We live in exciting times. Not so long ago there was pretty much just one option in off-road wheel sizes: 26″. That was a good standard for a long time; strong, quick handling, and light. But then along came the 29er, which is really 700c (road size) redesigned for mountain biking (the IBD, or internal bead diameter, of a 700c and 29er are identical). Those bigger wheels are great at rolling over things like rocks and logs (decreased angle of attack), have a bigger contact patch (tire to ground) and they maintain momentum well. But with these advantages come some disadvantages, namely slower acceleration and decreased control in tight technical situations; it’s harder to throw the bike around. There’s also the issue of flex, both in the wheel and frames made to accommodate a bigger wheel, though both of these issues are diminished by improvements in materials and manufacturing processes.
Enter 27.5, halfway between 26″ and 29er. 27.5, AKA 650b, is an attempt to balance the pros and cons of 26 and 29, allegedly retaining some of the agility and acceleration of 26″, and the roll-over and contact patch of 29er. Giant invested big in 27.5 for their 2014 line, and luckily produced a little literature to support it. Here’s a highlight blurb from Giant’s “Why Giant 27.5” flyer (all italics throughout the post are direct quotes from this document, which can be viewed in full at the bottom of the post):
- “Here’s why: Because whatever type of riding you do – XC, trail or enduro – you want the same things from your bike. Lighter weight, more efficiency, better control. Giant’s all-new 27.5 techinology nails all three, helping you ride faster and have more fun. Already proven with pro XC and enduro race wins-on bikes ranging from hardtails to Maestro full-suspension-Giant 27.5 technology delivers uncompromising off-road performance.”
Lest ye think this mere marketing, Giant conducted a series of tests and present 3 arguments for 27.5. They are
- Lighter weight
- More efficient
- Better control
I’ll go into each argument in detail.
1. Lighter weight:
“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.
The overall weight of a 27.5 wheelset (wheel, tire and inner tube) is only 5% greater than that of an identically built 26-inch wheelset. Compare this to the 12% increase of a 29-inch wheelset and you can see how a seemingly small increase in diameter results in substantial weight gain – and poorer performance when climbing or accelerating.
STATIC WHEEL WEIGHT
Lighter wheels/tires result in quicker acceleration and lighter overall bike weight – a win-win combination.”
So lighter wheels/tires are better than heavier ones; no arguments there. Additionally, while 27.5 is halfway between 26 and 29, the weight favors 26 rather than cutting the difference in half.
2. More Efficient
“Snappier acceleration and a reduced angle of attack for a smoother, more agile ride.
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.
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 [referring to a graph in the flyer, see attached scan of flyer], 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.
So 27.5 favors the angle of attack of 29er and the wheel weight properties of 26″.
3. Better Control
“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 [again, see the diagram on the scan; it shows that a 26″ tire contact patch is 6cm, 27.5 is 8cm, and 29er 9cm, demonstrating that the 27.5 is nearly as good as the 29er and much better than 26″ in this respect], a 27.5-inch wheel has a similar contact patch to the 29.
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.
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.
Meat and potatoes: the contact patch of the 27.5 is almost as large as that of a 29er tire and 2 cm larger than a 26″; more contact means more control. Additionally there’s less flex in the bottom bracket and headtube on a 27.5 frame when compared to the 29ers, and geometry is easier to optimize in smaller frame sizes relative to 29ers (this is irrelevant to tall riders who ride larger bikes).
To view the scanned original of Giant’s Why 27.5? document:
Page 1: Giant Why 27.5? p1
Page 2: Giant Why 27.5? p2
Look for posts in the near future that cover the models.