Tony_G

Bike (2018 Cannondale Synapse disc 105 SE) has about 42,000 miles on it. Suddenly it would not downshift to small chain ring. Shift cable is good. Small lever that was previously inside the shifter now sticks out and moves with no resistance. Maybe just a broken spring?

This shifter has cable brakes, not hydraulic. Reputable e-bay sellers have the R7000 shifter pair for $210.

Can the broken shifter be disassembled and repaired, or do I need to replace it?


Broken part circled in red.

Iride01 

They don't make them to be taken apart and rebuilt anymore. So a lot of that will be all on you and your ability to figure it out and do what's needed. Perhaps something has gotten in there and jammed the return spring or that little arm itself. So maybe jiggling it while blowing compressed air at that area might free it up. If the spring is broken, then it's probably a spring that won't easily be found anywhere for sale.

You might see if anyone has made a youTube video about it. I don't know how much more you'll be able to see if you remove rubber hood covers and the shift levers. But if you don't do anything you will have to buy another STI. So depending on what you feel comfortable putting back together, then disassemble to your desire. At the worst, you'll just have to buy another STI.

https://si.shimano.com/en/pdfs/ev/ST...-5800-3711.pdf

https://si.shimano.com/en/manual/sea..._model=st-5800

Tony_G

Thanks for the pdf. I might try the replacement myself, but I am leery of the internal cable routing. I’ve read that if things go off track it can require the removal of the steering head (I’ve done that before) and the crank (mine needs special Cannondale tools and I haven’t done that).

Iride01 

If you have a friend that is better at tinkering, you might see if they want the challenge.

Crankycrank

As Iride01 mentioned, check out some YouTube vids. Skip the coffee and bring your patience if you want to try a rebuild. Shimano 105 5800 (left side) shifter rebuild - YouTube