It depends on the manufacturer.

The most common method of measuring is c to c (center to center) and c to t (center to top) Things get kinda weird with sloping top tubes though as some people measure the actual seat tube while others draw an imaginary line where the seatube would have been and measure that intersection...


translated, check with the manufacturer

The convention for measuring seat tube length on frames for ALL manufacturers is exactly the same: Do it as weirdly and cryptically as possible so no one knows what the measurement means. Then, after two years with the same geometry when people are starting to figure out 51 cm. actually means 53.4cm., change the names of the sizes to "Small, Medium, Medium-Long, Small-Large-Short,Large-Tall, etc." Then publish your geometry on your website and in your catalog, but be sure to make certain they don't match. Then, finally, when someone calls your company on the phone to find out the center to top seat tube measurement on a 54cm frame- make sure your voice mail is on and you're out to lunch. But other than that, it's pretty straight forward. Bottom line- you have to measure it yourself if you want to know the dimensions.

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Tom Demerly
The Tri Shop.com

Tom,

You left out the part about throwing in a few gratuitous English measurements rather than metric.

it all went to hell a long time ago, when we got off the standard lugged steel tubing. with oversized tubing center to center became less useful, and center to top became more important, because what you REALLY needed to know was where the top of the head tube was (assuming a level top tube).

then, with compact geometry and dropped top tubes (on tri bikes) forget it. when we went to dropped top tubes we adopted our own nomenclature, which basically was "center to air." we just named the bike the size you ought to ride. a 59cm QR measured 59cm no where on the bike. but it worked amazingly well. if we'd called it a 52cm bike, which it was, center to center, we'd have had no end of explaining to do.

all you need to know for fit is:

1. seat angle
2. effective to tube
3. head tube length
4. wheelsize

technically, BB height, or drop, is necessary, but that's minor. there are certainly other measures that matter to how the bike handles, but the above four are all you need to know when it comes to fit. the seat tube height is of no real value.

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Dan Empfield
aka Slowman