Splain Me Frame Sizing, Please
 

How come when I measure my Klein 55 cm frame, the seat tube, from the center of the BB to the top of the seat tube comes out to 50.5 cm?

Whazzupwitdat?

1) Modern bike frame sizes are most closely related to the effective top tube, not the seat tube.
2) Bike frame sizes are like women's clothing. The number gets you in the general vicinity, but ultimately doesn't mean jack squat.

Compact geometry.

http://www.ebicycles.com/bicycle-too...d-bike/compact

The sticker on the tube says 55 cm. It's a compact frame. This measurement on my frame is 50.5 cm.

What am I missing? (other than 4.5 cm).

4.5 cm

Too late, m x 3.

So I guess we're going with "doesn't mean jack squat."

Actually, I'm trying to figure out the size of my daughter's '08 Trek 1.2 frame which doesn't even have a sticker. We've upgraded everything else on the bike, so I'm guessing the frame is next. I'd like to know what size to start looking for. Thought I'd check a known quantity (my Klein) just to make sure I don't go off half cocked. But, alas, that's where I find myself.

Measure the headtube and seat tube then lookup the geometry for that frame. You should be able to find the exact size given by Trek.

There is no standard for compact frame sizes. For example a Specialized 56 cm is closer to everyone else's 57 or 58. More important are the stack and reach which you can measure on her frame. You can also use head tube length, the effective top tube length, and the seat tube angle to derive the stack and reach for other frames when the mfg doesn't publish those.