To preface this blog, it is about general sizing guidelines for mountain bikes. Sizing for MTB is very different than sizing for road bikes and this blog is about trail/enduro bike sizing and not about techy pro-level XC bike measurements.
Sizing is very important to think about when buying a new mountain bike. Riding a mountain bike that is fitted improperly can result in incorrect posture and poor technique. If your bike is either too large or too small, it could lead to feeling less comfortable and confident while riding on the trail. That's not what you want to do, you want your bike to fit and feel dialed in so you can maximize fun :)
When it comes to mountain bikes, sizing between all available models has much more to do with how the bike handles than just the roominess (or lack thereof) in the top tube length. While the top tube length, or “reach,” is an important measurement to consider, understanding how the handling characteristics change with a longer or shorter wheelbase is also very relevant. In general, a longer wheelbase will be more stable at high speeds, but also harder to maneuver in switchbacks and tighter sections of trail. On the contrary, a shorter or smaller bike will be easier to move around on the trail and act more lively when getting through tight turns. However, a shorter bike will also be a bit more unstable at high speeds. This is why it is important to think about what handling characteristics favor your riding style and tendencies.
A lot of riders including myself (I am 5 foot 10 inches tall) may fit in between sizes medium and large, and could be happy on either size. In order to make sure you go with the correct size frame, first think about your riding style, current size bike you ride and how you like it. For me personally, I grew up riding mountain bikes and didn't come from riding BMX or motocross, and that's why I prefer a longer bike that may feel harder to maneuver for other riders of the same height. What it comes down to is that each rider has his or her own individual style and general preferences. The riders that come from riding cross country and are looking for an enduro bike may be happier with a smaller frame because that is what feels more comfortable to them. On the other hand, riders that come from riding downhill looking to buy that same enduro bike may favor a larger frame that may be sized more comparably to the longer wheelbase on the downhill bike that they are used to.
All of us come in different shapes and sizes, and that is an important reason to be open-minded when thinking about the way your bike “fits” your particular body. For example, if you are a rider who prefers a size medium frame but has relatively long legs, getting the correct length dropper post is something to consider. If your dropper post features 100mm of travel, it may not be tall enough even at the minimum insertion line in the seat tube. On the other hand, if you are a relatively small rider who prefers a larger frame, you may need a dropper with less travel in order to get your seat low enough in both the upright and dropped positions. Companies like Fox, Rockshox, and KS all offer different travel dropper posts that will accommodate these situations. Oh and by the way, most people ride a 125mm dropper post, but keep in mind that is just "most," not everyone.
At the end of the day, there is no crystal ball answer to the question, “What frame size do I need?” There are however a couple great ways to ensure you get the right size mountain bike. The best option is to actually get out there and ride the bike you considering. If you live locally, our demo bike program at Worldwide Cyclery in Newbury Park CA allows you to get a feel for all the different sizes that may be available. The second best option is to closely compare the geometry charts of the bike you are riding now and the one you are considering buying. There is some significant size variance among brands so keep that in mind. But if you really like the fit of your current bike, match it up as closely as you can to the bike your considering and you'll be set. If you think your current bike is too small or too big, look at the geo charts and make some judgments. We know you have to be a bit of bike nerd for this stuff but if you need help we are always happy to chat on this topic to help out. Feel free to give us a call at the shop if you have any questions or need a suggestion on what frame size you need!