Birdy upgrade by integrate Tiagra Crank

I take the original Shimano Tiagra teeth disk out, replace into the Birdy original 56T disc.
There is nothing change but increase the  pedal in efficient. and take some weight out~ and cost not much!

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中文版 各種零件的安裝與爆炸圖 包含零件編號 都可以找到
Shimano Manual Web Site