Not since dual-suspension mountain bikes debuted has there been so much controversy and confusion surrounding a mountain bike technology. We are talking about the wheel-diameter war. Mountain bikes were originally designed around 26-inch diameter wheels, and these wheels had a long, unchallenged reign as mountain biking’s recognized “standard.” This meant that tire, rim, suspension and frame design and technology all started and ended with 26-inch wheels.

Then came a group of frame designers who questioned the superiority of 26-inch wheels. The best-known advocate for breaking the 26er mold was Gary Fisher, who championed a larger-diameter wheel that would deliver more contact with the trail, be less affected by the trail surface, lower the bottom-bracket height in comparison to the wheels’ axles, and, once up to speed, roll like a runaway freight train.

The early 29ers had very limited support from tire companies and even less support from suspension suppliers. Accepted frame geometry, intended to work around 26-inch wheels, needed modifications to take advantage of the larger hoops. Early adopters of 29er technology got bikes with heavy wheels, heavier steering response, hook-and-ladder-long wheelbases, mediocre fork performance and tires that were too big and heavy. Most riders went right back to their 26ers after a demo ride on one of these early creations. There were, however, enough riders who rode in desert (sandy), winter (snow) or wide-open (fire road) conditions to keep the glowing ember of the 29er alive.

Today, 29ers are no longer an ember, but a forest fire. They enjoy plenty of wheel and suspension choices, and their geometry has been dialed to take full advantage of the 29-inch format. There are even companies that only produce 29er bikes. Niner and Fisher Signature Bikes from Trek are only available with 29-inch wheels. The final proof that 29ers have arrived may be their growing acceptance by our European counterparts, who avoided 29ers like the bubonic plague. Riders who speak negatively of the larger wheels have not ridden one recently or prefer more than four-inches of rear wheel travel.

THE NEW GUY

The MBA wrecking crew fields more questions about 29- vs. 26-inch wheeled mountain bikes than any other technical feature, and that includes suspension and drivetrain questions. With so much confusion and misunderstanding about the two wheel sizes, certain powers in the mountain bike universe have decided to make things even more con- fusing by offering a third wheel size, halfway between the 26er and the 29er wheel—the 27.5-inch wheel (also referred to by bike geeks as 650b). The advantage of the mid-sized format is that it is supposed to deliver the benefits of a 29-inch wheel while making it easier to eke out clearance for rear suspension travel and for the action of the front derailleur.



WHEEL TECH 101

Think of the wheel as an inclined plane—a ramp. When the wheel contacts a bump, say a two-inch block of wood, a triangle is formed from the top edge of the block to the ground and back to the point where the tire contacts the trail. The steeper the triangle, the harder it is to get the wheel up and over the block. The smaller the diameter of the wheel, the steeper the triangle becomes, until it reaches the point where the wheel is too small to get up and over and stops dead in its tracks. The difference between the strike angle of a 26-inch wheel and a 29-inch wheel (over the wooden block) is only about 5 percent, but the cumulative effect of rolling up and over a 5-percent steeper angle thousands of times a mile adds up quickly. The larger-diameter 29-inch wheel has noticeably less rolling resistance.

Another benefit of a larger-diameter wheel is the relationship between the bottom bracket center and the wheel axles. A lower bottom bracket lowers the center of gravity, and gives the chassis a more stable feel while cornering. The bottom bracket heights of 26-, 27.5- and 29-inch designs are about the same, but a closer look reveals a secret that might explain the highly lauded cornering ability of 29-inch chassis. At 12.5 inches, the bottom bracket center is only a half inch lower than the axles of a 26-inch bike, but the bottom bracket is 2 inches below the wheel axles of a 29er. Weighting the pedals of a 29er puts much of the rider's weight well below the axles, which has a profound stabilizing effect.

26 VS. 27.5 VS. 29

Jamis Bikes covers their bases by offering steel-framed hardtails in the three wheel size formats. When they offered us three similarly priced bikes to try back to back, we jumped at the chance for the ultimate demo ride. You can argue with hype, crunch all the numbers, and show up with years of bias, but when you ride bikes back to back, things become clear very quickly.

The 26er

Although all the Jamis frames measured 17
inches from the center of the bottom bracket to
the top of the seat tube, the 26er felt like a minibike compared to the other two bikes. It was the easiest to straddle and had the nicest fork. Jamis spec’ed this 26er with a 6-inch-diameter front brake rotor, the smallest of the three.

Pros: Excellent on steep and stepped climbs. Easy to accelerate out of momentum sapping situations. Braking was strongest, especially at the bottom of
steep descents. By far the
 most responsive to pumping
the trail. A light-feeling that encourages the rider to hop
over or around obstacles.

Cons: Descending is intimi
dating, especially after spending
time on the other two wheel sizes.
This bike requires more rider input and a
bigger commitment. Rider position has a bigger effect on how the bike handles and responds. While a rider can “roll” a techie section on the larger wheels, the 26er rider has to attack and maintain enough speed to stay on top of the ruts, rocks and flat-edged bumps. The rider has to work on looking ahead, as the 26er places the rider lower over the front.

