Road Cycling - How do you measure a frame?

In all the ads, bikes are described as being 55cm or 58cms or whatever. I'd always assumed this was the length of the cross bar - from centre of stem to centre of seat post, but a few articles and ads hint at something different.

Given that some of the frames I've seen from GT and others have non-traditional geometry - with a steep sloping cross bar, is my assumption correct. I was wondering if perhaps frames are measured from bottom bracket to seatpost instead?

Stew :confused:

Folks - I now see that I should have posted this in the Road cycling forum, and I guess its one of those perenial questions that newbies like me are forever asking.

I'll browse the archive for similar posts, to save you answering it AGAIN.

cheers

Stew

I move the thread to the road cycling forum for ya...

Frame sizes are usualy measured in two ways, c-c and c-t. c-c stands for center to center, that is the measurement from the center of the bottom bracket to the center of the top tube. c-t measures the distance from the center of the bottom bracket to the top of the top tube or seat lug.

Check out this link for a picture: http://www.coloradocyclist.com/bikefit/frame_dimensions.cfm

Like joe said most frames are measured from center of bottom bracket to either the center of the intersection of the top tube and seat tube or to the top of the seat tube collar. While the bicycle industry uses this type of measurement as "standard". Top tube length and "effective" top tube measurment makes more of difference in the way a frame should fit. Now dont get me wrong having a little room between your boys and the top tube is a good thing but also keep in mind "effective" top tube measurement. Recently I learned that some people think "effective" top tube measurement is the distance from your handlebars to your seat I strongly disagree with this and think "effective" top tube measurement is from the center of your headtube and top tube junction measured horizontaly to where that intersects with your seat tube. This way you can get an "effective" top tube measurement on almost all bikes weather they have sloping top tubes or not.

I think most standard (i.e. non-sloping top-tube) frames are measured c-t these days. I assume the reason is that tubing sizes vary considerably. Ti or Al frames tend to have fatter tubes than steel (don't know about carbon), which results in a larger frame for a given c-c measurement. In any case, it's something to keep in mind if you see a c-c measurement...