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Bike Size Investigation #18   BackToList   Print
Written: 2020.06.08   Review Date:2020.06.08    LastUpdate: 2020.06.09

A detailed analysis of how bike sizes compare.

1. Introduction
2. Seat Tube Size vs Frame Size
3. Seat tube versus Stack Ratio
4. Stack-Reach Limits
5. Marin four Corners

1. Introduction
Almost all bikes are made in different frame sizes. The traditional way to specify frame size for traditional frames was to give the seat tube length. Eg: People would talk about a 24" frame (600mm). or just letters like S=small, M=medium, L=Large, etc. Some manufacturers have "sizing charts" that show the height as compared to

For example, the Trek 520 page has a chart called "Sizing and Fit" which shows the following:

  48 51 54 ... 60 63 ----------------------------------------------------------------------------------
  Rider 158-163cm 162-168 168-174 ... 185-190 190-197
  Height

  Inseam 74-77 76-79 78-82 ... 86-90 89-93

The numbers above each column are their sizes. Eg: Size 63 is for riders 190-197.

The StandardBikes database has two fields: Title and FitSize. The "title" is the nominal number. Eg: Size 60. For FitSize, I reduce it to a single number at the mid point, because the reader is obviously capable of picking the closest size. Therefore my fit sizes are as follows:

  48 51 54 ... 60 63 ----------------------------------------------------------------------------------
  Rider 160 165 171 ... 188 194
  Height

The next column to compare is the specs for a given size, to see how consistent various manufacturers are. I'll use size 60, which is a rider about 188 cm tall. So I look up that size in StandardBikes (Frame #23) and using the "Stack" format report I get the following:


  Title FitSize Stack Reach SRHypo --------------------------------------------
  60 188 625 392 738
The question is: what is it that makes a given frame be designated as a given size? Traditionally when the top tube was level, sizes were determined by the height of the seat tube. But the trek has a sloping seat tube so that won't work.

It is generally stated that the stack and reach alone determine the fit. Bike Radar

Bike Exchange Here is what they say: "The two most fundamental elements of frame geometry are stack and reach. Knowing the bike's stack and reach will allow you to compare between different manufacturers and immediately know if it will fit you, regardless of how it is described or what the seat tube measurement is. "

  So my theory is that we can select similar sized bikes from our database simply by looking for frames that have a similar stack and reach. One way to find those frames is to search for frames that fall within a certain stack range. Eg: All the frames with a size within 1 cm of the Trek 625, such as between 615 and 635.

The answer is a number of frames.


  Blue Norco
  Brodie Romulus
  Marin Four Corners
  Miyata
  Salsa Marrakesh
  Surly LHTrucker 26"
  Surly LHTrucker 700c
  Trek 520

I notice that all are the 60cm size, and have a seat tube of about 600.

3. Seat Tube Size vs Frame Size
In this chapter I will investigate how seat tube size relates to the manufacturers frame size. First of all, there are several ways to specify seat tube size. Center to center, or center to top. The only one that makes sense for comparison is when ST length is effective seat tube length: crank center to intersection with the center line of a virtual horizontal top tube. This prevents useful comparison with bikes like Trek 520 which only give the actual seat tube.

In the Bike Size Lister, search for 60cm, and you will see: Eg:


  Size ST ---------------------------------------
  Brodie Romulus 60cm 600
  Salsa Marrakesh Sora 59cm 620
  Trek 520 60cm 188
A simple report of all the 60cm sizes shows that it is still used for all frames where the top tube is level.

The Brodie Romulus numbers don't seem to make sense. It is a downsloping top tube,yet they give the seat tube as 600mm for XL.

Brodie Romulus: This is a mystery. In the photos, it is clearly a sloped top tube, yet for their 60cm size, the seat tube is a full 600cm. Is it possible they adjust those seat tube numbers to be effective, as if you had a level top tube?

If so, then I need a field called "Effective Seat Tube".

3.5. Seat tube versus Stack Ratio
The stack ratio itself does not give you any lengths. It would seem that the number to compare would be the Hypotenuse of the stack-reach triangle. So when you run that report, and compare it with manufacturers size, you see a range from 698 to 748.

Looking at it the other way, let's search for all the bikes within 20mm of the trek 520 (from 718 to 758). That pulls up a list of 15 sizes. So narrow the range to 725 to 753. That pulls up 10 bikes, and all are the large size which is what I expect. Except the Marin Four corners, it seems to have selected the M size.

3.9. Stack-Reach Limits
(SRLimit). The idea is to select all the frames which fall within a certain stack and reach, plus or minus a certain amount. For example, my Miyata has a stack of 601 and reach of 390, so I want to look at everything within 15 mm of that size.

Miyata (Specs) 601,390,15 9 But all small sizes 54cm to 57cm Miyata measured 618,390,15 8 mostly smaller sizes
  " 618,390,20 15 All sorts of ranges
  618,390,30

Blue Norco 626,410,15 7 Has the Marin size M

 

* Marin is a really tall frame. Their Large size is 646 and the XL is 678! These are far above the 618 I measured on my miyata or the 626 of the Norco.

5. Marin four Corners
Marin Four Corners is an example of a frame that is not a traditional level top tube.

They give only the "effective" top tube. Eg: in their XS size, the