How to Get the Right Size Mountain Bike for You
Getting your first bike can be intimidating. Whether you are looking at one in person or shopping online, knowing which size you’ll need is very important. When I was a child, it was much easier. Children’s bikes are sized by simply standing over them. But we adults have to be more discerning!
Your mountain bike frame size is determined by two body measurements. How well these body parts are matched to sections of your bike will impact nearly everything about your ride. How comfortable you are, how easily you can control the bike, even mounting and dismounting are changed by the frame size.
The Basics of Frame Measurements
Now, what size mountain bike do I need? First, we will talk about frame sizes. Most mountain bike frames are sold as small, medium or large. A few makers also sell extra small and extra large frames. It can be hard to find mountain bikes sold with more exacting measurements. Most manufacturers just list offer up a list with various heights and a suggested size for each.
Most of the time you can find measurements for the bike frame sections. Here are the most commonly provided:
- Head Tube – the short tube at the front of the bike between the handlebars and the front wheel.
- Seat Tube – long vertical tube underneath the saddle, right before the rear wheel.
- Down Tube – lower long tube between head tube and seat tube.
- Top Tube – upper long tube between the head tube and the seat tube.
- Seat Stays – thinner tubes that run from the seat tube to the rear axle area.
- Chain Stays – thinner tubes that run from the rear axle along the chain to the bottom bracket.
As you can see, there are a number of measurements that you’ll need to think about when bike shopping. But they are not all equally important. The most important measurement is the seat tube length. This is the part of the bike that determines how high or low the saddle can go. The second most important is the length of the top tube. That determines how much you’ll be stretched out while riding.
Some of the measurements are more useful for understanding how the bike will handle. For example, the seat and chain stay lengths heavily influence how a bike handles at speed. You will find that slightly longer chain stays make your ride much more stable while going downhill.
About Top Tube Lengths
There are two measurements for top tubes in mountain bikes. If you look at a mountain bike size chart, you will notice that most manufacturers mention the top tube and the effective top tube lengths. This is because modern mountain bikes often have a sloped top tube. The effective top tube length is the length directly from the head tube back to the seat tube, the same as on a road bike.
A sloped top tube is safer on a mountain bike. Not only is it more easily mounted and dismounted, but it allows the frame to be made smaller. This improves handling on the trail. It also makes it far less likely to hurt yourself in a crash!
If you want to be more upright as you ride, you will prefer a bike with a somewhat shorter top tube. If you prefer a more ‘roadie’ posture, you will want to have a slightly longer top tube. Try to find a length that feels comfortable in a neutral position. You will be able to fine tune your fit with careful choices in stems and handlebars.
All About Seat Tube Length
You might notice that many modern mountain bikes have shorter seat tubes than road bikes or cruisers. There are a number of reasons for this. Standover gap, stiffness and weight are all factors. You may also notice that some types of mountain bike frames don’t even have a full seat tube! So what are we to do when we need to measure the seat tube? It doesn’t matter if the frame doesn’t have a seat tube that goes straight down to the bottom bracket. You’ll still measure it as if it is completely there. Most manufacturers will measure from the center of the bottom bracket up to the seat post clamp. Mountain bike frames have to be very adjustable. You need to be able to move your seat up and down depending on terrain. Standover gap is the space between your body and the top of the top tube. You want at least an inch of space between your body and that tube.
Starting Your Bike Fit Process
t’s generally said that the best way to get a good fit is to try potential bikes in person. This is true. But if you are buying a bike from an online seller, then you can’t try them out first! What you can do is to ask to try friend’s bikes. If you can’t, you can often try out bikes at box stores to check fit.
When you find a frame that you like, you must measure it. You’ll need a fabric tape measure. Plan to measure at least the seat tube length and the effective top tube length. These two measurements will help you the most!
- Measure the effective top tube length by measuring from the top of the head tube straight back to the seat post.
- Measure the seat tube by measuring from the center of the bottom bracket to the top of the post clamp.
- You may also find that measuring to the bottom of the saddle itself can be helpful.
Now, the first time I read this advice, I was perplexed. I didn’t know any riders. Nor do I have easy access to a box store with lots of bikes. So I couldn’t try out bikes or get bike measurements! What I could do instead was measure myself.
Take Your Own Measurements
There are really only two body measurements we’ll need. You need an accurate measurement of your height and your inseam. Keep in mind that women and men of the same height will have slightly different body measurements.
