Why 27.5″ Wheels?

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For the past decade, wheel size has certainly been one of the most debated topics in the Mountain Bike Industry. And, the debate has probably peaked over the last 2 years with the main stream acceptance of 27.5” wheels. Although the 26” wheel has been (and still is) a good wheel size with some redeeming performance characteristics, it is for good reason that the 27.5” (or 650B) and 29” (700cc) wheels have basically taken over. To understand why it has taken so long to evolve it is helpful to have a little bit of history.

In the 1980’s, when Mountain Bikes first started being produced commercially, 26” wheels were a natural starting point for a few reasons. First, because of the inherent maneuverability, and easy acceleration, the 26” wheel provided a natural pairing to the heavy frames and less efficient drive trains available for mountain biking in those days. Second, the most trail appropriate early frames, tubes, tires, forks, and brakes the industry could produce economically for an emerging market segment were most naturally 26” wheel oriented. 29” wheel parts were primarily available for lighter weight road applications, and therefore not durable enough. The 27.5” wheel or (650B) was more of a European size and not very common in the United States, where Mountain Biking was growing the fastest. And maybe most importantly, as the U.S. market grew and, bikes were being imported from Asia and, with 26” wheels, they were treated as junior bikes by US customs, and therefore the importing tariffs were much less than ‘adult’ sized 29” wheels making these bikes more affordable to the end user. Despite efforts by Bianchi Europe to produce 29” wheeled bikes in the early 80’s, the establishment of the 26” wheel was the accepted standard, and the rapid evolution of suspension, brakes, frames, drive trains, etc. was naturally focused around the 26” wheel.

It was not until then industry powerhouse Gary Fisher (owned by Trek) started producing 29” wheeled bikes in the early 2000’s, along with existing niche brands Niner and Surly, that the 29” wheel finally created the demand required to produce a quality and diverse selection of parts and accessories. As a result the 29” wheel took off around 2004 and 2005 as it provided the upsides of a more stable ride, better role over capabilities, better straight line momentum, and better traction.

On the flip side, like most changes, the 29” wheel came with performance tradeoffs including loss of maneuverability, quickness, and additional weight. So, only a few short years ago, industry leaders Giant Bicycles, Santa Cruz, Scott and a handful of other leaders agreed that they wanted the best of both worlds. And, the 27.5” wheel became the refinement of its predecessors. Although the 29” wheel certainly has a strong following, and 26” wheels will exist for junior and other applications, the 27.5” wheel is only gaining momentum by winning world cup cross country races and downhill races alike while also seemingly being the sweet spot for recreational and enthusiast mountain bikers. The reasons are pretty convincing and out lined in Giants explanation below.

Why 27.5” wheels?

1. Lightweight

Significantly lower bike and rotational wheel weight helps you climb faster with less effort.

Overall Bike Weight

Compare the weights of identically equipped bikes with different wheel sizes and you’ll see substantial weight differences. As expected, the 26-inch-wheel bike is somewhat lighter than the 27.5, and substantially lighter than the 29 (up to two pounds of overall bike weight savings from 29 to 27.5). Every gram saved helps you ride faster.

Wheel/Tire Weight

The overall weight of a 27.5 wheel set (wheel, tire and inner tube) is only 5% greater than that of an identically built 26-inch wheel set. Compare this to the 12% increase of a 29-inch wheel set and you can see how a seemingly small increase in diameter results in substantial weight gain—and poorer performance when climbing or accelerating.

Weight Comparison

Static wheel weight
Lighter wheels/tires result in a quicker acceleration and lighter overall bike weight – a win-win combination.

Wheel Weight

More efficient

Snappier acceleration and a reduced angle of attack for a smoother, more agile ride.

Rollover

Increased wheel diameter decreases the angle of attack (the angle in which a round object intersects a square object). This is a good thing. A 29-inch wheel rolls over a 6-centimeter square-edge obstacle 14% more efficiently than a 26-inch wheel does. In comparison, a 27.5-inch wheel rolls over the same obstacle 9.8% more efficiently than a 26-inch wheel does.

Another way to analyze angle of attack is the degree of impact—where 26-inch equals X degree, 27.5 equals X-4 degrees and 29 equals X-6 degrees. Again, a shallower angle is better—so 29-inch takes the win, with 27.5 exhibiting nearly the same performance but without the weight penalty.

Rollover

Acceleration

Arguably the most important benefit of 27.5 over 29 is quicker acceleration. This is the “snap” that a rider feels when they push hard on the pedals. It is affected not just by overall static weight but also where the weight is distributed throughout the wheel. The farther the weight is from the center of the hub, the slower the acceleration. So a similarly constructed 1000-gram 29-inch wheel is slower to accelerate than a 1000-gram 26-inch wheel—because the larger diameter rim and longer spokes place weight farther from the hub. The key to snappy acceleration is minimizing the weight of the outermost components (rim, nipples, spokes, tire, tube). As you can see, a 27.5-inch wheel is only 1.5% slower to accelerate than a similarly constructed 26-inch wheel, but a 29-inch wheel is 3.6% slower than a similarly constructed 26-inch wheel.Acceleration

3. Better Control

A larger tire contact patch, increased stiffness, and optimized frame geometry improve traction, braking and handling.

Traction

The larger the diameter of a wheel, the greater the contact patch of the tire. A larger contact patch results in better traction, which leads to improved acceleration, deceleration and cornering. As you can see, a 27.5-inch wheel has a similar contact patch to the 29.

Traction

Frame

Stiffness

Lateral (side-to-side) frame stiffness can be affected by wheel size. To accommodate larger wheels, frame dimensions must be elongated. Therefore, a size medium 29-inch wheel frame has more lateral flex (bottom bracket and headtube) than a size medium 27.5 or 26-inch wheel frameset. Additional flex compromises handling under heavy pedaling or sharp cornering.

Frame

Frame

Geometry

The larger the wheel, the more difficult it is to optimize geometry, especially on smaller frames. As the frame size decreases, headtube heights become higher (in relation to saddle height). On 26 or 27.5-inch frames, it’s less of a problem, but geometry limitations can affect smaller 29-inch-wheel frames.

 

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