BB-Seat Diff.

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Air Drag

An important value for the efficiency of the pedalling force and the air drag.

There is an article made by the Dutchman Bert Hoge. He measured air drag in relation to different factors. One of these factors was BB-Seat Difference. The ideal is that the feet move inside the "forward shadow" of the rider. If you have the BB-Seat Difference too small then the feet will protrude below; too high then the feet will protrude above the riders body. Bert Hoge mentions a ideal of +15cm to +25cms for a seat reclining of 21degrees (extremely flat). Most have the seat a little more up (e.g. 30degrees), and thus the upper bound of +25cms could be a little larger.

Design Choices

Apart from air drag, another reason for not going too high is comfort: with the feet so high your toes will be proned to become numb.

On the other hand, choosing a low BB is not always an option for large wheel pythons, because choosing a low BB will shorten the distance between the front axle and the BB. This might get you into trouble with the chainline.

References

 http://www.ligfiets.net/redactie/meetligfietsengels.pdf