Old 01-10-19, 04:09 PM
Steve B.
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Join Date: Jul 2007
Location: South shore, L.I., NY
Posts: 3,189

Bikes: Flyxii FR322, Soma Smoothie, Miyata City Liner, Specialized FSR Comp, Fuji Professional

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Some thoughts;

1) It almost sounds like a compatibility issue between the Oval crank and the 5800 system, maybe the positioning of the crank is not to a Shimano spec. I was wondering if a mismatch is causing the big ring to change the entry angle of the chain to the lower jockey wheel and possibly pulling the rear derailleur in such a way as to exhibit the "too much cable tension" symptom. Except many of these bikes have been sold with these components installed so that "maybe" rules out compatibility as the problem.

2) Shimano does not show a compatibility between the 5800 system and 46/36 rings, as noted here: 2018-2019 SHIMANO Product Information Web

3) I applaud the OP's desire to learn and do your own work, but sometimes the lack of experience is going to drive you nuts as you try to troubleshoot. I build up bikes from frames but on occasion have to head to the shop for some expert help. My local guy caught that I was attempting to use a 9spd system with an 8 spd. triple crank, was why the chain was dropping between rings when I shifted to a smaller ring. I would never have thought of that as I had no clue Shimano had changed spacing between rings.

4) It really sounds like a cable tension issue. Maybe a housing is not seated correctly ?, got moved when you disattached cables ?, thus some friction at certain points in cable travel ?. Maybe just enough dirt in the system to cause mis-shifts in that combo ?, even though the bike is only a few months old ?, been riding gravel and dirt ?.
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