For many years mountain bikes came with 26″ wheels. However, in recent years mountain bikes with larger wheels have been introduced and have become quite popular. In fact, other than some fat bikes (bikes with very wide tires), it seems that most new mountain bikes are being designed around either 27.5-inch or 29-inch wheels. The differences between the various wheel sizes and their associated frames result in differences in several factors including acceleration, traction, angle of attack, weight, maneuverability, and fit. Let’s discuss the 27.5-inch and 29-inch wheels in chronological order (29-inch wheels were introduced before 27.5-inch wheels).
Mountain bikes designed around 29-inch wheels are known as 29ers. One of the disadvantages of 29-inch wheels is that they don’t accelerate quite as quickly as smaller wheels because of higher rotational mass (i.e., much of the weight of the wheel is further from the center of the wheel). Wheels that accelerate more slowly can be perceived as resulting in a bike that is somewhat less “responsive” to pedaling forces. However, larger diameter tires actually have more tire area in contact with the ground and that should improve traction with all else being equal. Angle of attack refers to the angle formed when a round wheel touches a square object. Larger diameter wheels result in a shallower angle off attack, which allows the wheel to more easily roll over the object. Therefore, due to their larger diameter, 29er wheels roll over trail obstacles more easily than 26-inch wheels. As a result, many riders find that the larger wheels make them feel more confident on rough terrain. With regard to weight, larger wheels are of course heavier. Riders who aren’t particularly competitive may be more concerned with some of these other factors (e.g., traction and angle of attack) but for the competitive rider weight can be a deciding factor. As mentioned above, maneuverability can be another factor to consider. 29ers generally require longer frames in order to accommodate the larger wheels, but larger frames can feel less maneuverable not only because of their larger size but also because of the increased flexibility that is inherent in a larger frame. The longer frames associated with larger wheels can also be a factor with regard to how the bike fits the rider. Shorter riders may find it difficult to find a 29er that fits them particularly well, but riders who are quite tall may prefer a 29er.
In summary, 29ers have become very popular as many people have decided that the advantages of the larger wheels significantly outweigh the disadvantages when compared to 26-inch wheels.
27.5-inch (aka 650b)
Mountain bikes with 27.5-inch wheels were introduced after those with 29-inch wheels and were intended to bridge the gap between 26-inch and 29-inch wheels, supposedly offering the best of both worlds. Therefore, 27.5-inch wheels will accelerate more quickly than 29-inch wheels but may provide slightly less traction. They provide an angle of attach that is better than 26-inch wheels but not quite as good as 29-inch wheels. 27-inch wheels obviously weigh less than 29-inch wheels and the resulting bikes are probably considered to be more maneuverable than 29ers. Mountain bikes with 27.5-inch wheels can probably also accommodate a greater variety of rider sizes than can 29ers.
Since 27.5-inch wheels are newer than 29-inch wheels, the jury is still out to some extent. Time may tell us that the 27.5-inch standard will become the favorite as it represents a nice compromise between the traditional 26-inch wheels and the larger 29-inch wheels. In the meantime, and for good reason, it certainly doesn’t seem like either 27.5-inch or 29-inch wheels are going to go away anytime soon.
27.5+ and 29+
To muddy the waters, the bike industry has introduced “plus-size” wheels and tires in recent years. However, just like larger diameter wheels require a different frame design, the same is true for wheels and tires that are significantly wider. So, you can’t put a 27.5+ wheel on a 27.5 frame and the same is true for 29+. They’re completely different animals. With the hope of providing just a little more information but not too much, the basic premise of the plus sizes is that you get improved traction through a combination of the wider tire and the ability to run lower air pressure. For now, the plus-size bikes (and the associated new standard for frame and fork width) are still in the vast minority so it might be prudent to be a little cautious here.
What’s the Verdict?
Sorry, but the answer to that question is pretty subjective. If you’re particularly tall, gravitating toward a 29er probably makes a lot of sense. If you’re particularly short, perhaps gravitating toward 27.5 makes more sense. For the rest of us, it probably depends on the type of riding you like to do and how the individual factors discussed above relate to that type of riding.