The 27.5er

This bike looks and feels proportional. Other
riders didn’t even notice we were riding 27.5-
inch wheels unless they took the time to read the numbers on the American Classic rims. It doesn’t have the high feel of a 29er, while positioning its rider in a more comfortable riding position. This was the only bike of the three that used a 15-millimeter thru-axle, and there was no doubt this would give the bike an edge over a similarly equipped model with a 9-millimeter quick-release axle.

Pros: The bike has a
nimble feel that is way clos
er to the 26er than the 29er,
but it doesn’t require the
rider to work as hard. Your
body position always feels neutral.
You can pump the 27.5er along the
trail, and when the trail heads downward, it allows the rider to remain relaxed. You don’t need to attack rough sections. These wheels do a good job of staying on top of the rough stuff, and the front end goes where you want it to go.

Cons: We couldn’t come up with a ride negative. It doesn’t steer as fast as a 26er, but we never found a trail so tight that this made a difference. It didn’t roll over the rough sections as smoothly as the 29er, but again, it wasn’t a big enough difference to put the 27.5 rider off the wheel of the 29er.

The 29er

There is no getting around the fact that this bike feels way different from the other two. The front
end feels tall, and the rider sits inside the bike
rather than on top of it. From the first crank rotation, you realize different forces are at work. The quick response of the smaller wheels is gone.

Pros: The big wheels are great for descending. They smooth out
nasty trail that would have you puckered on the 26er. The added confidence inspires
you to remain more relaxed
as the wheels float over
rocky terrain. The big
hoops offer the best trac
tion of the bunch, both in
corners and on loose
climbs. This bike requires
the least amount of body
English to stay hooked up (going
up or down). Crewers noted they 
remained seated longer in the 29er. Once up to speed, the bike holds momentum well—and holding momentum is what the 29er is all about.

Cons: Getting up to cruising speed takes more effort. Steep climbs or regaining speed following a momentum- zapping misstep is noticeably tougher. The only plus here is out-of-the-saddle efforts work great because the rear tire maintains traction. The large wheels don’t respond as well to pumping the trail, and lofting the wheel requires more effort. This bike doesn’t have that lively squirt of acceleration you feel on both the 26er and 27.5er when working the backside of a whoop. The larger rotor up front wasn’t enough to give this bike the braking power we wanted.

THE RANKING

Win: This 27.5-inch-wheeled bike proved to be the most versatile of the three hardtails. It did an excellent job of blending the best traits of the 29er and 26er with- out ever feeling like a compromise between the two. This bike scampered away from the 29er on steep uphills or after a speed-zapping mistake. It then gapped the 26er on the other side of the mountain on the way down—all the while delivering a very resilient ride that didn’t beat up the rider.

Place: This 29er couldn’t match the 27.5 in a drag race from a standstill or up a steep ascent. If your riding is wide-open trails with few surprises, the large wheels will hold an advantage over the 27.5. Our 29er held a slight advantage on the descents over the 27.5 (and blew the 26er into the weeds), but the advantage was less than what the 27.5 could throw down going up.

Show: This 26er felt the lightest of the group (it was), and, in experienced hands, it would beat the other two in acceleration and slicing up or down a trail. It propelled its rider with pumpatude power and liked to manual, wheelie and hop around obstacles. The operative word here is “experienced.” An accomplished rider can pump and manual all day long. The rider who can’t do those things won’t have as much fun on this 26er as he would on either of the other bikes.

THE REALITY

The shootout winner earned its position by duking it out on the trail. In the asphalt jungle, there are other factors to consider. The 29er rider will have plenty of upgrades to choose from in tires, wheels and forks, and the 26er rider has even more. The 29- and 26-inch hoops are here for at least the lifetime of the bikes. The 27.5’s future is not so clear. While Jamis and KHS have both committed to the wheel size (as well as custom builders like Kent Erickson Bikes and Form Bikes), it is still a format that is found only on the fringes.

Mountain Bike Action stated previously that in a perfect world a series of quantitative tests comparing rolling resistance values over varied terrain could find a convergence point where the tire's air volume is optimized for sucking up big hits, the overall wheel weight is minimized, and the diameter of the tire delivers the lowest possible rolling resistance. This would result in an exact wheel and tire size that is best suited for the job.

Don’t hold your breath. That statement was made over five years ago and we still only have 26-, 27.5- and 29-inch wheels to choose from. The fact that many product managers have never ridden a 27.5-inch wheel is even less encouraging. Our shootout conclusion doesn’t duck the question in hopes of a yet-to-be-agreed-to, pie-in-the-sky solution. It is based on what we just rode and what you can buy today. You just have to understand that going with the 27.5-inch wheels makes you an early adopter of a design, and you are betting on a dark horse. Of course, that is one very fast, sweet-riding dark horse of a hardtail.



Reprinted from our March 2012 issue. Like us on Facebook