Originally Posted by
Lombard
modern bike bikes use the equivalent sizing.... ie a sloping seat tube bike of 58 will fit like a classic bike of 58 cm
see the size chart for cervelo, it links height to size
https://www.cervelo.com/en-US/bikes/caledonia
in fact the size chart for trek that you listed does the same thing , size is equivalent to classic frame size and most direct comparison is height
so to repeat, size is based on seat tube length, either actual lengths in classic designs or effective length in sloping tube designs.....
The effective seat tube length is the distance between the bottom bracket and the point at which a virtual horizontal top tube would intersect the seat tube.