You would think that the task of measuring a bike would be a very simple one but alas, as with most things in life, we have made it complex and ultimately very confusing. Over the years a few different ways of measuring frames, seat tube length specifically, have evolved. Sadly they have all become commonly used among the many frame manufactures out there, I have even dealt with manufacturers that have utilized 2 different methods of sizing within their range of bicycles. Needless to say this can get very confusing and needs to be kept in mind when purchasing a bike, especially when using an existing bikes measurement as the foundation for a new one. In my opinion buying a new bike in a size based solely on the fact that it is the size of the previous one is a bad way to go about things anyway but can be disastrous if the frames have been measured to different points.
Today’s modern frames are designed very differently from the older, horizontal top tube, bikes of yesterday. The new geometry of compact frames with sloping top tubes are designed to have lots more visible seat post and can vary a lot from design to design so, again, sticking with the size of a previous one for the new and expecting it to fit like the old probably is not going to happen. Always get a new bike fit done on the bicycle style and design you are contemplating.
The drawing below shows the various points that are used when quoting seat tube frame sizes. All have a starting point at the center of the bottom bracket but from there can be measured to the top of the top tube, the top of the seat tube itself or even to an imaginary line which represents the center of the top tube if it were a horizontal design
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There is actually another one that is only used by frame-builders when building custom frames and that is; to the center line of top tube where it will actually be. This is because we are building a bike to an individual’s body measurements and the tube dimensions are already factored in. We also work from center line dimensions when setting the frame jigs.
With all these methods you can see how varied the results can be. Looking at the frame in the picture you can see that there is quite a difference from the shortest measurement to the longest on the same frame. I have always thought that the most important measurement on a production frame nowadays is the effective top tube length. When comparing a favorite frame to a possible new addition this is a good comparison measurement to start with, but again a proper fit is the way to go.