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What Size Frame Do I Need for a Mountain Bike

I’m Buying a Mountain Bike – What Size Frame Do I Need?

One of the best ways to get biking regularly is to make sure cycling is something you want to do–and that all starts with getting a comfortable bike. Especially when it comes to mountain bikes, that means getting a bike with the right frame size. Large bikes can be too difficult to handle, leading to major risks when things get rough. Small bikes can cause back injury after a couple of hours hunched over your handlebars, skittering across rough terrain.

So how can you get the right size bike to make sure you are ready to hit all the trails ahead? Where do you even start? Use this handy guide to begin finding the right size bike for you.

Frame Size on Mountain Bike Explained

When you start browsing bikes online, there are a few key terms you will need to know. First is the “effective top tube.” The top tube is the horizontal or nearly-horizontal bar at the top of the bike. However, since most mountain bikes have slanted top tubes, the effective top tube is the distance from the middle of the head tube straight to the seat post–as if the top tube was completely horizontal.

Next is the “reach.” Imagine a line going straight up from the bottom of the seat tube. The reach is the distance from this line to the middle of the head tube. Since you will spend a lot of your time standing on your mountain bike, this is a big factor in how well you are able to control your bike and how comfortable it will be to ride.

Finally we have the word “stack.” Now imagine taking a horizontal line straight out from the head post. The stack is the distance from the bottom of the seat tube to this line. This impacts not only how well you can handle the bike but also how much power you can generate. 

When testing out a bike, keep all three of these measurements in mind as you consider how comfortable it is and how well you can control it. You may want to ask yourself these questions:

  • When seated, how well can I reach the handle bars?
  • When standing, how well can I control the bike?
  • When standing, how fully can I extend my legs (to generate power)?

What Is the Best Size Mountain Bike for Me?

Mountain Bike What Size Frame Do I Need

These measurements are important and will help you talk to salespeople about bikes to ensure you get the right one for you. However, most bike manufacturers only publish the size of the frame, which is most often the total length of the seat tube (which does not include the adjustable seat post). This a helpful measurement because you can usually estimate what size frame you need with just your height and/or inseam measurement. Here is a simple table to help you estimate what size frame you need based on inseam and height.

Mountain Bike Size Chart

Your Height Your Inseam Seat Tube Length Frame Size
4’10” – 5’2” 25” – 27” 13” – 14” X-Small
5’2” – 5’6” 27” – 29” 15” – 16” Small
5’6” – 5’10” 29” – 31” 17” – 18” Medium
5’10” – 6’1” 31” – 33” 19” – 20” Large
6’1” – 6’4” 33” – 35” 21” – 22” X-Large
6’4” – 6’6” 35” – 37” 23” – 24” XX-Large

However, keep in mind this is just a way to find a good starting point–use this guide to pick out a bike to start with, but test it out and if it doesn’t feel right, move on to the next one, even if it’s a different size. You never know until you try!

Correct Size Mountain Bike for Me: The Perfect Fit

There are a few ways to get a feel for how well a bike fits. If the top tube of the bike is horizontal (or nearly so), straddle the top bar with both feet flat on the ground. You should have about an inch or two of space between the top bar and your groin. However, this really only works if the top bar is close to horizontal–if it is sharply angled, try the next method.

No matter how angled the top tube is, you can also get a feel for the fit of a bike by sitting on the seat. When seated, you should be able to touch the ground, but just barely–maybe only with the tips of your toes on one foot. If you flat-footed or have to lean the bike over to touch the ground, it is a bad fit. (Of course, you can experiment with adjusting the seat post, but keep in mind that the seat post needs to be at a certain height to optimize power.)

But the best way to make sure a bike fits is to ride it! If the store will allow it, take the bike for a short demo right, keeping in mind the questions from above. Every bike will feel different, even if their measurements are the same. Use this guide to get started, but try out every bike you can to find the right fit!

How to Choose Frame Size for Mountain Bike: Avoid Bad Fits

Even though you will probably feel the effects of a poorly-fitting bike pretty quickly, keep an eye for these common problems.

A bike that is too large can feel unmanageable, like you might lose control over even slightly technical terrain. You will also feel pain in your back caused by having to reach too far to grasp the handlebars properly. Additionally, if a bike’s top tube is too high, you won’t have enough standover clearance to protect your …sensitive areas.

A small frame has some similar problems. The biggest issue is that you will be cramped up for most of your rides, whether you are standing or sitting, This can lead to soreness, fatigue, and cramps. In some cases, you may even run the risk of catching your toe on the wheel!

Correct Frame Size for Mountain Bike: Get Help Now

If you are in the market for a new mountain bike and have additional questions that are not answered in this article, let us know! We will take user submitted questions from our readers to add to our cycling knowledge base to share among our community.  If you’d like some additional help choosing your new mountain bike, feel free to shoot us an email so we can help! Stay pedalin’!

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