You’re shopping for a new bike and frame size has you stumped. Some bikes have the standard frame design you’re accustomed to. You could simply buy one the same size as your old bike.
But the shop also has a selection of non-conventional road bikes with sloping top tubes or other exotic shapes, and these are offered in only 3 or 4 sizes.
A bike’s size is more important when it has a conventional diamond frame. With a horizontal top tube, stand-over height is more crucial than with a sloping top tube.
When all bikes had quill stems and threaded steerer tubes, seat tubes had to be fairly long in order to get the handlebar high enough in relation to the saddle.
Here’s What’s Important
If you’re getting a standard diamond frame with a horizontal top tube, the size is important, although a centimeter one way or the other shouldn’t matter. The difference can be taken up with the seatpost, by cutting the fork’s threadless steerer tube to the correct length, and by choosing a stem that angles upward, downward or is level.
With sloping-top-tube frames, seat tube length is all but meaningless. It’s the top tube length that’s important.
In fact, some makers of these “compact” frames offer only 3 or 4 general sizes. They fit customers by installing appropriate seatposts and stems.
The best way to make sure you’re getting a frame of the correct size is to have a pro fit. Full-service bike shops can do this for you, as can cycling coaches and some physical therapists.
Many fitters use tried-and-true commercial systems that have proven to work well for most riders. And several new fit systems have become popular in the past few years.
Again, tweaking is accepted and even expected following a fit that puts you into a good, basic position.