Okay, if the seat goes up 10, the bars should go up 10? But if one wants to get more aero should they be left alone?
Maybe. Moving the bar up keeps the relationship between the seat and the pads constant, but opens up the distance between your leg at the top of the stroke and your torso by 2X the difference in crank lengths. Leaving it alone makes that distance only 1/2 that amount…or, equal to the difference.
Yes…one can use those differences to also lower the torso relative to the ground in order to attempt to lower CdA.
Now when the seat goes up 10, the seat effectively goes back some also so does one need to adjust the net seat
back 10 even though it has moved some already.
Yes, it does…unless you have a vertical seatpost, like on a P3, P5, etc. But yeah, due to the seat tube angle, adjusting the seat post up will typically also move it rearward as a function of the cosine of the seatpost angle. For example, with a 75d seat angle, the seat will move rearward relative to the BB ~.25mm for every mm the seatpost is extended. That’s why I said it’s almost about the same…for small variations in crank length I wouldn’t worry too much. But, for larger variations, it’s something to consider.
And you are saying 10 up on the seat, then 10 back on the aerobars?
Let’s use your example of going from 200mm cranks down to 175mm. Here’s how I would go about it if I was interested in JUST changing the crank length to investigate JUST that effect:
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Document the current location of the saddle and the pads relative to the BB, along with the distance from the part of the saddle I sit on to the pedal spindle with the pedal the furthest distance away from the saddle (straight line measurement, i.e. “saddle distance, or height”.)
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Remove the 200mm cranks and install the 175mm cranks.
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Since the difference in crank lengths is 25mm, I know that in the end I’ll want to have the saddle 25mm further back horizontally on the bike relative to the bottom bracket. This will place my lower leg in the same orientation to the crank when it is in the highest torque part of the pedal stroke (i.e. ~ horizontal). With a bike with a vertical seat post, that’s easy…I just move the saddle rearward on the rails 25mm. For bikes with an angled seatpost, it’s only slightly more involved (see below).
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For a bike with a vertical seatpost, I would then extend the seatpost until the saddle to pedal distance (“saddle distance”) matches what was measured originally. For a bike with an angled seatpost, one can either do the trigonometry, or probably more easily, just iterate on extending the seatpost and moving the saddle until you end up where the “saddle distance” is correct AND the seat is 25mm further aft horizontally.
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Raise the bars vertically 25mm and move them rearward 25mm horizontally as well. This maintains the relationships between the pads, the saddle, and your lower leg in the power stroke portion of the pedal cycle. Boom. Done.
OK…now that you’ve established that ONLY the crank length has been changed in your fit, you can evaluate that effect in isolation. I personally find that even with differences up to 7.5 to 10mm, when following the above protocol, the different crank lengths are almost imperceptible from a “feel” standpoint.
Other fit options that moving to a shorter crank length allow (i.e. changing saddle-bar drop, etc.) can now be investigated separately.
