Assuming my aero position is pretty dialed in with a 172.5 crank length, how does changing to a 165 crank length affect my position?
Seat is going to have to go higher… this moves the nose back and increases your drop unless you raise up the front end. Generally, shorter cranks allow you to have a more open hip angle at the top.
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Seat would have to move forward to accommodate. Think about it like you are rotating your entire position clockwise around your bottom bracket.
Generally, shorter cranks allow you to have a more open hip angle at the top.
How is it opening the hip angle if the seat is moving up? I’m assuming here that the bars stay in the same place and that the drop has increased.
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The seat is being raised based on the leg angles at the 6 o’clock position. At the 12 o’clock position, since the crank arm is shorter, the angles of the leg are more open compared to a longer arm crank, and so is the hip angle. It’s trigonometry, really. Think of it as a system made of triangles.
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Ahh, I understand where he was coming from. I was confused since I normally see hip angle being defined in Empfields articles as when the crank arm is down near 6 o’clock, not near 12 (http://www.slowtwitch.com/Bike_Fit/F.I.S.T._Tri_bike_fit_system/Hip_angle_24.html). What you said makes sense for the angle at the top of the stroke though.