You’d have to live under a rock to be unaware of the wheel size debate that has gripped mountain biking for the past few years.
Since you’re reading this, you probably don’t live under a rock, unless said rock has Wi-Fi. And while we’ve all heard countless arguments favoring each size, what’s certain is that the classic 26-inch wheel size has lost significant ground, which means that most everyone who’s looking for a new ride is choosing between a 27.5- or 29-inch bike. Your choice will largely depend on your priorities, as each size offers distinct advantages. Let’s dig through them.
Claims of increased contact patch and rollover aside, arguably the largest reason that 29ers have earned such a devoted fan base is due to bottom bracket drop. Many 26-inch bikes locate the bottom bracket above the centerline of the axle in order to provide adequate ground clearance. This gives the sensation of being on top of the bike. A 29er with an identical bottom bracket height, as measured from the ground, will typically locate the bottom bracket far below the centerline of the axle.
This creates a sensation of being “inside” the bike. It has the additional benefit of making the bike more stable by lowering the rider’s center of gravity relative to the wheels. 27.5-inch wheels open up the opportunity to build a bike with bottom bracket drop, except the smaller wheels have a lower centerline, which means that the drop will be less pronounced. Accordingly, at comparable bottom bracket heights, a 27.5-inch frame will be less stable than a 29er but more responsive to rider input.
Trail is a function of head angle and fork offset, and it’s the largest determining factor in how a bike corners. 29-inch forks are built with a longer offset than their 27.5-inch counterparts, which means that 29ers require a steeper head angle to achieve the same trail measurement. A more kicked-out head angle yields a bike with greater pitch stability, or fore-aft stability. This means that when things get wild, you’ll be less likely to be ejected over the handlebars. This is mostly a factor for high-speed descending and steep terrain. Accordingly, it’s more of a consideration for the gravity-obsessed than the XC racers among us.
Considering the impact of steering geometry is critical for making comparisons between bikes of different wheel sizes. A 29er with a 68-degree head angle will maintain composure at high speed, but low-speed handling, particularly cornering, will tend to be sluggish. By comparison, a 68-degree head angle on a 27.5-inch bike will yield a noticeably more responsive handling feel, but will be less predictable when initiating turns at high speeds. So, if you’re comparing bikes between wheel sizes, expect a 29-inch bike to be roughly two degrees steeper than a 27.5-inch bike with similar cornering characteristics.
Conventional wisdom holds that 29 inch wheels roll faster than their smaller brethren, and in some cases, it’s true. However, as you’ve no doubt been told, there’s no such thing as a free lunch. First, let’s assume that we’re comparing 27.5 and 29 inch wheels, all built with the same rim, tire, and pressure. Apples to apples, so to speak.
At a given speed, 29’ers do roll slightly more efficiently, and it’s most noticeable on flats or gentle grades, either up or down, with successive small bumps. The difference comes down to angle of attack. In other words, the larger wheel approaches a given obstruction at a shallower angle, enabling it to roll “through” the object, rather than “over” it. However, this example assumes a constant speed, and mountain biking is a dynamic activity.
There’s a price to pay for this improved roll over. Larger wheels weigh more, and that added weight is placed farther from the hub, which further increases the wheel’s inertia. In other words, 29’ers require a stronger effort to get moving. This phenomenon is most noticeable when exiting turns, and during technical climbing, where bursts of acceleration are part of the game. Big-wheel evangelists will of course point to the fact that you’ll run lower gearing to compensate, but gearing doesn’t change the laws of physics. The fact remains that 29’ers maintain speed more effectively, where 27.5 inch wheels accelerate more efficiently.
The larger wheel diameter offers a larger contact patch. In other words, all things being equal, a 29-inch wheel will put more rubber on the ground at once. This means that you’ll have more available grip for cornering, braking, and climbing. The added diameter also reduces the angle of attack when rolling over a given obstacle, which enhances the sense that your bike is rolling through trail features, rather than up and over them. And as previously mentioned, the increased bottom bracket drop yields a bike that feels more planted, which is of particular benefit to both beginners and endurance racers.
Riding a bike with less bottom bracket drop results in a more responsive handling feel. Along these lines, the smaller diameter wheel frees up more room to shorten chainstays, especially on long-travel bikes. With shorter chainstays, a bike tends to turn on a tighter radius, helping the rider make quicker changes in direction.
Conversely, the use of smaller wheels requires slacker head angles to achieve the same trail measurement. This balance of slacker head angles and shorter chainstays gives the 27.5-inch wheels greater pitch stability compared to a 29-inch wheeled bike with similar handling traits. Ultimately, this is advantageous to aggressive descenders. Additionally, the smaller wheels are both stiffer and lighter, which enhances the lively feel that’s inherent with 27.5-inch bikes.
The most common misconception we hear about 29ers is that they aren’t responsive. Broad generalizations tend to skirt that fine line between exaggeration and flat-out nonsense, and this is a perfect example. Sure, there are plenty of 29ers that are highly stable and handle best when ridden wide open, but there’s no shortage of quick-handling 29er XC whippets. Granted, when comparing similar bikes, a 27.5-inch bike will tend to be more responsive, but there are other factors to consider, mainly suspension design and your position on the bike. And if someone tells you that 29ers don’t jump well, consider the source. Big wheels love to fly. So most likely, it’s not the bike’s fault.
What we’ve found is that, by and large, XC racers, beginners, and fans of all-day trail rides tend to prefer the stable ride provided by a 29er. Meanwhile, aggressive riders with a taste for descending generally favor the more responsive handling offered by 27.5-inch bikes. 27.5 is also an amazing option for those who are hesitant to give up their beloved 26-inch steeds, since it promises both forward compatibility of high-end parts and a ride that’s similar to the old standard. It also provides some distinct advantages in terms of both grip and overall speed. So, if you’re on the fence, do yourself a favor and ride as many bikes of each wheel size as you possibly can. Generalizations aside, there are bikes of both wheel sizes that we absolutely love and that cater to most any taste. Naturally, there are those who will be resistant to the changing tides, but from what we’ve seen, it’s being driven by a genuine desire to make mountain bikes better for all.