To get your height without a helper, you’ll need a wall and some chalk. Remove your shoes, stand up straight and make a mark at the crown of your skull. The easiest way to get your inseam is to place one end of a tape measure flat on the floor and measure up your leg to the place where it joins your body.
Men’s frames are generally made with male proportions in mind. Think longer legs and a shorter torso. Women’s frames are often made for those of us with shorter legs and a longer torso. On the bike frame itself the difference is generally noticed most in the effective top tube length. Woman’s frames will have a slightly longer effective top tube measurement.
Unisex bikes do exist. They are generally able to fit both body types. They will use measurements that are an average between male and female lengths.
Don’t just try bikes made for your biological sex! Some men are more comfortable on women’s frames. Some women like men’s bikes better. It just comes down to how your body is made.
How to Tell if You Have the Wrong Frame Size?
It isn’t always obvious at first when the frame is the wrong size. There are a few things that even a novice rider will notice. There are also less obvious clues that you might only notice while out on the trail.
Too-big frames are hard to mount. You might also feel like you’re going to tip over. You’ll be more likely to smack that top tube when you finally do, too! Other clues that aren’t so obvious include:
- An Over-Extended Torso. You’ll notice this when your back aches while you ride.
- No Standover Gap. You won’t be able to adjust the seat much.
- Control Problems. You do a large amount of steering with your body. If the frame is too big, the balance between you and the bike is thrown off.
Small frames can be almost as bad as a too-big frame. A too-small frame will have more than a few inches of standover gap.Your arms and torso might feel cramped or constricted. You could also feel silly, like you’re riding a child’s bike. Other too-small clues include:
- Bad Toe Clip. This means the front tire will hit your toes as you turn. A little is OK, hitting the side of your foot is not!
- Pain and Injury. A too-small frame will hurt your back as you hunch over. Over time, your knees will also suffer wear and pain.
- Bad Knee Clipping. This means that your knees will hit the handlebars. This is normal in extreme turns. Not so much for regular turns.
Now, your bike fit doesn’t have to be perfect! A slightly too large or too small frame can be adjusted to perfect the fit. You can change the saddle position, the seat post, the handlebars and stem. These changes cannot safely take a small frame to a point where someone who needs a large frame can ride it. What they can do is fit someone who might be between a small and a medium frame onto that small frame.
Finding Your Size
Most manufacturers will have mountain bike sizing guides for their own frames. But I’m going to give you a general guide. I’m also going to give you a equation that you might find helpful in your search.
Take your inseam measurement in centimeters. Multiply it by 0.66. Round it up or down, as needed. That number is the size frame that will fit you. As an example, my inseam is 76 centimeters. Multiply that and round down and we have 50 centimeters. You’ll then convert it to inches. I’ll probably want a frame around 15 inches, or a small.
- If you’re 4’10” to 5’1”and your inseam is 24 to 29 inches, you’ll need a 14 inch extra-small frame.
- If you’re 5’1” to 5’5” and your inseam is 25 to 30 inches, you’ll want a 15 inch small frame.
- If you’re 5’5” to 5’9” and your inseam is 26 to 31 inches, you’ll be comfortable on a 16 inch medium frame.
- If you’re 5’9” to 6’0” and your inseam is 27 to 32 inches, you’ll like a 17 inch large frame.
- If you’re 6’0” to 6’3” and your inseam is 28 to 33 inches, you’ll need a 18 inch extra-large frame.
- If you’re 6’1” to 6’6” and your inseam is 29 to 34 inches, you’ll want a 19 inch extra-extra-large frame.
Notice that there is overlap in the sizes? That’s because a perfect bike fit is more of an art than a science. Each one of us is unique in how we’ll fit into a particular frame.
Going back to my measurements, I could also fit into an extra-small frame. So, if I got the chance, I should try both and keep the one I found most comfortable. I’d then dial in the fit by adjusting or changing various parts of the bike.
One Last Word
These sizing guides and tips will hold true no matter what size wheel you prefer. A 29er frame should fit just as comfortably as a traditional mountain bike or a road bike!
If you are trying out a new wheel size or suspension, be sure to give yourself some time to get used to how your new bike handles. As long as you are comfortable and properly fitted onto the frame, you’ll adapt to the big wheels of a 29er or the handling of a suspension bike.