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Di2 chain slipping - then suddenly gone. Any ideas/explanations?

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Di2 chain slipping - then suddenly gone. Any ideas/explanations?

Old 07-24-19, 09:53 AM
  #1  
NoWhammies
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Di2 chain slipping - then suddenly gone. Any ideas/explanations?

Yesterday I was doing interval training on one of the local mountains. While shifting in to a harder gear, I mistimed my shift and heard that awful 'grinding of grears' that happens when I apply power and shift at the wrong time.

Right after this, the chain kept skipping on the rear derailleur. I was surprised by this because I thought Di2 was indexed and this couldn't happen. Anyway... I hoped off the bike and tried to get the skipping to stop by going through the gears/rear cogs. Nothing. So I decided to head down the mountain and get to the LBS to see if they would fix it.

As I was descending, I put the bike in the large chain ring and the middle of deck on the rear cassette. During the descent I started peddling and after 30secs - 1mins, the chain jumped/skipped a couple of times. I could also hear a clicking every revolution or so. I thought "oh crap. How much is this going to cost me".

Now here's the strange part. When I was down the mountain and on the flats, everything was fine. The skipping was gone, the clicking was gone and my bike was riding as normal. So why the post here? I'm curious to know if a) anyone has experienced this before or b) if someone can tell me what the heck happened.

Thank you.
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Old 07-24-19, 10:34 AM
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Did you happen to bend your rear derailleur hanger with that mistimed shift while getting on it?

Take a picture from the rear with the derailleur centered in the cassette.
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Old 07-25-19, 08:56 AM
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Any update on this?
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Old 07-25-19, 09:28 AM
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Originally Posted by NoWhammies View Post
I'm curious to know if a) anyone has experienced this before or b) if someone can tell me what the heck happened.

Thank you.

This doesn't sound identical to the problem I discovered yesterday, but it sounds somewhat similar, so FWIW:

I experienced some rare bad shifts and chain drops after 5 years of trouble-free Di2 shifting. I eventually located the problem with the bike on the stand. Briefly, one of the pins (that works in conjunction with the ramps) on the large chain-ring was pushed out too far, and was snagging the chain when it was not supposed to. This became more obvious when I put it into the smallest cog on the rear and the smaller chain-ring up front (i.e., I cross-chained it). Once per revolution (or less, as it did not always happen), the chain would snag on this one pin, and partially ride up, as if trying to shift to the large chainring by itself. When the chain would ride up, it would impart a twist, and that propagated to the rear, and the chain appeared to be skipping on the cassette as well. It took me about 15 minutes to figure out the over-extended pin was the cause of the problem, rather than a bent chainring or something with the chain or cassette. I could only see what was going on when I turned the crank very very slowly. I pushed the pin in all the way with a vice grip, and everything is fine again.

The moral of that story is that you have to take a very long careful look at everything, and run the chain through all the gearing. It seems counter-intuitive, but the problems often only show their origin when you do everything in slow motion.

Last edited by wgscott; 07-25-19 at 09:35 AM.
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Old 07-25-19, 03:23 PM
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@jadocs Thanks for the follow-up. I'm trying to find the time to get a photo or two of the rear derailleur centered in the cassette. Hopefully tonight or tomorrow.


@wgscott what is a pin working in conjunction with the ramps? I've never heard of this before. Good description of your issue/problem though. Thanks for the input.
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Old 07-25-19, 05:18 PM
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@NoWhammies: They are on the inside of the large chain-ring, and are designed to hook onto the chain and help lift it up to the larger chain-ring in the course of a shift.



Last edited by wgscott; 09-18-19 at 04:24 PM.
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Old 07-25-19, 05:26 PM
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Have you looked at your cassette for broken or damaged teeth?
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Old 07-25-19, 08:33 PM
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Always do the easy and cheap thing first.

Is the rear wheel installed into the dropouts and tightened down properly?
Is the cassette lockring tight?
Are the derailleur mounting bolts and derailleur hanger to frame bolts tight?
Worn chain or cassette?
Is there lateral, side to side play in the freewheel indicating a worn bearing?

