The effective top tube length (ETT) is an important measurement for every serious cyclist when choosing their bikes.
To understand what an ETT is, we must first know the definition of a seat tube angle. The seat tube angle is the angle between the seat post and the horizontal (please refer to diagram for reference). Seat tube angles range from about 70-75 degrees. A steeper (bigger) angle moves the saddle forward while a shallower (smaller) angle moves it back. The ETT is essentially the length the top tube would be if it ran horizontally from the top of the head tube to the seat post.
While a difference of 5 degrees may seem insignificant, this difference will greatly impact how a cyclist performs or fits on the bike. For instance, if we compare 2 bikes with identical top tube length of 52cm, the bike with a shallower seat tube angle will have the saddle further away from the handle bar as compared to the bike with a steeper seat tube angle. This will mean that the same cyclist will have to crouch and bend more on the former bike, making him/her adopt a more aggressive position. However, if the cyclist has tight hamstrings or poor flexibility in the spine, he/she will end up having more discomfort during a long ride. This explains why buying road bikes based on top tube length is highly inaccurate and one can end up buying a road bike with the wrong size.
When fitting a cyclist, the ETT is one of the key measurements we need to consider. Along with measurements of the cyclist’s upper body flexibility, these 2 values will determine how we can position the cyclist in the most comfortable position to optimise his/her performance on the bike.
The ETT length will also determine how the cyclist distributes his/her weight on the bike. A short ETT will cause the bulk of a cyclist’s weight to be transferred to the front wheel while a longer ETT will encourage the cyclist to shift more weight to the back of the saddle. The significance of weight distribution of a cyclist is that of safety and ease of handling. Too much weight towards the front wheel is dangerous especially when riding down a slope, while too much weight towards the back wheel decreases efficiency of riding and makes climbing more difficult.
Good bike fitters will always have to take these factors into consideration. Contrary to popular belief, bike fitting is not always about performance. The safety of the cyclist should always be the first priority for bike fitting. Getting a bike with the correct size and dimensions enables the cyclist to handle the bike with ease and therefore feel safe to ride on.
It is therefore crucial for cyclists who are shopping for their next bike to know the relevance of buying a bike with the same ETT as their previous bike (provided a proper fit was already done). Otherwise, they will be in for a ‘rough’ ride.
Besides ETT, there are many other aspects bike fitters consider during the bike fitting session. Physical attributes such as hamstring flexibility, mid and lower back stiffness, leg length differences as well as physical fitness level play integral roles in performance and comfort.
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