How To Measure Bike Frame Size? A Complete Guide

If buying a new bike or even your first is on your mind, then understanding the measurement that fits your frame is of paramount importance. This ensures that you get the bike that works for your type and fits you perfectly. Understanding the size of the bike’s frame is the most important factor that would assist you in buying or renting a bike that you can ride with comfort and ease.

Every bike manufacturer uses a different measure for the frame sizes. So you need to understand the fundamentals of measuring a bike. Read this blog to know how to size up your mountain bike or road bike frame.

The Key Numbers That Define Your Bike:

  •     The Top tube length
  •       The Seat tube length
  •       The Reach and stack – and why they’re important numbers
  •       The Wheelbase
  •       The Chain stay length
  •     The Front center
  •       The Seat tube and head tube angles
  •       The Bottom bracket drop
  •     The Bottom bracket height

We would be discussing each to understand how to measure the bike:

Element Needed to Measure a Bike Frame:

  1.   A measuring tape
  2.   Clinometer to measure angles or any relevant smart phone app
  3.   Long spirit level or a straight piece of wood and clinometer app
  4.   One Plumb line

All bikes whether mountain or road are measured in metric units. Very few manufactures though still size mountain bikes in inches. Brands like moots even use the combination of both.

We would be using metric units to keep uniformity of measuring. You can find the geometry charts for every current frameset on every bike manufacturer’s site. If your bike’s size chart is there you can take a copy of the same. It would be more precise than our measurements and would also act as a reference.

How To Measure Top Tube Length?

How To Measure Top Tube Length

There was a time when all the bikes had horizontal top tubes, but now many new bikes are designed with slope tubes.

When you see the geometry table of a bike it would definitely have a real top tube length. But if you want to take the accurate measure you must measure the horizontal top tube length. This is known as the virtual top tube or effective top tube length and various geo charts.

That is the horizontal distance between the seat post’s central line and central line of the head tube. To make sure that the measure is horizontal, you should use your clinometer app or spirit level so you can measure it correctly.

 The top tube length is the measure that many bike manufactures take to size road bikes. Although this does not hold true for mountain bikes, as in this the frame size is S, L, and M etc. This type of system is used in road bikes too. The S, S/M, M/L and L is used in Merida’s bikes.

The size measurement differs with each bike as it is up to their own interpretation. The top tube of Ridley’s size S frame is around 54cms, which is medium size in several other brands’ frames.

The virtual top tube length is also not uniformly measured by all brands either. In Colnago the horizontal distance is measured from head tube to the vertical projected up on the seat tube. This leaves the count for further backward position of the seat post, so its numbers come out small to other makers. A 50s Colnago is equal to one 54cm top tube.

How To Measure Seat Tube Length?

How To Measure Seat Tube Length

The straight line distance between top of the seat tube and center of the bottom bracket is known as seat tube.

It may sound easier but in reality it is trickier. In some bikes like the Trek Madone there is substantial extension in the seat tube which is above the top tube junction. In others there is a seat mast, so it makes it really hard to compare the alternative’s dimensions.

Other than this the mountain bikes in particular in the seat tube have a kink. So in this case you don’t follow the line of the tube as it would be longer.

For measuring, with the bottom bracket center and the top of the seat tube you need to line up your straight edge, only be sure to follow the right line.

How To Measure The Stack And Reach?

How To Measure The Stack And Reach

Now we know that to compare the frames seat tube lengths and top tube are a bit of a minefield. To get more consistency most bike makers now show case stack values and show reach for the bikes. The reach and stack are very crucial and important.

The crux is that the horizontal distance between the two is reach. To measure this you would need your spirit level.

A plumb line needs to be attached to the end of the level. When using the blue tack, you should make sure the blob on the end of the string is symmetric and the string hangs down perfectly straight or your measure can be off.

Now you should align the top level of the edge to the centerline of the head tube’s top. Now you should move the level forwards and back till the plumb line interacts with the bottom bracket’s center. After this you need to measure the total distance between the head tube and top of the plumb line and you would get your reach.

