There are a dazzling array of numbers that confront mountain bikers but one of the most important numbers is wheel size. Although the choice is quite narrow - with only three options - wheel size has an enormous influence on your riding experience. 

• 11 things to put in the ends of your handlebar
• Big Read: Big mountain pairs racing with Enduro2 Les Arcs
• The best hip packs you can buy for MTB and gravel - tried, tested and reviewed

The history of the best mountain bike wheels is a simple one. Since the late 1970s, when the first mountain bikes were produced and marketed, everyone rolled the same wheel size: 26-inches. The industry standardised around the 26-inch rim size, influencing corresponding tyre sizes. 

For decades, everything was 26-inches, relating to mountain bike wheels and tyres. But in the late 2000s, something happened. After much analytic thought and peripheral experimentation, mountain bike wheels got bigger. A lot bigger and the industry upsized to 29-inches. 

Today, most mountain bikes roll 29-inch wheels and tyres. It has become the default wheel size for mountain biking. But it is not the only wheel size you can choose, because there is a third mountain bike wheel size: 27.5-inches or 650b is it’s often labelled. 

In principle and practice, the choice for mountain bikers has become binary: you either ride 27.5- or 29-inch wheels. But which size is best for you, and how does going bigger or smaller, influence your riding experience? 

The benefits of a bigger mountain bike wheel

Why did mountain biking significantly increase wheel size from 26-inches to 29-inches in the late 2000s? And perhaps more importantly: why has 29-inches become the default standard in mountain biking wheel sizes, instead of a mere trend? 

Mountain bikers count grams and bigger wheels, with their matching tyres, add rotational weight to a mountain bike. Something experienced riders spend a lot of money to avoid because a mountain bike with increased rotational weight requires more energy to pedal. But for the sacrifice of more rotational weight, there are genuine benefits. 

The most significant advantage of larger mountain bike wheels and tyres is the relationship between contact patch and terrain. A larger diameter tyre will have a lot more tread in contact with the terrain it is rolling over, than a smaller diameter tyre – even if both tyres are of the same width. That bigger contact patch equals more grip when riders are climbing, braking or cornering on a descent. 

Beyond the benefit of a bigger tyre having more grip, there’s also enhanced ride comfort. Larger tyres have more air volume and can be run at comparatively lower pressures. And a mountain bike tyre which can roll at lower pressure conforms better to the terrain it is rolling over. Plainly, bigger tyres, at lower pressures, will perform better at absorbing terrain impacts like rocks, roots and general trail texture. And that means reduced rider fatigue and more steering control, especially on long rides over rough trails. 

If bigger wheels are better, why would you choose 27.5-inch?

The demand for mountain bikes rolling 27.5-inch wheels, is much smaller than 29ers. But that doesn’t render 27.5-inch wheeled mountain bikes irrelevant. 

For some riders, the smaller of mountain biking’s two ‘standardised’ wheel sizes has real benefits. But if the 29-inch wheel happened first, and came to dominate, how do 27.5-inch wheels fit into the evolution and present reality of mountain biking?

Proportionality is a thing with any bicycle design. Frame shaping’s foundational principle remains two triangles, joined, with a wheel attached to each. But below a certain frame size, on the tiny end of the rider size spectrum, bigger wheels don’t fit. Especially when they are expected to move, as part of a mountain bike’s suspension system. 

Extra-small and extra-extra-small mountain bike frames aren’t really compatible with 29-inch wheels. But they can integrate 27.5-inch wheels with ease. 

Smaller wheels are more agile

The first 29-inch mountain bikes weren’t great. Designers didn’t reasonably calculate and anticipate the influence of those larger wheels on frame geometry and riding characteristics. This was especially true for dual-suspension mountain bikes, many of which were still accommodating front derailleur compatibility in the late 2000s and early 2010s. The result was early dual-suspension 29er mountain bikes which were very unwieldy and awkward to ride on tight and technical descents. 

In reaction to these ungainly early 29ers, some riders wanted a mountain bike wheel size that offered some of the 29-inch wheel’s rolling benefits and enhanced grip, but less agility impediment. And that solution was the 27.5-inch wheel size. 

So what do 27.5-inch wheels do better, than 29ers? It’s all about the difference in physics between a 27.5-inch mountain bike wheel and one which measures 29-inches. For a given composition of rim material and tyre width, a 27.5-inch wheel will be lighter than a 29er. And the lighter a mountain bike wheel is, the less lateral force it requires to lean and turn. 

Mountain bikers who prefer 27.5-inch wheels are those who value the agility of a smaller wheel size. Those riders who enjoy threading their bikes around the tightest switchback corners with speed and style. 

When designing a new long-travel mountain bike frame, having adequate clearances for 29-inch rear wheels is challenging. On a mountain bike with 160-180mm of suspension travel, the rear wheel moves greatly when that rear shock fully compresses. 

Shaping a mountain bike frame that can accommodate a 29-inch wheel and 160-180mm of rear suspension travel, can result in a wheelbase that is too long for some riders to enjoy. Smaller riders who prefer mountain bikes with 160-180mm of suspension travel, still find packaging advantages with the 27.5-inch wheel size. 

Smaller wheels make a big difference in the air

Not all mountain bikers graduate to become confident jumpers, capable of clearing road gaps. But if riding off big drops and landing those A-line double on your trail are what you like doing, 27.5-inch wheels make sense. 

Even in mild side winds, a larger wheel is much more susceptible to becoming a ‘sail’ in the air, unbalancing the entire bike. That is why the world’s best extreme mountain bikers, who launch huge drops and land even bigger jumps at events like Red Bull Rampage, prefer 27.5-inch wheels.  

The legacy of smaller wheel sizes being preferred by riders who launch big jumps and tricks isn’t a late reaction to the context of 27.5 versus 29. It goes way back to the late 1990s and early 2000s, when some downhill mountain bikers were using 24-inch rear wheels to radically optimise bike feedback and responsiveness. Not to mention having the smallest possible wheel and tyre surface area, when launching jumps. 

What has happened to the original mountain bike wheel size - 26-inch?

If you are given to riding extreme trails and love jumping, the 27.5-inch wheel has value. Especially for compact riders, on small and extra-small frame sizes. For most riders, the rolling smoothness, stability and enhanced braking performance of a 29-inch wheel are ideal, hence the popularity of 29-inch wheeled mountain bikes. 

But what about 26-inch wheels? There are precious few new mountain bikes which feature 26-inch wheels. They are primarily specialist dirt jumping hardtails, or small and extra-small frame downhill brakes, from boutique brands. 

Does that mean 26-inch wheel mountain bikes are entirely irrelevant? No. Because there is a trend toward sophisticated dual-suspension kids’ mountain bikes, the 26-inch wheel size is ideal for that application.

When designing a tiny dual-suspension frame for kids, there’s not a lot of space for the wheels to move through their suspension travel. For a specific age range, 26-inch is the best wheel size option for a dual-suspension kids’ bike.  

You might also like:

• Who makes Micro Spline wheelsets for Shimano's 12-speed mountain bike groups?
• The best trail and enduro mountain bike wheelsets, tried and tested
• Which MTB freehub body do I need? XD, Micro Spline or HG?