How to Measure A Mountain Bike Frame

by Kevin Stard

Generally, there are different sizes of mountain bikes. Therefore, when buying a new bike, you must choose the right size that fits your particular needs and physique. To select the right size, you have to measure some particular parts of the bike accurately. Hence, you need to know how to measure a mountain bike frame.

In this article, you will be shown how to measure a mountain bike frame and find the perfect bike for you.

Why do you have to measure the frame size of a mountain bike?

Perhaps you are still wondering why you need to measure the frame size of a bike. Here are the three reasons for measuring mountain bike frame size:

  1. To guarantee the comfort of the rider
  2. To ensure the safety of the rider
  3. To choose a bike that suits the biking expertise of the biker

Tools for measuring mountain bike frame

To measure a mountain bike frame size, you need the following tools:

Notably, mountain bikes can be measured in either centimeters or inches. However, for consistency, you are advised to utilize only metric units for measuring your mountain bike.

How to measure different parts of a mountain bike

Although you can find the top tube length in the geometry table of your bike, you should still measure it to ensure consistency. Notably, some top tubes are horizontal while others have slopes. The shape of the tube will determine how you will measure it.

The horizontal top tube length is the measurement of the distance between the centerline of the seat post and the centerline of the head tube. You will need a clinometer app or a long spirit level for accurate measurement.

As you learn how to measure a bike frame, you should note that mountain bikes are marked S, M, L, etc. based on their top tube length. However, some manufacturers use other methods for interpreting their measurements of mountain bikes.

Seat tube length refers to the measurement of the size of the seat tube. You have to measure it from the center of the bottom bracket to the top tube.

The major issue with seat tube length comes from the difference in the placement of the seat tube in many bikes. While the seat tube of many bikes extends above the top tube area, others get to a seatmast. Resultantly, you may not be able to compare the dimensions of these bikes easily.

You have to measure the seat tube length along the distance from the bottom bracket center to the upper part of the seat tube.

The reach is the horizontal distance between the bottom bracket and the upper part of the head tube. A spirit level is required for measuring this part.

You need to get started by placing a plumb line at the end of the level and then align it with the center of the top of the head tube. Continue moving the level until there is an intersection between the plumb line and the center of the bottom bracket. To find the reach, you have to measure the distance between the head tube and the top of the plumb line.

Alternatively, you can measure the reach by placing your bike against a wall. Firstly, measure the length from the wall to the top of the head tube. Afterward, you should measure the length between the wall and the bottom bracket of the bike. Now, just find the difference between these measurements to determine the reach.

On the other hand, the stack refers to the vertical distance between the bottom bracket and the upper part of the head tube. In other words, the reach measures the horizontal distance between these two parts whereas stack measures the vertical distance between the same parts.

To measure your stack, you can simply measure the vertical distance between the top of the head tube and the ground. Afterward, measure the difference between the ground and the bottom bracket. Then find the difference between these two measurements to find your stack.

You may want to consider repeating the processes to ensure that you get accurate measurements for the reach and stack.

The next thing you need is to learn how to measure wheel base of your bike. This is for determining the distance between both the front and rear axles. You just have to place your tape measure on one axle and extend it to the other one. This will give you the measurement of the wheelbase.

Although measuring the wheelbase is easy and straightforward, you should consider repeating the measurement a couple of times to ensure consistency in the number.

For accuracy, you can also measure the wheelbase from each side of your bike. Afterward, take the average of the measurements to determine the size of the wheelbase.

Notably, the wheelbase has two essential components – front center and chainstay length. Both components play a key role in determining the handling of a bike. So, you need to measure these two components along with the wheelbase.

The front center is the distance between the front dropout and the axle of the bike. Carefully use your tape measure to find this distance and record your measurement.

The chainstay length is the distance measured from the center of the rear dropout to the center of the bottom bracket axle. It involves only a straight line. Therefore, you shouldn’t have any problem with measuring the chainstay length as long as you know what to measure.

This refers to the distance between the center of the bottom bracket shell and the ground. It does not require any technicalities as you only need a tape measure to get the job done.

However, to ensure the measurement is accurate, you need to make sure your bike stands straight. Also, tires play a major role in the bottom bracket height, so you have to inflate them to the normal pressure.

The other measurement of the bottom bracket is the bottom bracket drop. This is for the distance between the center of the crank axle and the height of the wheel axles. You can simply check the reading on the geo charts of your bike to find it.

As an alternative, you can measure the height of the bottom bracket as well as the height of the rear axle. Afterward, subtract the first reading from the second one to get the bottom bracket drop.

When it comes to how to measure a mountain bike frame, the last but not least things to measure are the seat tube and head tube angles. These angles affect the handling of the bike; therefore, you must get them right.

For bikes with a straight seat tube, just place your clinometer app on it and get the measurement on the app.

However, if the seat tube is not straight, you need to line up the distance between the top of the seat tube and the bottom bracket shell with a straight edge. Then, put the clinometer on your smartphone on the line to measure the seat tube angle.

The next thing is the head tube angle. Place your smartphone with the clinometer app at the end of the centerline. This will give you the head tube angle.

Nonetheless, some bikes have fork legs that don’t have an angle at the crown. In this case, both the head tube and the legs will have the same angles. Therefore, you can just measure the straight fork legs and use the reading as your head tube angle.

What bike size is best for various individuals?

In addition to knowing how to measure mountain bike frame size, you should also know the ideal bike size according to the size of each individual. A simple mountain bike size chart is shown in the table below.

Rider Height in cmFrame Size in cmFrame Size
148cm – 158cm33cm – 37cmXS
159cm – 168cm38cm – 42cmS
169cm – 178cm43cm – 47cmM
179cm – 185cm48cm – 52cmL
186cm – 193cm53cm – 57cmXL
194cm and above58cm and aboveXXL

With this chart, it will be easier for you to find the perfect bike.

Conclusion

In a nutshell, mountain biking is a highly engaging sport that you can enjoy alone or with your loved ones. However, you must choose the right size for you by learning how to measure a mountain bike frame as described above. Dedicate some time to learning the steps above and you will get the best bike for your biking adventures.

Kevin Stard - professional sportsman in category MTB. Participant of the Downhill World Cup #1