It is essential to know the key measurements of a mountain bike that determines the frame size. So, if you’re choosing your mountain bike for the first time, then you should ensure that the bike fits you well.

Mountain bikes are measured in inches or sometimes in small, medium, and large sizes.

Factors That Defines Your Bike Frame

  • Top tube
  • Down tube
  • Seat tube
  • Seat stay
  • Chainstay
  • Reach and stack of frame
  • Wheelbase

What instruments do you need to measure a bike frame?

  • Measuring tape
  • Long spirit level
  • Clinometer
  • Plump line or a piece of string

Measurement of Top Tube Length

To determine the frame size of a mountain bike by the top tube method, align the top edge of the level with the centerline of the head tube.

Afterward, move the level back and forth until the plumb line intersects the bottom bracket spindle.

Afterward, measure the distance between the edge of the plumb line and the head tube, and you will know how far you are from the top of the tube.

How to measure a mountain bike frame

Measurement of Down Tube Length

We can measure the down tube length by measuring the distance between the two points (C-T).

This is the distance between the top of the seat tube and the center of the bottom bracket.

how to measure mountain bike frame

Measurement Of Seat Tube Length

This measurement is generally taken from the bottom bracket (where the cranks pass through) to the seat tube (where the seat post exits the frame).

When it comes to bike sizing, it is the defining measurement.

P.S: A shorter seat tube would give you more clearance over the frame because seat tube length is closely tied to top tube height.

How to measure mountain bike frame

Measurement of Seat Stay Length

The measurement of a seat stay’s length is taken from the rear axle to the point where its centerline intersects the surface of the seat tube.

It is the distance between the seat tube centerline and the seat stay centerline that represents the length of the seat tube.

Measurement of Chainstay Length

Using a ruler, you can measure the straight line distance between the bottom bracket axle and rear dropout, which shows that the chainstay is fairly easy to measure.

Shorter chainstays are better than longer chainstays.

How to measure mountain bike frame

Measurement of Reach and Stack Length of Frame

Stack is a vertical distance between the center of the bottom bracket and the center of the top of the headtube. 

Reach is the horizontal distance between the center of the bottom bracket and the center of the top of the headtube.

For the measurement of mountain bike frame size, Place the mountain bike frame diagonally across the corner of the room or wall. Take a horizontal measurement from the back wall to the center of the brackets.

The angle separating the bottom bracket center and the floor should be measured vertically. And now measure the distance between the top of the plumb line and the head tube, and you’ll have your reach.

how to measure mountain bike frame

Measurement of Wheelbase of the Mountain Bike Frame for Sizing

The wheelbase of your frame is the distance between the front and rear axles. A frame’s ride quality is largely determined by this, which also varies with frame size.

I found it very simple to measure, although you need to set the fork straight ahead, or you won’t get the correct measurement of your wheelbase.

As well, you can increase accuracy by determining the wheelbase on both sides of the bike and taking the average.

how to measure mountain bike frame

Measurement of Seat and Head tube angle

Seat tube and head tube angles are key factors in determining to handle, with more upright angles leading to more nimble handling.

Clinometer or android application(Clino App) will be useful here.

When you have a straight seat tube, you can measure the seat tube’s angle with your smartphone by lining it up and reading the clinometer app’s number. 

For an accurate reading, make sure your bike is upright and on a horizontal surface. The straight line between the bottom bracket shell and the top of the seat tube is the boundary between the top of the seat tube and the kink in your seat tube, so you need to use a straight edge here to align your phone.

In the case of straight fork legs, the angle of these legs will be the same as the angle of the head tube, so it is best to measure this.

Before doing all these measurements, make sure that your bike is standing vertically.

how to measure mountain bike frame

FAQ's

Is it better to buy a bigger or smaller mountain bike frame?

If your frame is bigger, your wheelbase will be larger (the distance between your wheels is greater). A Mountain bike with a bigger frame gives you the feel of a monster truck instead of a razor that cuts the trail and will make you feel confident while riding.

What is the reason that mountain bike seats are high?

The high seats of mountain bikers allow them to pedal more efficiently and with more power. During pedaling, it will enable you to extend your legs completely.

This helps you keep your knees from getting injured. You will be able to handle your bike better since the center of gravity is elevated.

Does frame size matter in mountain bikes?

The frame size affects your riding comfort and safety. Which frame size suits you best depends on many factors like height and riding style. Moreover, it is also a matter of personal opinion that which frame size suits you best.