brown's 69er". Cycling News. Retrieved 2006-08-22.
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