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Mountain Bike Sizing Explained

One very popular topic that’s always asked about online is mountain bike sizing. But here’s the funny part – you need to get off the internet and spend some quality time with some real mountain bikes to learn about sizing them!

But since it is so important to get it right (your comfort and safety depend on it,) I’m going to give you some tips to point you in the right direction. So let’s get started right now!

 

cannondale scalpel

Mountain Bike vs Road Bike Sizing

(Feel free to skip this section if you have no interest in road bikes.)

In case you are used to sizing road bikes, I must warn you – sizing a mountain bike is different.

With road bikes, you get technical and take very precise measurements to dial in the correct frame size and riding position. With mountain bikes, the process is a lot more laid back. That’s because of the nature of mountain biking – you encounter so many different scenarios and move around on the bike so much, it would actually be counterproductive to have such a precisely fitted bike!

Here, it’s not about the perfect fit. It’s about a good fit and accommodating the special needs of mountain biking that you don’t have to deal with on the road.

The point is, don’t fret if a bike shop employee takes a laid back approach to sizing your mountain bike.

 

mountain bike frame sizing

How Mountain Bikes are Sized

When looking at mountain bike sizes, you will first be presented with the frame sizes. The frame size is so important that bikes are labeled just by this size.

Frame size is based on the frame’s seat tube length. It is generally measured from the center of the bottom bracket shell to the center of the top tube, where the top tube bisects the seat tube. The measurement is stated in inches, so you’ll find frames sized 15″, 17″, 18″, 19″, etc.

There are two ways companies state bike sizes, though. The first is in inches, as mentioned above, but what is becoming quite common today is labeling bikes Small, Medium, and Large, etc.

What are we forgetting? Top tube length!

Equally as important to the seat tube length is the top tube length. This will vary based on the seat tube length, manufacturer, and style of bike.

Generally, the XC race-oriented bikes have longer top tubes than recreational or all-mountain bikes, but all brands of bikes will vary in this regard. You might try a size medium Giant and a medium Specialized, and guess what? Their positions will feel different!

Does any of that matter? Not really!

Seriously, don’t get bogged down with any specific numbers. I’m not going to give you any sort of chart that plots your correct bike size based on your height or inseam because they just don’t work. (I already went on a rant about choosing a frame size, so I don’t need to do it again.)

There’s not a lot you can do here except test ride the bike and see how it works!

Then test ride another one and see if it is better or worse!

 

Do Your Height and Inseam Length Matter?

There’s a lot of garbage information online about bike sizing, and that leads people to believe they can purchase a bicycle based on their height and/or inseam length.

Sorry, it’s not that simple!

You have to take into account your height, inseam, arm length, torso length, flexibility, riding style, and a myriad of other factors when choosing a mountain bike. So if I told you that you need size X bike since you are Y height, I would be doing you a grave disservice!

So yeah, your height and inseam do play a role, but you have to take your individual body into account. Everyone has different proportions (short but with a long inseam, for example,) and this means everyone will have slightly different requirements for sizing.

I’ll say it again – you need to fit the bike in real life to be sure it fits. Don’t just rely on a chart. And – test ride before purchase!

 

Sizing The Bike for You

I don’t want you to blindly go bike shopping, since some shops aren’t the smartest at proper sizing, so I’ll give you some tips to make sure a bike is the correct size for you.

This is still generalized advice for when you sit on a bike, though, so take it with a grain of salt.

Standover clearance

When standing over the bike, straddling the top tube, you’ll want at least 3-4″ of clearance at your groin. This should be self explanatory for any guys!

Seat height

To get a quick, rough estimate of your proper seat height, sit on the bike and place your heel on the pedal (with the pedal at the 6 o’clock position.) Raise the seat to where your leg is straight in this position.

This should put you in a position where your knee is slightly bent at the bottom of the pedal stroke (assuming proper foot placement) and your hips don’t rock side to side when you pedal.

Keep in mind, you also want the saddle low enough you can get your butt way back behind the seat for steep downhills. So this isn’t a conclusive test for your exact seat height. It’s just to see if the bike is potentially going to fit you.

Reach

If the bike passes the standover and seat height tests, now it’s time to make sure you have a comfortable reach to the handlebar.

For this, just sit on the bike and hold the handlebars, and see if you’re comfortable. The bike shop employees will see if you’re too crunched up or too stretched out.

Toe overlap

Common on some road and cyclocross racing bikes, toe overlap is something you do not want on a mountain bike! So just do a quick check where you twist the handlebar side to side and make sure the front wheel does not hit your front toe when you’re in the riding position.