One more option is to put the bike against a wall, now measure the distance to the bottom racket and top of the head tube and minus one figure from the other. One thing to keep in mind is that the measurement should be horizontal.

The stack is the vertical distance of top distance from the bottom bracket. So if you have measured your reach, you can easily measure your stack, through the plumb line.

One more method to know is to measure the vertical distance from the head tube to the ground and then measure the bottom bracket’s height from earth and minus this.

You must keep in mind that both stack and reach are quite hard to capture. So you may need help when using the plumb line method and you should always repeat to check the accuracy and consistency.

How To Measure The Wheelbase?

How To Measure The Wheelbase

The frame’s wheelbase is the total distance between rear axles and front axles. It is one major aspect of measuring ride quality of the frame and this would be different for different frames.

The wheelbase is pretty easy to measure but you need to keep the fork straight ahead otherwise it would spoil the measurement.

As the stack and reach here also it is important to repeat the measure many times to get the accurate figures. When you measure the bike from both sides of the bike the accuracy also increases. You should take the average of the measures as it would compensate for the fork if not straight.

How To Measure The Length Of The Chain Stay?

How To Measure The Length Of The Chain Stay

The chainstay length is among the two components that contribute to your frame’s handling characteristic and make up your wheel base. The frame which has a shorter chainstay would feel livelier than the longer ones.

The chainstay length can be defined as the line between center of the rear dropout and center of the bottom bracket axle. This makes it easier to measure through a ruler.

How To Measure The Front Center?

How To Measure The Front Center

The next component of the wheelbase is the front center. That is equal to the length of the chainstay but can be measured from front dropout to the axle.

This affects the toe overlap with the front wheel and also affects handling. This point is often omitted by the bike manufacturers, but BMC as an exception shows this in its geometry charts.

You should remember that the wheelbase is not the total of front center and chain stay length as none of these is calculated horizontally.

How To Measure Head Tube Angles And Seat Tube?

How To Measure Head Tube Angles And Seat Tube

The head tube angles and seat tube are the most prominent factors that determine handling. The more upright the tube angles the more nimble would be handling. Here having a clinometer app would come handy.

By lining up your smartphone and reading the number on the clinometer app you can measure the seat tube angle (if you got a straight seat tube). For this you should make sure your bike is standing on a horizontal surface and vertical to get perfect reading.

If your seat tube has kink then to follow the line between the top of the shell tube and bottom bracket shell you would need a straight edge and then need to line up the phone with it.

The angle of the centerline head tube would not be the same as the front of the head tube as most of the new bikes have tapered head tubes.

You can get closer to the angle of the central line by holding your phone to the same or this can be done by using a straightedge to be in line with centers of the bottom and top of the head tube.

If you are having straight fork legs that do not have an angle on their crown. The head tube angle would be similar to the angles of the legs so that you can measure this one more time. Here also the important point is that your bike should be vertical.

The head tube angle can also be measured by lining up the steerer extension with a clinometer above the head tube.  

How To Measure The Bottom Bracket Drop?

How To Measure The Bottom Bracket Drop

The difference between the centerline of the crank axle and height of the wheel axles is known as the bottom bracket drop.

This can be measured by height of the bottom bracket and height of the rear axle and then subtracting each other. It is one of the key measurements which is quoted by the bike manufactures on their respective geometric charts.

How To Measure The Bottom Bracket Height?

How To Measure The Bottom Bracket Height

The distance between the center of the bottom bracket shell and the distance from the ground is the bottom bracket height. It’s comparatively easy to measure, the only thing to keep in mind is that your bike should be kept straight upright, only then the readings would be precise.

In contrast to the bottom bracket drop this would be slightly influenced by the tires too, so the tires should be inflated to the accurate running pressure.

So now you know everything to measure the bike correctly. Now you know perfectly to size up your frame. You should always keep your numbers safe. As losing them would mean that you have to repeat the whole process again, which I am sure you would not like.

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