I get this might be a dumb question but I was looking at some geometry charts for bikes and I cannot figure out what a size 46 (or any other) bike means. I thought it was effective top tube or maybe seat tube but looking at a geometry chart for a Cannondale there is only 1 measurement that matches to the size.
The size 54 has a 54cm effective top tube but the 46 has a 51cm ETT and the 61 has a 59.4cm ETT.
Idk if it’s clearer with older bikes and the geometry of newer bikes is just different, but I don’t understand it.
by Expert-Tension7327
4 Comments
Great timing for this question because [Bike Farmer did a video on it.](https://youtu.be/1oV4E1EPX18?si=SiTxnNs2OKpbCeRC)
Historically, that dimension was Seat Tube Length, but with sloping top tubes and more non-conventional geometries nowadays, you’ll find that that measurement is no longer representative.
Manufacturers keep that sizing dimension because it approximates legacy frame geometries.
Once you do a professional bike fitting you will find what measurements suit you. Stack and reach will become more important when shopping. You can also determine the kind of riding the bike will be best for as well as handling characteristics from looking at things like head tube and seat tube angle, wheelbase, stand over, bottom bracket height and fork rake.
In that diagram the seat tube length is less than the frame size since the top tube is not horizontal. 46 probably means the old way of measuring the size of a bike which was the distance from the bottom bracket to the top of the top tube. With sloping top tubes though that measurement probably means something like the distance from the bottom bracket to a theoretical top tube which is horizontal. For modern frames standover height and reach are probably more relevant and the frame size is just given to give a rough idea of what a similar frame with a horizontal top tube would be like.