Measuring a bike's size using the Center-to-Top (C-T) method involves determining the specific vertical distance along the seat tube.
The exact answer, based on the provided reference, is:
Center-to-Top (C-T) is measured from the middle of the bottom bracket, but the upper endpoint is at the top of the seat tube.
Understanding the Center-to-Top (C-T) Measurement
When bike manufacturers specify frame sizes, they often refer to a measurement along the seat tube. Center-to-Top (C-T) is one of the common ways to take this measurement.
Here's a breakdown of the process:
- Starting Point: Locate the bottom bracket. This is the cylindrical part of the frame where the crankset (pedals and crank arms) is attached and rotates. The measurement begins at the middle of the bottom bracket shell. This is often interpreted as the center of the axle bore.
- Endpoint: Identify the seat tube. This is the frame tube that the seatpost slides into. The measurement extends vertically upwards along the seat tube to its very top.
Therefore, the C-T size is the straight line distance from the center of the bottom bracket up to the highest point of the seat tube.
Center-to-Top vs. Center-to-Center
It's important to distinguish Center-to-Top (C-T) from Center-to-Center (C-C), another common frame measurement. The primary difference lies in the upper endpoint:
- Center-to-Top (C-T): From the middle of the bottom bracket to the top of the seat tube.
- Center-to-Center (C-C): From the middle of the bottom bracket to the intersection of the seat tube and the top tube.
This means a frame measured as 54cm C-T will typically be slightly larger than a frame measured as 54cm C-C, as the C-T measurement includes the portion of the seat tube above the top tube junction.
Understanding these specific measurement points helps in accurately comparing frame sizes, although modern frame geometry often relies on additional measurements like effective top tube length and stack/reach for a more complete fit assessment.
