The components of a bike have a huge impact on how it rides – and most people are well aware of that. However, the influence of frame size and the great effort manufacturers put into delivering sensible sizing systems are often underestimated. Additionally, there isn’t just one “correct” way to do things, as frame size and proportions both depend on your preferences and riding style. We’ll explain what you need to consider with modern bike sizing and how it affects your experience on the trail.
Choosing a new mountain bike can quickly turn into a “first-world problem” – with too many options to choose from (where we’re happy to assist you with our detailed buyer’s guide). Once you’ve settled on the intended use, brand, model, and, of course, the color, there’s still the tricky question of the frame size. Many riders make this decision too hastily – “I always ride a size L” or “My old bike was an M.” However, frame size shouldn’t be chosen based on habit, as this could significantly affect control, comfort, and the bike’s overall ride qualities. Additionally, seemingly identical frame sizes can vary greatly between manufacturers and even between model series and production years.
And even though many riders wish for it, there’s no such thing as a simple size chart. Clear assignments based on height or inseam length are outdated, and more relevant factors now come into play. We’ll explain below which measurements manufacturers use to categorize their bikes and what you should pay special attention to when choosing the right size – whether the bike is labeled S4, XLong, Extra Medium, or Size 12. But how did bike sizing suddenly become so complicated?
How do Mountain Bike Manufacturers Interpret their Sizes?
Most manufacturers follow a similar sizing system, which often ranges from XS to XXL, much like clothing sizes. However, just as a Levi’s jeans in size M doesn’t necessarily fit the same as an M from Wrangler, the same applies to mountain bikes. A modern Canyon in size M, for example, can be bigger than a YT or Santa Cruz in size L – and there are even differences within the same brand: a Canyon L from two years ago might now correspond more to an M – confused yet? We have been too at first.
Most manufacturers offer four to six frame sizes per model to accommodate a wide range of rider heights. However, this also comes at a cost – literally. Each size requires its own carbon molds, which, amongst others, is a reason for high bike prices.
In the past, frame sizes were traditionally given in centimeters, based on seat tube length – the distance from the bottom bracket to the top of the seat tube. This system is still common for road bikes, where leg length is crucial for fit. In mountain biking, however, other factors play a much bigger role – partly due to the widespread use of dropper posts and the fact that frame size is more relevant when riding in a standing position. As a result, modern bike sizing is now determined by frame length rather than height.
Manufacturers like Specialized with their S-Sizing, and Norco with their numbered size classifications were pioneers in switching to a new sizing nomenclature to encourage buyers to engage more deeply with geometry values and the brand’s sizing concept, rather than just purchasing an L as they always had.
Today, the most important value for defining frame size is the reach – the horizontal distance between the bottom bracket and head tube. In other words, how far your hips are from your hands, which largely determines how big or long the bike feels on the trail. Since we always ride in a standing position when descending, this measurement is far more relevant for trail feel than seat tube height. When climbing, however, additional factors come into play, particularly the seat tube angle: a steep seat angle places you in a compact, upright position, while a slacker angle shifts your weight further back into a more stretched, sporty posture.
Most manufacturers design their frames with consistent reach increments of 20 to 25 mm between sizes – for example, Specialized, Propain, YT, and Canyon follow this approach. Santa Cruz, on the other hand, use uneven increments to ensure finer reach adjustments in the most popular sizes M and L. Ibis and Trek take a different route, offering intermediate sizes like “XM” or “M/L.” Ibis follow a unique sizing philosophy, categorizing their bikes based on top tube length, which indirectly determines reach through other geometry values. Atherton Bikes take customization to the extreme: with 22 available sizes offering 11 different reach options in 10 mm steps, you can get a nearly tailored fit.
The biggest advantage of modern reach-based sizing? In most cases, it allows you to choose between different size options, so you’re not limited to one that is based solely on your leg length. This allows you to factor in personal preferences, trail characteristics, and specific riding styles – a huge advantage!
