This post compares the seat tube C-T and seat tube C-C measurements from the perspective of frame sizing.
Seat tube C-T
The distance from the center of the bottom bracket to the top of the seat tube. (C-T = Center to Top)
Seat tube C-C
The distance from the center of the bottom bracket to the middle of the junction where the seat tube meets the top tube. (C-C = Center to Center)
Why Is The Seat Tube C-T Is More Popular Now?
The seat tube C-T measurement is more widely used for two reasons:
- It measures the entire seat tube and tells the user how much seat post will be needed to satisfy his inseam requirement.
(In cycling, the inseam is the distance from the rider’s foot soles to the crotch. The measurement is critical for acquiring a proper bike fit.)
- The vast majority of modern bikes (road, MTB, gravel..etc.) have a sloping top tube (a top tube that isn’t horizontal) that attaches lower on the seat tube.
This frame style leaves a portion of the seat tube sticking above the top tube and thus using the C-C measurement wouldn’t provide the most accurate assessment of the seat tube’s length.
When Is The Seat Tube C-C Useful?
- Horizontal Top Tube
If the top tube is horizontal as in the case of classic road bikes, then the C-C measurement is close to the same as the C-T. However, there’s still a slight difference because the C-C doesn’t go all the way to the top of the seat tube.
The discrepancy depends on the tubing’s circumference. If the frame is made of still and therefore uses thinner tubes thanks to the high-tensile strength of the material, the difference between C-T and C-C in this case could be as little as 0.5cm.
If the frame relies on aluminum and therefore has thicker tubes to make up for the lack of tensile strength, the difference could be slightly bigger.
That said, the vast majority of aluminum bikes have sloping top tubes pushing the gap between C-C and C-T further away.
- Frame Building
If you know both the C-T and C-C, the C-C could be useful as an indication of where the top tube meets the seat tube.
What Do I Do Once I Have The Measurements?
The primary purpose of those measurements is to size a bike accordingly.
Moreover, the length of the seat tube is the measurement used to classify different frame sizes. For instance, a producer could label all of his 56cm frames as medium.
A classic way to size a frame is to use the inseam and the seat tube length along with a formula from Greg LeMond, an American professional cyclist of the 1980s.
The formula is:
Seatube C-C = Inseam Length x 0.67
So, if your inseam is 84cm, the output is C-C = 84cm x 0.67 = 56.28cm.
In other words, you will need a frame that has a seat tube C-C of about 56cm.
Realistically, however, the manufacturer may not list the C-C value.
If you need the C-T number, you can guestimate it by adding about 1cm to the formula’s output. In our case, we would need a frame that has a C-T of about 57-58cm.
That being said, even the C-C value is good enough as a guideline. 1cm of seat tube length does not make or break a frame in the vast majority of cases, especially in the world of recreational cycling.
You can always use the seat post to compensate for insufficient length. This is what the pros do after all.