Measure A Bike Frame
The usual sizing measurement relates to the seat tube length, given in centimetres for road bikes and inches for mountain bikes. The best way to measure the seat tube is from the centre of the bottom bracket shell to the centre of the top tube.
Unfortunately, with the advent of compact sizing, sizing has become even more difficult. As you can probably tell, I’m not a big fan of compact geometry bikes. The main reason for their introduction by the Giant bicycle corporation was to reduce the number of frame moulds required for the construction of their new composite bikes. Increasing frame stiffness, reducing frame weight and improving fit certainly weren’t the first considerations when the idea of compact frame sizing was conceived. I’ve come to loathe them even more when people don’t know how to quote their effective top tube & seat tube lengths.
If you add a longer or shorter stem, the steering changes noticeably. If you put the seat too far forward or backward your knees and hamstrings are more prone to injury. On the other hand, it’s extremely easy to raise and lower the saddle without affecting the geometry of the bike.
Finding your ideal bike size and position then takes time. Know that buying a frame over the internet is inherently risky, you probably don’t risk buying a pair of $100-200 shoes online without having first tried them on.