lyrrad wrote:
Crank length affects far more than just the knee to chest angle.
A shorter crank whilst opening your hip angle at the top of the stroke, closes your knee angle unless you move your seat back.
This is approximately the same as simply moving your seat forward to open the angle, but different.
I run two crank lengths on my two road bikes, 5mm different, yet my seat height is not 5mm higher to compensate.
The setback is slightly different as is the stem reach and height.
You need to consider the whole system when changing lengths, not just the simplistic shorter crank/lift the seat.
I am not saying you are wrong, just trying to work it through in my head. And on paper. Maybe I am saying you are wrong but just not sure I am right. One of the two. :)
I understand moving the seat forward to open the hip angle, you increase height accordingly, otherwise the 'circle' around the BB, which comprises the bike fit, becomes squashed for want of a better word. So I get the example you mentioned.
But in the case of a shorter crank, and a raised seat, at the top of the pedal stroke you would be 2 x the difference in crank length away from the pedal (if crank diff was 10mm this would mean 20mm away), with both your bum and your foot in the same vertical plane as before.
With the vertical plane of your saddle being the same horizontal distance away from the vertical plane of your pedal but now with a greater reach to the top of the pedal stroke if anything your knee angle would be more open.
To decrease the knee angle I thought you have to either decrease the seat vertical distance in comparison to the top of the pedal stroke - which we are not doing - or you need to decrease the horizontal distance from the seat to the top of the pedal stroke, which we are not doing.
Where am I going wrong?
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