Still Wondering about 27.5? Giant Breaks it All Down For Us.
Mountain Biking industry leader Giant Bicycles doubles down on 27.5 inch wheels
Giant Bicycles tossed all personal doubt about the 650b / 27.5 tire phenomenon away back in 2014 when they re-shod their entire MTB bicycle line with the relatively new (don’t tell Jamis that) tire size. This wholesale change in tire & wheel sizing essentially phases out all their traditional 26″ MTB sized bikes in favor of the newer tech.
If you haven’t made the shift to 27.5″ for yourself yet, either because you can’t decipher the benefits, haven’t read Pinkbike’s review, or you are still skeptical of the benefits… well wonder no more. Giant has thankfully broken it all out for our thoughtful consumption via the infographics included below.
Ultimately, it all comes down to a personal sense of bike-feel (is it fun, is it lively, does it react to your inputs well, etc.); but not all of these mysterious factors needlessly remain unquantified…enter Giant. They have put together the following report, broken out into 3 categories in their attempt to do define the benefits of the 650b.
- Weight differences between 26, 27.5 and 29 inchers,
- Efficiency differences, and
- Control-ability differences.
Read below for the full transcript (edited for brevity and readability):
1. Lighter Weight
The Principle: Lower bike and rotational wheel weight can help you climb faster with less effort. Compare the weights of identically equipped bikes with the three different wheel sizes and you’ll see differences. The 26-inch-wheel bike is somewhat lighter than the 27.5, and substantially lighter than the 29 . The 27.5 incher only comes in at 5% greater than the 26 inch wheel counterpart and 7% lighter overall than the heavyweight 29. These seemingly small increases in result in noticeable weight gain which equate to poorer performance when climbing or accelerating. In addition, lighter wheels/tires are quicker accelerators – a win-win combination.
The Principle: Snappier acceleration and a reduced angle of attack combine for a smoother, more agile ride. The increase in wheel diameter decreases the angle of attack (the angle in which a round object intersects a square object). This translates to a smoother ride on the trail. 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 perhaps arguable benefit of 27.5 over 29 is the 650b’s quicker acceleration over its bigger brother. 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). Looking at the graphic below, 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. The middle man strikes a good balance here.
3. Better Control
The Principle: A larger tire contact patch, increased stiffness, and optimized frame geometry improve traction, braking and handling. 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 benefits from a similar contact patch to the 29, but without all the extra weight gain.
Lateral (side-to-side) frame stiffness can be affected by wheel size. To accommodate larger wheels of the 29er, the frame dimensions must be elongated to make room for the extra girth. Therefore, a medium size 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 and can make a bike feel noodly or unstable when pushed.
Lastly, the larger the wheel, the more difficult it is to optimize geometry, especially for smaller frame riders. As the frame size decreases, the headtube height becomes higher in relation to saddle height. On 26 or 27.5-inch frames, this is less of a problem.