Standard frame measurement terms
ST: Seat Tube: Measured from the center of the bottom bracket along the center to a point near the seat post binder. The exact location of this point may vary between manufacturers depending on the frame design. The main purpose of this would be as a reference point and, if located at the seat binder, would indicate whether your favorite seat post would be clamped above the minimum insertion line.
TT: Top Tube: Measured from the frame size point horizontally to the steering tube axis.
STA: Seat Tube Angle: The angle of the line passing through the bottom bracket and frame size point relative to the horizontal.
SB: Setback: (used as a more accurate way to measure seat tube angle) the horizontal distance from a vertical line passing through the bottom bracket to the frame size point.
HTA: Head Tube Angle: The angle of the steering axis relative to horizontal.
HT: Head Tube length: The length of the head tube (not including headset components)
How to compare frame geometry:
1: Measure the seat tube from the center of the bottom bracket to the point where you want the seatpost to exit the seat tube. Subtract 1 cm. That is the seat tube length and reference point for the next two measurements.
2: With the bike on a level floor, measure horizontally from the seat tube length point to the center of the steering axis. That is the effective top tube length.
3: If you don’t have the published seat tube angle for your frame and you don’t have an accurate protractor or “Angle Finder”, you can measure setback. To do this, hang a plumb bob from the top tube so it passes through the center of the bottom bracket axis. Put a piece of tape on the top tube to mark the spot. Measure horizontally from this line back to the frame size point. That is the frame setback. Seat tube angle can be calculated from setback and seat tube length or you can simply compare setback directly.
4: Measure the overall length of the head tube, not including any headset parts. Measure the spacers between the headset and the bottom of the stem that you will be using. On a new frame, increase the head tube length by the total thickness of spacers you wish to remove.
5: Measure from the floor to the top of the top tube at the point where you would stand over the bike. That is the standover height for that bike.