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Shimano Di 2 rear derailleur question
Last Post 05/03/2021 11:07 AM by Orange Crush. 5 Replies.
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thinline

Posts:323

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05/01/2021 01:34 PM
What does the little "on/off" switch do on the Di2 rear derailleur?  I just changed tubeless tires for the first time (think I got it right) and the Di2 manual says to flip that switch to "off" before removing the rear wheel.  So I did.  Oddly, the manual did not say anything about switching it to "on" after reinstalling the rear wheel nor was the switch in "on" position to begin with.

Put everything back together and flipped it to "on" to take the bike for a quick spin to help spread the sealant in the tires, and it wouldn't shift properly.  Got on line to try and self-educate but couldn't find answers.  Flipped the bike over to pedal and shift.  The RD would shift between 3 or 4 gears mid-cassette at best and would hesitate and struggle to go to larger cogs.  Out of curiosity I flipped the switch to "off" and everything works perfectly.  It looked like when it was in the "on" position that it was restricting the derailleur's movement and not letting it extend downward freely when I shifted to the larger cogs.

Anyway, working fine now.

Any insights?

Many thanks!
Orange Crush

Posts:4499

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05/01/2021 02:04 PM
Sounds like you you figured it already and likely something to keep chain tension.

Sounds similar to on off switch on GRX gravel derailleur. When I got my bike last year I was unaware of this switch and first two rides chain was bouncing like mad on bumby gravel descends. I complained about it on a gravel ride page on FB and someone immediately piped up is your switch “on”. Doh.
zootracer

Posts:833

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05/01/2021 08:22 PM
I've changed a few rear flats with my Ultegra di2 and never flipped the switch 'off' and encountered no problems. Baggiest PIA was dealing with the thru axel skewer. Things used to be so simple...
6ix

Posts:485

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05/02/2021 12:25 PM
That switch controls the clutch. With it set to the "on" position, it's difficult to rotate the bottom pulley forward. I always turn it off when removing the rear wheel, otherwise you end up fighting it when trying to get the derailleur to rotate around the cassette when installing.

Due to the increase spring tension, having it set "on" all the time does slightly degrade shifting performance but it's a moot point with electronic. Definitely makes a big difference in all but eliminating chain slap.
thinline

Posts:323

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05/03/2021 07:50 AM
Thanks, gang.

I stopped to chat with the local mechanic and he explained it's use and pulled out a new derailleur and showed me the difference in tension with it on and off.  It is on a gravel bike so I spend time on dirt roads with it but nothing so rough that chain slap is an issue.  But, if it ever is, I now know what to do!
Orange Crush

Posts:4499

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05/03/2021 11:07 AM
What happened in my case is that the chain jumped from roughly middle cog to smallest cog and immediately the derailleur cable got yanked by several inches. With that I could only use the smaller cogs in rear. Not an issue on either ride as it happened near end and the return was mostly downhill. First time it happened I could not figure out what was going on until I noticed the cable yank at home. Fixed it but funnily I still did not notice the clutch on/off. Not sure what the equivalent punishment would be with di2 since there is no cable.

Lots of rough FSRs here. Downhills are often pothole surfing and the downhill trajectory will resemble a slalom course. Hitting one of these things bang on at some point is pretty much a guarantee particular with the sun and trees shadow play.
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