However, for this system to work smoothly, the seat tube length, which used to define the frame size, still plays a crucial role. The seat tube must be short enough to ensure sufficient standover clearance, even with bigger sizes. If it’s too long, the saddle might still be in the way when fully dropped – a major issue on technical descents. At the same time, the seat tube has to have enough insertion depth to accommodate a long-travel dropper post, so that riders choosing a smaller frame can still achieve their proper seated position. This requires careful balance, making modern frame design a true technical challenge.
Which is the Right Size for me? A Guide for Different Rider Types
Okay, so reach is the key factor in determining a bike’s frame size, and if the concept is well thought out, it allows you to choose from multiple suitable sizes. But which reach length is right for you? This is where things get tricky because there’s no generic answer. Needless to say, a good starting point for selecting a size is your height and body proportions, which is why manufacturers categorize frame sizes on their websites based on these metrics. However, these alone aren’t enough – your riding style and intended use play a crucial role too.
Shorter bikes are more agile, respond quicker to steering input, and make it easier to playfully interact with the trail, while longer bikes offer greater stability, improved composure at high speeds, and a more confident feel on steep, technical terrain. The goal is to find the perfect balance between agility and stability – tailored to what you enjoy most with your mountain bike.
If you love blasting through rough terrain at full pelt, and bombing straight through rock gardens, a longer bike can be a great choice – or if you ride a trail bike but occasionally hit the bike park. If you focus more on playing with the trail, popping off every ledge, and making quick direction changes, you might be better off sizing down. Even if you primarily use your enduro bike on moderate home trails, a shorter bike could be a better fit.
Another important factor is your riding style: if you ride aggressively and like to keep a lot of weight over the front wheel, a shorter bike may suit you better. If you prioritize comfort and want a more stable feel in steep sections, a longer bike might be the way to go. But what if your dream bike doesn’t have the perfect reach for you? No problem – there are plenty of ways to make adjustments. The stem length can influence your reach to the handlebars, but you shouldn’t overdo it, as this quickly affects the handling. Handlebar rise and spacers under the stem allow you to fine-tune your cockpit position. The most effective solution is reach adjustment via special headset cups, though not all bikes are compatible with this modification.
A good reference point for the right reach can also be your current bike, as long as its geometry is still relatively modern. However, if your bike is more than five years old, mountain bike geometry will have evolved significantly in that time, so we wouldn’t recommend using its reach values as a reference.
The Future of MTB Sizing – Where is the Trend Moving?
Over the past ten years, mountain bike geometry has evolved significantly. “Longer and slacker” was the motto – longer reach values, slacker head angles, and the widespread shift to 29-inch wheels have fundamentally changed how bikes handle. However, it took some time for manufacturers to dial-in the optimal geometry for bigger wheels. Now, the geometry revolution has slowed down considerably, with most brands having found their sweet spot, which is why reach values have remained fairly consistent in recent models.
One element of mountain bike geometry that is still evolving is seat tube length, which is getting consistently shorter. Looking at enduro bike tests from recent years, seat tube length has steadily decreased, while the usable dropper post travel has increased. In 2022, the average seat tube length was 443 mm, paired with a 179 mm dropper post. By 2023, this had already dropped to 434 mm, with an average dropper travel of 191 mm. In 2024, many bikes now feature seat tubes around 435 mm, while dropper posts with up to 197 mm travel are becoming common. The same trend applies to modern trail bikes. The direction is clear: longer dropper posts provide greater freedom of movement on the bike, significantly improving downhill performance. If you want to learn more about dropper posts, check out our latest dropper post comparison test.
So if you come across size labels like S3, XLong, or just a number, it usually means the manufacturer is using a modern sizing system. The goal is clear: instead of blindly choosing your usual size, you’re encouraged to engage with the geometry values and find the bike that best suits your riding style.
While you get plenty of options with the choice of frame sizes these days, this also means that this topic has become a lot more complex. The big geometry revolution is over, and most manufacturers are now focusing on fine-tuning and increasing freedom of movement through shorter seat tubes. This development allows riders to choose between multiple sizes and tailor the bike to their personal preferences and riding style – for better control, comfort, and fun on the trail. Now that you know what matters, you can navigate the geometry jungle with confidence and find the perfect size for you.
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Words: Simon Kohler Photos: diverse