Make sure all fasteners are properly torqued, check the chain, cassette and freewheel bearing for wear and look for cracks in the frame while things are apart.

Clean everything up, put it back together and test.

Proceed from there.


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Old 07-25-19, 10:16 PM
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Originally Posted by NoWhammies View Post
Yesterday I was doing interval training on one of the local mountains. While shifting in to a harder gear, I mistimed my shift and heard that awful 'grinding of grears' that happens when I apply power and shift at the wrong time.
What's a "mis timed shift" ?

I shift my Campy (non electric) rear under power all the time. That's a main benefit of modern drivetrains.
Shifting to a smaller cog is not very demanding.
I shift while climbing, out of the saddle, into larger cogs without problems.

Does Di2 have a "reset" type adjustment, so it self-corrected after a bit ?
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Old 07-26-19, 06:03 AM
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I'm positive this isn't a Di2 issue.

It is an issue with the mechanics of the system, not the electronics.

Something is bent, loose or worn.


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Old 07-26-19, 12:19 PM
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@wgscott oh, that's those things are. I have always wondered that. Thank you.

@TimothyH well I was doing hill interval work last night. Didn't do any shifts under load, but I was in and out of the saddle a fair amount. No issues. So whatever was bent might have un-bent itself. Or whatever the case/issue was.

@jadocs, I took some photos after my ride last night, so the bike is a little dirty. Hope they help.




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Old 07-26-19, 10:58 PM
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Originally Posted by NoWhammies View Post
@wgscott oh, that's those things are. I have always wondered that. Thank you.

@TimothyH well I was doing hill interval work last night. Didn't do any shifts under load, but I was in and out of the saddle a fair amount. No issues. So whatever was bent might have un-bent itself. Or whatever the case/issue was.

@jadocs, I took some photos after my ride last night, so the bike is a little dirty. Hope they help.
Hard to see directly from the rear but it does not appear bent from the pics. Did it happen in a gear that you are in most of the time? If so, I’m wondering if that particular gear on the cassette might be worn. I assume your chain is good to go? Either way I would check to make sure things are not loose like Tim mentioned above, and I would put it on the stand and make sure everything is indexed properly...since you tweaked it out in the road when it happened.
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Old 07-27-19, 02:58 PM
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@jadocs The chain is, according to my chain wear indicator, at 50% life. The rear cassette is OEM, so I am hoping to get another chain life out of the cassette before replacing it. That said, the cassette has been making some strange noises of late, so I am thinking the casette may be coming to the end of its life. Although to look at it, I don't see any shark tooth happening.

I cannot recall if the occurrence happened in a gear that I use all the time. I'll give the bike a look over on the stand to make sure everything is running properly after dinner tonight. Will give me something to do as the sun goes down.
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Old 07-28-19, 12:57 PM
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Originally Posted by NoWhammies View Post
@jadocs The chain is, according to my chain wear indicator, at 50% life. The rear cassette is OEM, so I am hoping to get another chain life out of the cassette before replacing it. That said, the cassette has been making some strange noises of late, so I am thinking the casette may be coming to the end of its life. Although to look at it, I don't see any shark tooth happening.

I cannot recall if the occurrence happened in a gear that I use all the time. I'll give the bike a look over on the stand to make sure everything is running properly after dinner tonight. Will give me something to do as the sun goes down.
Keep an eye on it. I’m thinking that possibly it happened in a gear that you are in most of the time in combination with a slightly worn chain (even though it’s still within the spec range). It has happened to me, but it just felt like a skip...it did not make a huge racket afterwards though. I switched chains just to see what would happen and it has not happened since.

Not saying this applies to you: One thing I have learned observing some of my buddies who also have Di2, is stop messing with it. Set it properly per tech manual. By that I mean, they have experienced a slight delay when up shifting (ie. not as crisp as they think it should be...a little racket before it finally engages), and they end up “tweaking” it. When that happens the pulley wheels end up not being centered properly...and that can cause skipping as well. I really think that a worn gear plus a slightly worn chain causes some of the delay/racket when trying to upshift to it.
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