 

In Between Sizes?

What if you’re in between sizes? Or maybe two different size bikes can be made to fit you?

This happens all the time with mountain bikes. Especially with the variety of seatpost and stem lengths. The general school of thought is that you should get the smallest frame that still allows a proper fit.

The smaller frame means three things:

  1. The frame is lighter (so a lighter bike overall.)
  2. The frame is stiffer for better performance.
  3. More standover clearance for safety of the family jewels.

Assuming the smaller bike fits you just fine, and feels just as comfortable and responsive as the larger bike, go for it.

 

trek 69er bike

Choosing a Wheel Size

Choosing a wheel size used to be a no-brainer – almost exclusively (some kids bikes came with 24″ wheels,) you’d get 26″ wheels on a mountain bike.

Nowadays, there’s a legitimate case for 26″ or 29″ wheels, or even 650b (which is right in between the other two sizes at 27.5″.) There are even “69er” style bikes with a 29″ wheel up front and a 26″ in the rear!

Why go bigger than 26?

The bigger the wheel, the easier it rolls over obstacles. This makes 29ers very fast on rough terrain and gives them a bit of a full-suspension feel, without needing rear suspension.

However, the bigger wheels can take longer to accelerate and be harder to maneuver, so they aren’t well-suited to tight, twisty courses with lots of speed changes. So for this sizing question, you are mainly considering the type of riding you plan to do.

[Usually, 29ers start in a Medium frame size, so if you’re on the shorter side, you might not be a candidate for 29″ wheels anyway.]

 

Miscellaneous

Sizing your bike is what matters, but there are a few parts sizing guidelines I want to mention in this little miscellaneous section.

Crank arm length

The standard crank arm length for mountain biking is 175mm.

170mm cranks are common on road bikes, but when mountain biking, riders usually prefer the extra leverage gained from a slightly longer crank arm.

Stem length

Stem lengths typically range from 90-120mm, with shorter stems like 90-100mm being the most common.

Changing the stem on a stock bike setup is common and very helpful, since it’s a quick way to change the “reach” to the handlebar.

Handlebar width

Handlebar width typically matches shoulder width, although it is personal preference more than anything.

Keep in mind a wider handlebar can offer better control, but is harder to squeeze between trees on tight singletrack.

 

Custom Built Frames

This is not common, but if you are NBA player tall, or your body has some very odd proportions, it can be hard to set up a normal bike to fit you well.

If this is the case, you might need to order a custom built frame from a small builder or a company such as IF.

If you go this route, you’ll fill out some forms with all your body measurements and be interviewed about your riding style and goals. It will take quite a bit of effort, and it won’t be cheap, but there is always a way to get the right size of bike!

 

Mountain Bike Sizing Summary

Getting the correct size bike is very important. The frame is the most expensive and hardest to replace part of your bicycle. so get the sizing right the first time!

Take your time and do it right. That means talking to shop employees for advice, asking “why” they recommend something, and above all, test riding multiple bikes before making a final decision.

Good luck!

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3 Comments
  1. Awesome bike sizing explanation, thanks for sharing!

  2. you state

    “Assuming the smaller bike fits you just fine, and feels just as comfortable and responsive as the larger bike, go for it.”

    Am I the only one that thinks this statement makes no sense? How can a larger AND smaller frame feel just as comfortable?

    My 30+ years in MTB suggest to go with the LARGER bike and fit a shorter stem/wider handlebars if needed….MTB companies have been shoving smaller bikes down our throats for too long..go bigger! get more comfortable!

    • @Martin

      I already explained this above but hopefully I can re-word it and it will make sense for you.

      Let’s say you have an ideal riding position that a bike fitter would aim for. Sometimes that’s only possible on one frame size, but sometimes it’s right on the line where that position can be achieved on two frame sizes. It’s quite common with mountain bike frames because they come in fewer sizes than road frames. Me personally, I’m often on the line between medium and large frames.

      There will be different stem lengths, different amounts of seatpost showing, etc. The frame is just one component of bike fit. If you can achieve your ideal position on either one, comfort and power output will be equivalent. But if both frames ride equivalently, and you can’t decide… the smaller one will weigh less. It’s that simple.

      If you happen to be doing test rides and getting a fit, and there’s a clear frame size that’s going to work, then you’re all set and won’t even be encountering this issue.

      Let me know if you need any more clarification!

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coach levi
Hi, I'm Coach Levi. I'm a USA Cycling Certified Level 3 Coach as well as Level 1 Certified with Precision Nutrition. Want to feel better, ride faster, and look great? Let's work together!

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