shortened my cranks and raised the seat, now bouncing at high cadence
 

I've been using the same bicycle and fit for about 4 years. After switching from 172.5 to 170 cranks, I raised my seat height by a little under a centimeter a couple of weeks ago and, 200 miles later, I'm still bouncing at RPMs above 120 (which didn't happen before). Tried single leg pedaling and focusing on forward-backward instead of up-down on the pedal stroke; these have helped slightly, but I still bounce. My hips don't rock at lower cadences. My legs feel like they're extending more fully than with my previous fit, but definitely don't feel like they're over-extending.

Lowered seat by 5 mm this morning, rode for 25 mi, and my stroke was smoother. Legs felt under-extended, though.

Do I need to spend more time on re-learning my stroke, or did I set the seat too high after changing crank length?

Why did you change crank length?

You shortened the cranks by 2.5mm, but raised the seat about a cm (10mm) effectively raising the seat height 7.5mm. If it was perfect before, then it's too high now.

bored--
- threads on this forum and other bf.net forums discussing crank length for various rider heights--i'm 5'10" with shorter legs than average for my height (30" pants inseam)
- online formulas to calculate crank length, which routinely put me in the 160s?!
- wanted to see how shorter cranks felt

turkey--
yes, i realize that. wanted to increase height more than the decrease in crank length to see if my legs would feel more fully extended at the knee, which they admittedly do. the knee extension feels good, just not the bouncing.

but i see what you're getting at...fixing something that ain't broke? :P

I'm not sure if you can feel any effects from changing the crank length by 2.5 mm. When I shortened my crank by 15 mm, my cadence went up 15 rpm. Your change by itself would be almost undetectable. Things you're seeing are principally the effect of the higher seat.

"fully extended" sounds very much like "over extended".

+1

When I change cranks (I go from 175 to 170 and back again it seems) I'll change my saddle height by the crank length difference. 175 to 170, raise the saddle 5 mm.

There's debate on if you should change saddle by the whole difference (5mm in my case) or half (2.5mm). I find that my leg extension needs to be constant, and how far I bring my foot up is less important. So I change my saddle height by the full crank length difference (5mm).

Incidentally it's easier to go to a shorter crank length than a longer one. Shorter crank lengths decrease range of motion. Increasing your range of motion is more difficult so a longer crank arm takes some adjusting.

Hope this helps.