How to choose a correct bicycle frame size

How to choose a correct bicycle frame size

Updated: 16/11/2018.

What is a correct, “real” (i.e. effective) bicycle frame size? How to measure it, tell which one will fit? While most other things can be “tweaked” to fit a rider, if a frame is too small, or too big (especially by more than “one size wrong”), it is very hard “fix” – since it usually requires changing the frame itself. If a new frame has different fittings: headset, bottom bracket shell, seatpost diameter etc, it calls for changing many othre components as well  – costing a lot of time and money.

That is why getting the frame size right is important both when buying a new bicycle, or buying a second hand (used) bicycle.

Important note: this post explains reasons why old sizing methods are unreliable and provides one better, more universal method. Author strongly recommends that after reading this, readers check a newer method – “stack and reach – effective frame size measurement“.

 

1. History

Up to ’80s of the twentieth century, bike frame producers made quite similar geometry frames. Top tube was horizontal, going from almost the top of the seat tube, to the top of the head tube. Here is where most of the conventional wisdom and measurements in selecting a correct frame size for a cyclist come from.

Those methods can still often be heard. E.g: “stand over the bicycle and a fist should fit between the top tube and the rider’s crotch”. And similar. Many things have changed since the “old days”. Bottom brackets are no longer at a standard height, they vary often from bicycle to bicycle. Top tubes are no longer horizontal, they are often at an angle, sometimes even curved, just to make measuring a bit more complicated.  🙂

 

2. Why standard measures are not reliable

Today, frame producers give measurements in two ways:

  1. Inches (or centimetres for road bike frames)
  2. Sizes, like S (small), M, L etc.

Because of the afore mentioned differences in frame design, these measurements are also not very useful. The only thing to rely are exact measures of a particular frame or a manufacturer table. Most frame manufacturers give tables with recommended frame size (out of their product lines) for rider height. These tables are a good starting point. They differ from frame to frame – that is not all the 19″ frames of a particular manufacturers are the same. Depending of the production year, model, designs differ.

For some decent starting reference, this site is also useful:

Bicycle frame size calculator – ebicycles.com

Here is why these measures should never be taken for granted:

 Two differently designed frames of the same relevant sizes, for the same rider size.

Two differently designed frames of the same relevant sizes, for the same rider size.

Dark grey frame will have for about two inches smaller nominal size, than the frame with a horizontal top tube. E.g. if the darker frame is noted as 19″, the other horizontal top tube frame will be 21″. Rider who fits one frame, will be positioned exactly the same on the other frame – the only difference being visual, the amount of seat post coming out of the seat stay.

 

3. Which measures are then relevant?

The only important measures for fitting a bicycles are the points in which a rider comes in contact with the bicycle: pedals, saddle, and handlebars. Contact points. Link for: detailed explanation of contact points and setting up a comfortable bicycle riding position.

Contact points, marked
Contact points, marked

When saddle is set to a correct height (relative to the pedals, i.e. bottom bracket), it is left to measure distance to the handlebars. This is called reach.

Saddle height and reach
Saddle height and reach

Reach can be determined through trial – finding a comfortable position. Arms should be at least slightly bent and, rider should feel comfortable. Not cramped, not stretched.

Depending on type and use of a bicycle, fit will differ. Also, it depends on the desired handlebars height. However, it is important that the fit is comfortable and that arms are slightly bent at the elbows.
Depending on type and use of a bicycle, fit will differ. Also, it depends on the desired handlebars height.
However, it is important that the fit is comfortable and that arms are slightly bent at the elbows.

This is why one of the most important measures is the effective top tube length:

Effective top tube length.
Effective top tube length. Picture tells how it is measured. Horizontally, from the top of the head tube – to the line coming from the seat stay – where they intersect.

Table below gives recommended effective top tube lengths for a rider height, as well as more-or less standard frame size that fits. This should also be taken with a grain of salt. Riders of the same height can have different proportions of legs and torso lengths. Women usually have a bit longer legs (and shorter torso), then men of the same height.

Effective top tube legth table.
Effective top tube legth table.

A 176 cm high rider with a shorter torso might look for a 55 cm effective top tube length, instead of a 56-58 cm as the table suggests.

 

4. How complicated is this, really?

Lots of numbers, data, but it basically all comes down to the last table. Fine tuning can be achieved with different sized stems, seatposts etc, but one should find a frame with roughly fitting effective top tube legth.

Useful link with a video:

Also do read: “Stack and reach – effective size of a bicycle frame“:

Stack and reach - effective bicycle frame size
Stack and reach – effective bicycle frame size

 

Sources: 

Leave a comment