BILL BOSTON CYCLES

The top tube length is defined as the horizontal distance from the centerline of the head tube to the centerline of the seat tube. This distance is the actual measurement of the top tube on bicycles that have a horizontal top tube. On bicycles that have a sloping top tube (as on Mountain Bikes), this is a theoretical number. In this instance, the effective top tube length would be measured horizontally from the center of the head tube at the height of the top tube intersection to the center of the seat post. While the actual length of the top tube will be different, it is the effective length that influences fit.

The top tube length is the most difficult to calculate and most important dimension on the bicycle. The top tube length, in conjunction with the handlebar stem length determines the hand position on the bicycle. Once we have established the hip position and the hand position, the top tube must be long enough to allow for good breathing and short enough to prevent excessive bending of the lower back. Additionally, the shoulder angle should never exceed 90 degrees. If the shoulder angle exceeds 90 degrees, the upper body weight is not supported properly by the skeletal structure. This situation leads to shoulder and upper back pain and fatigue. Comfort is the key here. If it is not comfortable just sitting on the bike for a short period of time, you will not be efficient while riding.

      Things to watch out for!

As I mentioned in seat tube angle, don't just compare top tube lengths from bike to bike. Always look at the effective top tube length and the seat tube angle. If two bikes both have the same top tube length and one has a significantly steeper seat tube angle, there is a good chance that the one with the steeper seat tube angle will have a longer effective top tube length once the saddle is established in the right position.

Lower Limb Inequality

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Last modified: July 09, 2014