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What Size Mountain Bike Do I Need?

What Size Mountain Bike Do I Need?

Choosing the proper size for your new bike is an incredibly important step that can make the difference in your performance and comfort down the road.

While it may be tempting to squeeze onto a bike that is slightly too small or settle for a bike that is too large, this mistake will only continue to cause problems for you as a rider.

Starting with understanding mountain bike sizes can feel daunting. Fortunately, there is significant information from manufacturers as well as in-person bike shop assistance to help you find the perfect size bike.

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I’ll talk you through the process I would go through in order to pick the proper frame size for me.

Suppose I’m looking into the Trek Top Fuel. This is a full suspension XC mountain bike that is more trail-oriented than some cross country bikes.

The first thing I would do is look at the Top Fuel on Trek’s website. This bike comes with a size guide with plenty of information.

I’ll first look at the suggested rider height as well as the suggested rider inseam to ensure I’ll feel comfortable standing over the bike.

Interestingly, looking at the suggested height and inseam can also give you a sense of if you tend to have longer legs or a longer torso. For me, I have longer legs than many people my height which will help our decision-making process down the line.

Looking at the Top Fuel, the Medium size bike is recommended for riders who are ~5’3 to ~5’7 with an inseam of ~30” to ~32”. My height is 5’5 and my inseam is 31.5”.

As you can see, I am perfectly in the middle of the height range but my inseam is towards the upper end of the recommended inseam length for a medium frame.

It is important to note that the Small frame does say it accommodates riders who are up to 5’5.6” tall and the Medium/Large frame accommodates riders who are as small as 5’5”. According to this chart, I could technically fit well on a Small, a Medium, or a Medium/Large frame.

With this information, we can look at the suggested inseam height in order to dial in which size will fit me best. For me, my inseam of 31.5” is longer than the suggested inseam for the Small bike, which ends at 31”.

In the Medium/Large frame, the suggested inseam is a minimum of 31.3”, meaning I technically fit in this range as well.

Now that I’ve looked at the suggestions, it is also important to think about the reach length of each frame. The reach is measured by drawing a line straight up from the bottom bracket and then measuring the horizontal distance from this line to the center of the stem.

At this point, I’ll think about my old bike, a Scott Spark which had a reach length of 40cm. While I was riding this bike, I continually felt like I was squished and would often crash, falling over the bars.

This indicated to me that the reach was too short and the frame was too small for me. As I look at the Top Fuel size chart, I see that the reach on the Small frame is 40.5 cm and the reach on the Medium frame is 44 cm.

Knowing that I would like a slightly longer reach, I can confirm again that the Medium frame will suit me better than the Small frame.

In order to differentiate between the Medium and the Medium/Large frame, I’ll think about my riding style. I typically like my bikes to feel as snappy and agile as possible with my preference of 29” wheels.

Considering this, the Medium frame is likely a better choice. If I did like for my bike to feel more stable instead of more agile, I might lean towards the Medium/Large frame.

Noting that I fit squarely within the Medium frame size did make this process easier. However, if you do feel like your height and inseam are truly between sizes, you can start to consider the reach of a bike you feel comfortable on as well as your riding style.

Some other components to consider are the changes you can easily make.

The easiest change you can make is your saddle height and position. It is important to ensure that you will be able to comfortably stand over the top tube and that your seat post will not need to be excessively long or excessively short.

Another consideration in regards to your saddle is to take note of the dropper post height if you plan on using one. Some frames do not have much space in the seat tube to accommodate a dropper post if your saddle height is lower.

Some riders who are between sizes will choose a smaller frame size to ensure the frame will accommodate the dropper size they’d like.

If you are intrigued by a larger bike for the sake of stability, ensure the dropper post you are looking to use will have enough space to drop low enough into the seat tube.

Other changes you can make are adjusting or replacing your stem or bars. This change can help you adjust to the reach you want.

Another consideration in finding the proper reach length is to consider how long your arms are. If you have shorter arms, perhaps opting for a bike with a slightly shorter reach will be more comfortable.

While there are numerous factors to consider, start small by checking the manufacturer’s suggestions, then consider your riding style. It can also be particularly helpful to test ride the bikes if you have access to a bike shop.

Often, bike shops are more than willing to help you find the proper size bike before you commit to purchasing a bike.

Take your time, test ride if you can, and remember that choosing the right size bike is incredibly consequential and can help ensure you have fun for years to come.

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