Bike sizes are primarily determined by measuring the length of the seat tube.
Bike sizing is most commonly based on the length of the seat tube, which is a key measurement to determine the appropriate fit for a rider.
According to standard practice, bikes are measured by their seat tube length. This crucial measurement helps cyclists find a bike frame that aligns with their body proportions, specifically their leg length and standover height.
The measurement point for the seat tube length typically originates from the centre of the bottom bracket (BB). The bottom bracket is the component that connects the crankset to the bicycle frame, located at the lower part of the seat tube.
However, it's important to note that the exact termination point of this measurement can differ depending on the bicycle brand. While often measured from the bottom bracket centre, some brands may measure to:
- The top of where the top tube meets the seat tube.
- The centre of the top tube at its intersection with the seat tube.
These variations mean that a '56cm' bike from one manufacturer might have slightly different dimensions compared to a '56cm' bike from another, requiring potential reference to specific brand sizing charts or geometry data. Despite these variations, the seat tube length remains the fundamental dimension used to categorize and market bike sizes.
