Today, I had a weird problem with my road bike's rear derailleur. During an event (which was a 2-hour climb up an Alpine summit), I got a noise when riding in the smallest gear. It has a R5800 105 groupset, pretty standard... Bike was serviced before, got new cables and shifts were fine, it started during the ride, after 1 hour - and I didn't have the problem on previous rides.
Here's a short video: https://photos.app.goo.gl/fuPLYwb4DsFnNsQL7
Chain always made the shift on the largest cog but there was some noise I couldn't really identify
(Note: Sorry for the bad filming, I originally didn't take it with the intent of using it here. It looks like a bad shift but I actually didn't get the lever action right, the chain actually shifted quite reliably but then had the noise running on the cog)
First, I tried some minor adjustment with the barrel adjuster, tightening the cable because I thought there might not be enough tension for the RD to fully go in. That worked on the largest cogs but then I had bad shifts on 1-2 cogs in the middle of the cassette. I ruled out the lower limit screw by turning it until the chain didn't make the shift but there was quite some margin, the bike worked before and had a service 2 months ago.
Then I checked the inner cable and found that it was actually clamped outside of the intended groove and had a bit of fraying - I took that as a signal the cable might have slipped and I need to tighten it properly. I'm no pro mechanic, so it took me several attempts and quite some time until I got good shifting through all gears and no noise on largest cog, but in the end I think it was the cable and I got it right, somehow...
Judging by the video and the initial symptoms described, do you think that was problem or did I just fix something else by accident? Do you have any advice how to approach this next time in a more methodical way, i.e. what to check first?
I know, that's all a bit confusing and perhaps not the most precise question but I feel I need to learn a bit since I'd have to wait about 3-4 weeks for a service appointment at my dealer during riding season.
Since I fixed the issue and slept over it, I'm trying to sort my thoughts a bit and how it would approach it next time
- Consider the history of the bike and what is likely to happen: If shifting worked fine before, it's unlikely that derailleur limits or B-screw have changed mid-ride (or over night)
- Can you rule out impacts/damage? Thinking about it twice, I can recall a slight touch of wheels during the race but it felt like pure tyre rub and was at very slow speeds ascending. If that had bent the hanger, it should have been inwards and probably impact more than one gear, plus it would feel more like "over-shifting" the last cog because of the way the RD gets natrually bent.
- Only touch one thing at a time and think logical: I was very erratic and also tried low limit screws/b-screw but the only thing that (partially) mitigated the issue was messing with the barrel adjuster, so focus on that. In my case, the problem went away when I slightly pressed against the inner cable while pedaling, so it is likely to be related to that.
- Inspect: Before making any changes, it is worth having a look at your bike. I already played around with adjustments when I figured out that the cable wasn't properly clamped, which is never a good sign. Not sure if it came loose or the bike shop did that in a hurry and I was good for the first 1000 kilometers...
- Prepare: Before touching anything, look up instructions and videos instead of messing around. At a certain point, it was clear to me that I had to fix the cable routing (and probably tighten it), but checking out some videos how to get a baseline adjustment when re-attaching a cable would have sped up the process.
- Test it on the road: Before going on the next ride, make sure that stuff also works in real-world conditions. I have a steep driveway on which I can test 10 percent gradient to verify if the issue is gone under load.