Buying a new mountain bike these days can be intimidating. There are so many options, variations, and "innovations"; but what's really going to affect the way your new bike rides?
Whether you're new to mountain biking or a seasoned rider, surely the wheel size debate has consumed a conversation or two about your current or future bike. The terminology alone can be exhausting. Now combine this with Downhill Dan swearing you'll win World Cups if you get a 27+ bike, and Chris the Climber assuring you the ability to get to the top of anything if you’re on a 29er, and you may be wondering what the truth is.
Let's take a deeper look at what exactly the viable wheel size options on a modern mountain bike are and what they really mean for your riding:
Making their mainstream debut in the early 2010's, 29ers were first designed and marketed for cross-country applications. This is the largest wheel size available, and has since been adopted by many genres of cycling beyond just XC, including both downhill and trail/enduro.
29" wheels are known to be fast. They have more rolling mass and therefore inertia, helping to keep you rolling once up to speed and carrying you over small rocks and bumps on the trail. Due to the larger circumference, they also have a larger contact patch with the ground, which can provide more traction while climbing, braking, and cornering. This larger contact patch also allows the wheel to roll over rather than into obstacles easier, as compared to the smaller/steeper contact patch and curve of a 26” wheel.
BONUS: Most 29” XC wheels can fit a standard 700c road tire. This makes utilizing your mountain bike on a stationary trainer or rollers a breeze during winter months.
Simply due to size, 29” wheels/tires will always be heavier than 26" or standard 27.5” comparable options. Further, due to the larger diameter, flex in lower end wheelsets will be a more serious issue than in smaller wheels of comparable level.
There was a time when the items I’m going to list in this section would have been put under “cons” instead, but with how far 29” bikes have come, I can confidently say these are myths now.
Geometry: When 29” wheels first came out, frame designs were still modeled after what worked for 26” wheels. As you can imagine, this caused some problems and made a lot of people weary about the capabilities of 29ers. However, over the last 5 years, the 29er frame has been completely reinvented. While the feel of a 29er is still different than that of a smaller-wheeled-bike, the same performance, handling, and rigidity that mountain bikers knew and loved on their 26” is alive and well on 29ers today.
Flex: Another problem with early 29ers was wheel flex. Again, the same technologies and standards that worked on 26” bikes just did not translate well over to the bigger wheels. However, with things like Boost Spacing and massive thru axels, combined with improved 29er geometry, wheel flex really isn’t an issue anymore. Of course, lower-end wheels will always flex in any size, but rest assured that a high-quality, reliable 29” wheel will have a nearly identical strength to any other size.
Fit: Several years ago when the 27.5 wheel came out, we started seeing a debate of selecting the right wheel size based on the riders height and size. In theory, it sort of makes sense, However, in application it really doesn’t. Here’s why: 29 and 27.5 wheels are incredibly similar in size when you hold the two next to each other (as can be seen in the photo at the top). When 29ers first came out and flex and geometry were still an issue, there was argument for factoring in height and size a bit more when selecting a wheel size. However, on a modern 29er, the pros and cons listed above still stand regardless of rider size. The best proof of this is watching an XC World Cup. You’ll see 4’11” riders on an XS frame 29er, and big 6’ athletes running the exact same wheels on an XL frame.
Also known as just “27” or 650B, this wheel size came out right after we started to see 29ers gaining traction in the industry. It was the middle ground between the classic 26” and new 29” wheels. However, it was also meant to be a solution to the early issues of the 29er that were nearly all resolved over the years.
Pros and Cons:
I will keep this section brief as it essentially is a punctuated version of the 29er pros and cons. They’re really the exact same, but the ‘good’ isn’t quite as good, and the ‘bad’ isn’t quite as bad.
27.5 wheels are a bit more agile and accelerate quicker due to the smaller size, but they also do not roll as fast or over things as smoothly as a 29er. At first they were being largely pushed as XC wheels, but have since evolved into a more suitable solution for downhill and enduro/trail riding. This is largely due to the minor increase in acceleration and maneuverability.
The 27+ option is a new style that has really exploded across the cycling scene in the last several years. By definition, plus tires are between 2.8" and 3.25" and mounted on a 40–50mm wide, 27.5” rim. Now that is a lot of numbers, so to put it simply, 27+ is a real big tire on a 27.5” wheel.
27+ wheels typically share the exact same outer circumference (when measured around the tread of the tire) as a 29er. This is because of the extra girth of the tire. With that, they share many of the positive attributes of the 29er in that they maintain rolling momentum well over obstacles.
Further, the larger tire volume allows you to run lower air pressure. This provides additional traction and dampening to small bumps.
My personal favorite attribute of Plus bikes is the combo of lower pressure, more tread/traction, and heavier wheels. This mix provides an incredibly stable and confidence-inspiring ride that largely reminds me of riding a dirt bike.
Having this much tire is heavy. Even more, pushing so much tread across the dirt may be a lot of traction, but it is also a lot of resistance. While Plus bikes can still be setup pretty light with a good wheelset and XC tires, it’s hard to completely avoid this effect without sacrificing what a Plus bike was designed for in the first place – providing a ton of traction, confidence, and fun.
Fat bikes are great because they’re so straightforward. They’re not trying to be excellent downhill machines or efficient climbing rigs. They are built for one purpose and one purpose only: gnarly terrain. Fat bikes typically start at 3.8” tires and go much ‘fatter’.
These bikes are designed for and used on terrain like snow and sand. While pedaling through powder is still tough, you’d be surprised how incredible it is to glide over the top of a groomed fat bike or XC ski trail on 5” tires.
Fat biking is an adventure sport that has been on the rise for several years now, as it allows for year-round cycling and riding in places that a regular mountain bike could never go before.
I hope this helps provide some clarification on the various MTB wheel sizes. Feel free to leave any questions you may have in the comments below. See you on the trails!