Mountain bike frames are primarily measured from the centre of the bottom bracket (BB) to the top of the seat tube.
While this is a common method, understanding how it applies to mountain bikes is important due to differences compared to other bike types like road bikes.
Standard Measurement
The most traditional way to measure a mountain bike frame size involves a specific point-to-point distance:
- From the centre of the bottom bracket: This is the point where the crankset pivots.
- To the top of the seat tube: This is the upper end of the tube where the seat post is inserted.
This measurement gives a basic indication of the frame's size, often correlating to historical sizing charts where frames were more square.
Why Mountain Bikes Differ
Mountain bike design often prioritizes agility and rider comfort on varied terrain. A key difference highlighted in the reference is the design of the top tube:
- MTB top tubes often slope down.
- They typically join the seat tube lower than on many traditional road bikes.
This sloping design isn't just aesthetic; it serves a practical purpose.
Standover Height
The sloping top tube directly contributes to more standover height. Standover height refers to the distance between the top tube and the ground when you are straddling the bike with both feet flat on the floor. More standover height is crucial for safety and maneuverability on technical trails, allowing a rider to dismount quickly without hitting the top tube.
Practical Sizing Advice
Because of the variations in frame design, especially the sloping top tube, relying solely on the traditional BB-to-seat tube measurement can sometimes be misleading or insufficient for determining the best fit.
According to the reference, it's highly recommended to:
- Take your inside leg measurement.
- Compare this measurement to the manufacturer's model specifications.
Manufacturers often provide recommended inside leg ranges for their specific frame sizes, taking the bike's overall geometry and standover height into account.
Summary of MTB Frame Measurement Key Points
- Primary Method: Centre of the bottom bracket to the top of the seat tube.
- Design Note: MTB top tubes often slope down for increased standover height.
- Practical Fit: Always compare your inside leg measurement to the manufacturer's recommended sizing.
While the BB-to-seat tube measurement is a foundational element, understanding the bike's overall geometry and matching it to your body measurements is key to finding the perfect fit for a mountain bike.
