Mountain Bike Fit Guide: Size Chart & Frame Geometry


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Fit! The most important thing to figure out before you drop in on a shiny new ride! Just like that pair of sweet jeans in the evo outlet for 70% off, it’s not a good deal if it doesn’t fit. Unlike denim, you can’t really alter aluminum or carbon, so it’s that much more important to get it dialed in from the get go.

For those of you coming from another cycling background, it’s important to note that mountain bikes have a much different intended purpose and therefore will fit and feel significantly different. Riding styles and trail types have also heavily influenced bike frame design in the past decade.

If you are interested in learning more about today's spectrum of mountain bikes, and what to consider when making a purchase, check out our Mountain Bike Buyer's Guide!

Sizing

The best place to start a fit discussion is by looking at manufacturer’s fit guidelines:
 

As you can see, mountain bike builders use the classic Small, Medium, Large sizing style. Seat Tube length is the traditional sizing method for bicycles, but with all the interesting shapes that mountain bikes come in these days, it has become less relevant to finding the right size. Most mountain bikes focus on an extremely low stand over height, so the traditional method of standing over the bike flat-footed doesn’t really work either. This is why mountain bike manufacturers use Small, Medium, Large sizing and give their own size recommendations based roughly on your height.

Just like a t-shirt, sometimes you’re in between sizes. Here’s a few tips on when to pick the larger or smaller of the two sizes when you’re right in between.

What you get for sizing up


When it comes to sizing, Reach tends to be what changes more from Small to Large than Stack. Almost all mountain bikes are trying to get as low as possible to buy the rider range of motion in the cockpit of the bike so you’ll see a lot of bikes with an increase in Reach. There’s also a minor corresponding increase in Wheelbase as the whole bike itself gets a little bit longer, so you get the benefits (and pitfalls) of a longer bike. Also, if you happen to have a proportionally longer torso, you’ll want to size up in order to get a neutral fit, since Reach is primarily what’s affected by the size. Flexibility and range of motion in your hips is also something to consider. If bending over and touching your toes sounds like a tall order, a longer bike is going to be a bit more taxing to maintain a low and aggressive riding position. On the other hand, if you know the difference between your Tadasana Pose and your Lotus Pose (AKA: you’re a supple yoga leopard), a larger size will reward you with a more aggressive riding position.

To sum up: If you’re flexible, longer in the torso and like to monster-truck over your terrain rather than whip or flick your way around it, size up.

What you get for sizing down


Conversely, a smaller sized bike will have a shorter Reach and a slightly shorter Wheelbase. Stand-over height will be a tad lower and you might have to extend your seat-post a bit in order to get to your pedaling position, but it’s a pretty minor consideration when it comes to being between sizes. If you’ve got proportionally longer legs than average, lack a bit of flexibility or generally like to be more upright , you may be a bit more comfortable on a shorter size. Additionally, a shorter Wheelbase will ride more nimbly and while the longer cockpit provides a really aggressive riding position, low and aggressive riding positions are tiring to be in, so consider your fitness, endurance and ride length. Shorter bikes are easier to be on for long periods of time.

To sum up: If you like a more nimble and whippy ride, are a bit less flexible, are shorter in the torso or are looking for a more casual riding position, size down.

Now go out and ride!

Now that you know what happens when you size up and down, go test ride some bikes! Just like 2 medium shirts from different brands don't fit the same, similar bikes from different brands will also fit and feel differently. You may size up with one brand and size down for another so go out there with an open mind, a good idea of your riding style and how you’ll use the bike, and find the best fitting bike for you!

Notable Geometry Measurements


These following measurements are important when discussing the fit, feel, and style of a mountain bike, and are often proportional to the style and terrain your bike was intended to perform on. The 2 main measurements that will affect the way that your bike will fit are Reach and Stack. Keep in mind that while these terms come from the road, triathlon & time trial world, those numbers won’t port over from your tri-bike to your mountain bike.

