SIMPLE Guide – How to Measure a Mountain Bike Frame

Cycling ranks as one of the top low-impact wellness exercises for people of all ages [1]. Coming from a year that was full of different forms of lockdown, many people are more eager to get out and stay fit. This new trend has greatly contributed to the current boom witnessed in cycling [2].

If you’re just looking to get in on some of the action and it’s your first time to buy a bike, you must first understand the different measurements relating to your bike frame in advance. Having this in mind will help you pick the best pick for all your biking needs. This knowledge is additionally important when renting too, as it ensures that the rental is both comfortable and convenient.

Unfortunately, different bike brands come with varying bike frame measurements. To make sure that you compare apples with apples, this article will let you in on some of the most vital frame measurements for you.

Before discussing how to measure a mountain bike frame, here is a quick highlight of the key measurements for your bike as well as the necessary tools for measuring a bike frame.

Key Measurements for your Bike

Below are some of the most fundamental bike measurements that you should always keep in mind:

  1. Top and Seat Tube Lengths
  2. Reach and Stack
  3. Wheelbase
  4. Chainstay Length
  5. Front Center
  6. Seat Tube and Head Tube Angles
  7. Bottom Bracket Drop and Height

What You’ll Need to Measure a Bike Frame

The standard measuring unit for most mountain bikes is either metric unit or inches. In rare instances, some mountain bike brands mix and use the two while manufacturing but the most widely used measurement is the metric unit.

To size up your bike frame, you will need to have the following:

  1. Tape measure
  2. Clinometer app
  3. Spirit level
  4. Plumb line

How to Measure Top and Seat Tube Lengths

The top tube length of your bike is commonly referred to as the virtual or effective top tube length by different mountain bike geo charts. The top tube length generally refers to the horizontal distance from the middle of the head tube to the midsection of the seat post.

Several mountain bike manufacturers label their bikes’ top tube lengths as S, M, and L. The standard top tube length for most medium-sized mountain bikes ranges around 54cm but beware that not all manufacturers measure this length the same.

To measure the length of your bike’s top tube correctly, you’ll need to use a downloadable clinometer app or spirit level to get the right angle before proceeding to take readings from your tape measure.

On the other hand, the seat tube length refers to the straight distance running from the top section of the seat tube to the middle of the bottom bracket. Similar to the top tube length, some mountain bikes often include extensions while others use seat masts. These additional kinks in different mountain bike brands usually make it difficult to compare between alternative measurements.

Therefore, to measure the length of your seat tube correctly it would be best not to include the line along the tube. Instead, line up your tape measure along the straight edge running from the top of your seat tube up to where the bracket center ends.

How to Measure Reach and Stack

In most mountain bikes, the reach and stack usually fall between the head tube and the bottom bracket. The horizontal distance between these two forms the reach and stack refers to the vertical measure.
Normally, these two are usually independent of any bike frame design and you might need an extra pair of hands to measure them correctly.

To measure the reach of your mountain bike, you’ll have to tie a plumb line at one end of your spirit level. To be more accurate, ensure that the string is straight and symmetrical. Once in place, align the top end of your spirit level with the midline of the top of your head tube then dangle the level to swing back and forth until the plumb line meets with the middle of your bottom bracket. After getting this intersection, you can now proceed to get the dimensions between the head tube and the top of your plumb line. That will be your reach.

While at it, simply measure the vertical distance between these two features and that will be your stack. This is the most accurate way to measure both the reach and stack of any mountain bike. In case you don’t have a partner to help you out, simply lean the bike against a wall and continue with the measurements.

Always repeat these measurements several times, just to make sure that you have the correct and consistent dimensions.

How to Measure Wheelbase

The wheelbase refers to the space between the rear and front bike axles. It forms a crucial part of any bike’s frame and is used to determine the design’s ride quality.

Since its measurements vary with different frame sizes, you must learn how to measure the right fit for your riding needs. Before you start measuring, remember to straighten the fork then measure the distance between those two axles using a tape measure.

For accuracy purposes, repeat these measurements a few more times and take measurements from each side.

How to Measure Chainstay Length

As part of the wheelbase, the chainstay is a great determinant of your bike’s frame handling capabilities. Its length generally refers to the distance between the middle of the rear dropout and the bottom bracket spindle.

It’s usually a straight line and can be measured easily using a ruler or tape measure. Just remember that bikes with shorter chainstays feel livelier relative to those with longer ones.

How to Measure Front Center

The front center of your mountain bike usually is another part of the wheelbase. Similar to the chainstay feature, the front center refers to the distance between the bottom bracket axle and the front dropout.
To measure it, simply take readings between these features using a ruler or tape measure. Keep in mind that this also determines overall handling, making accuracy a top concern.

How to Measure Seat Tube and Head Tube Angles

The alignment of both seat tube and head tube angles on your mountain bike influences handling and you can easily measure them with your favorite clinometer app. Here’s how to do it right.

If your seat tube is generally straight, open the clinometer app, align your phone with the seat tube then get the measurements displayed in the app. For accuracy reasons, remember that your bike must either be in a vertical or horizontal position.

In case your mountain bike has an additional kink, find a straight line running from the top of the seat tube to the bottom bracket shell, and align your phone with this edge for measurements.

Normally, most mountain bikes come with tapered head tubes, making angles along their length vary from position to position. To measure the angle along its centerline you’ll have to align your clinometer app with a straight edge running from the centers of both the bottom and top of the head tube.

You can also measure your mountain bike’s head-tube angle by getting the legs’ angle or by aligning your clinometer app with the steerer bar on top of the head tube. Just make sure that your mountain bike is vertically straight before measuring.

How to Measure Bottom Bracket Drop and Height

For starters, the bottom bracket drop refers to the difference between the midline of your bike’s crank axle and the height of its wheel axles whereas bottom bracket height refers to the measurement between the middle of your bike’s bracket shell and the ground.

To measure the bottom bracket drop, simply find and subtract the heights of the rear spindle and that of the bottom bracket. As for the bottom bracket height, simply measure the distance between the midline of your bottom bracket shell and the ground. Just make sure that your bike is upright and wheels inflated at running pressure.

Summary

Ideally, knowing the size of your desired mountain bike makes your buying process easier and guarantees to get the right fit for your riding needs from the start. Having the right measurements ensures that the bike’s setup is correct so you don’t have to worry about any health implications. The above tips are easy to follow and will get you on the right track in no time.

Citations

  1. https://www.betterhealth.vic.gov.au/health/healthyliving/cycling-health-benefits
  2. https://www.outsideonline.com/2420131/pandemic-bike-boom-here-stay
We will be happy to see your thoughts

Leave a reply

one × three =