04jtb

Don't know if this is the right place to post this or not, but I was wondering, which of the two lines in the picture below would be the c-t frame measurement?

Thanks in advance

JT

gfrance

^^ green line.

Scooper

The green one.

Ti-Fan

ditto

johnny99

I disagree. C-T is the blue line - to the top of the top tube, not to the top of the seat post collar.

However:
1. Some brands (like Trek) are measured to the top of the seat post collar (but this is not C-T)

2. Bikes with sloping top tubes (like this one appears to be) are also specified by their "virtual size" which is the distance along the seat post from the BB to an imaginary horizontal line from the front of the top tube to the seat post. The virtual size is the what your C-T would be if the top tube were horizontal.

mtnbiker310

Technically, from the past, I think johnny99 is correct, but I've been out measuring and test riding bikes recently, and all the ones I measured leads me to believe that the manufacturers have changed and are going by the green line now.

Scooper

Interesting. Historically, many manufacturers (Schwinn and Trek, for example) have used the term "C-T" to mean the linear dimension from the center of the crank spindle to the top of the seat tube, and the frame size was that dimension. Schwinn specifically stated in its Paramount catalogs that "Frame size is measured by the length of the seat mast from the center of the crank hanger to the top of the seat mast." The term "C-C" meant the linear dimension from the center of the crank spindle to the center of the top tube. For bikes with sloping top tubes, the effective seat tube length is the distance between the center of the crank spindle and the point at which the centerline of a virtual horizontal top tube would intersect the centerline of the seat tube.

Lately, though, it has become more complicated.

From Sheldon Brown's site:

- The old standard system was to measure from the center of the bottom bracket to the very top of the seat tube.

- Some manufacturers have decided that this is too easy, so now many bikes are measured instead to the intersection of the centerline of the top tube with the centerline of the seat tube.

- Some other bikes that have seat tubes that protrude farther than normal above the top tube measure as if they were measuring to the to the top of a seat tube with normal protrusion.

- Some bikes are measured to the top edge of the top tube, even though the seat tube protrudes higher up.

- Some bikes with slanting top tubes are measured as if there were a level top tube, they use the length that the seat tube would be if it was as high as the head tube.

Bottom line: seat tube "frame size" numbers are nearly meaningless unless you know how they are measured!

(Thanks, Sheldon.)

04jtb

Thanks for all of the replies

Sawtooth

Dispite the controversy, If I were ordering a frame online or something and I wanted the seller to give me a measurement, the only line that would give me meaningful information would be the blue line. I don't care about the absolute length of the seat tube. I only care about the length of that tube to where the top tube intersects. That gives me a much better understanding of frame size.

While we are at it, I strongly prefer top tube measurements to seat tube measurements.

merlinextraligh

Guess that means I ride a 77cm frame.

johnny99

Can you provide a reference where these companies are using the term "C-T" for the top-of-seat-tube measurement?

Scooper

I'll work on it. In the meantime, a very cursory Google search gave me these hits:

Surly:

Frame size generally refers to frame height, the length of the seat tube from the bottom bracket (crank bearing) to the top tube. There is a problem with this. That problem is that it tells you only a little about how the bike will fit you.

Long ago, bicycles were all shaped pretty much the same. That’s not true anymore. Also, frames are measured differently. Some companies measure from the center of the bottom bracket to the center of the top tube/seat tube junction. (also known as C-C) That’s a pretty good way of doing things, but doesn’t really take into account differences in tubing diameter which would make a steel 54cm bike “shorter” than an aluminum 54cm since aluminum tubes tend to larger in diameter.

Other frame manufacturers measure from the center of the bottom bracket to the top of the seat tube – like the point where you actually insert the seat post. (Call it C-T) This baffles us because if two frames had the exact same geometries and tube lengths, but one had more material sticking up above the top tube, the frame would be “larger” even though it would ride the same as its “smaller” compadre. Weird.

We measure our frames from the center of the bottom bracket to the top of the seat tube/top tube junction. (Uh oh, call this one C-T too) This gives you at least some small idea of how big the frame will seem, but in no way gives you the whole picture.

and

Wikipedia:

Frame size was traditionally measured from the center of the bottom bracket to the top of the seat tube. Typical "medium" sizes are 21 or 23 inches (approximately 53 or 58 cm) for a European men's racing bicycle or 18.5 inches (about 46 cm) for a men's mountain bicycle. The wider range of frame geometries that are now made have given rise to different ways of measuring frame size; see the discussion by Sheldon Brown.

and

Colorado Cyclist

Frame size refers to the length of the frame's seat tube. Pro frames are measured along the seat tube in one of two ways, center-to-top (C-T) or center-to-center (C-C). C-T measures the distance from the center of the bottom bracket to the top of the top tube or seat lug (See image below).

The image referred to shows C-T to the top of the seat tube:



and

Share the Road.org



I began my post with "Historically", and went on to say that seat tube frame size numbers are nearly meaningless unless you know how they're measured. Since the seat tube length as been traditionally used to indicate frame size, there are numerous instances of "C-T" being used to describe the seat tube length from the center of the crank to the top of the seat tube in the literature.

Scooper

I found some Schwinn and Trek geometry diagrams that clearly show the size is the seat tube length, and the "SIZE" dimension in the Schwinn diagram and the "A" dimension in the Trek diagram are both clearly from the center of the crank to the top of the seat tube. Since the seat tube length has historically almost universally been the frame size, the only remaining question is whether "C-T" in referring to the "frame size" means the top of the top tube or the top of the seat tube. Most (but admittedly not all) geometry drawings show the seat tube length to be to the top of the seat tube, so it's logical to draw the conclusion that "C-T" means from the center of the crank to the top of the seat tube, and "C-C" means from the center of the crank to the point on the seat tube where the centerline of the top tube intersects it.

Schwinn:



Trek:

Steel Man

Colnago also uses the c-t measure as the size of the frame