REACH


Reach is the horizontal distance between the bottom bracket and the center of the head tube. This is arguably the most important figure because it affects the length of your bike’s cockpit when you’re standing on the pedals and how much range of motion in your hips you’ll use up in order to achieve a good, strong riding position. Too long of a reach and you’ll be stuck leaned over and stretched out, too short of a cockpit and your weight bias will shift too quickly and leave you in a position that’s overly upright.

Stack


Stack is the vertical distance between the bottom bracket and the center of the head tube. This is primarily a gauge of seated pedaling position and relative handlebar height. This can be adjusted to a degree, with headset spacers & handlebar rise for increased stack height. For mountain bikes, where geometry is based on the aggressive riding (standing) position with the seat down, Reach trumps Stack as the primary fit dimension.

Head Angle


Head angle, or head tube angle, is the angle between the front fork and the ground. Although there are other parameters that go into what makes the front wheel behave, (fork offset, trail, etc) this is the key metric that the industry uses to determine front-wheel characteristics. A “slack” head angle is a lower number (eg: 65°) relative to a “steep” head angle (eg. 70°). A slacker head angle will, in general, be more stable at high speeds as well as feel more comfortable on steeper (downhill) terrain. Conversely, your bike will steer lazily and flop from side to side on uphill terrain - you will need to make more body position changes in order to tackle steep up hills, such as keeping your weight on the front tire so you don't wheelie yourself off the trail. Conversely, bikes with steep head angles feel like the front wheel is planted and steer nimbly on uphill terrain, while feeling twitchy and unstable on fast downhill terrain. If your inclinations (no pun intended) are towards pedaling uphill, you’ll pick a bike with a steeper head angle and vice-versa if you like to head down.

Chainstay Length


Chainstay length preference is personal to each rider. This measurement effectively determines the distance between the rider’s center of mass and the rear axle. Bikes with short chainstays have the rear wheel closer to being “under” the rider. Short chainstays allow the rider to be more dynamic with their weight. This means easier manuals & wheelies and a general ease of getting the front wheel off the ground. This comes at the expense of overall Wheelbase length, which contributes to stability at speed and over rough terrain. (Consider that unintentional wheelies while going uphill aren’t generally a desired trait.) If you want your bike to ride nimbly and like wheelies & manuals, short chainstays are your friend. If you prefer stability, go for a bike with longer chainstays.

Bottom Bracket Height


Bottom Bracket height is the distance between the ground and the center of your cranks. Some manufacturers refer to “Bottom Bracket Drop,” which is the vertical distance between the wheel axles and the bottom bracket. They effectively measure the same trait of the bike, which is how low your center of mass is. Just like your lowered sports car, bikes with low bottom brackets corner like they’re on rails and transition from turn to turn effortlessly. The tradeoff here is that the earth isn’t flat and trails aren’t typically paved smooth; the lower your bottom bracket, the more likely you’ll clip rocks, roots or logs you’re trying to hop over. If you’re a skilled and savvy rider, you can make sure your pedals aren’t down when passing over obstacles, but that means not pedaling over rough terrain and can be difficult to do when you’re brain’s starved for oxygen on a climb or you’re traveling at blinding speeds.

Wheelbase


There are a lot of factors that contribute to wheelbase including Head Tube Angle, Reach and Chainstay Length, which we’ve discussed previously. Total wheelbase is something of a culmination of all of these factors. The gist of wheelbase is that increasing the distance between your front and rear axles will increase stability, whereas shortening wheelbase will make a bike more maneuverable. Take note of where that increased wheelbase is coming from (Head tube angle? Chainstays?) and reference the above measurements to see how that’ll additionally affect the overall ride of your bike.

Conclusion


That’s a lot of information and though we've only covered the basics, this should give you enough background to pick a basic size and go test ride some bikes. When it comes to picking out a bike, there’s always a little bit of an unexplainable, intangible connection to a particular ride. It just feels right. So take this info with a grain of salt, throw your leg over a couple of bikes and go ride! If you’re in the Seattle, PDX or Denver area, check us out and come in for a test ride!

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