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Rear derailleur racing only when pedaling

Old 05-30-24, 01:55 AM
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Rear derailleur racing only when pedaling

Hi. Hope it's ok to post here. I have a 2020 Specialized diverge. The other week, the chain snapped. I replaced the chain, but when I peddled the rear derailleur 'rocks' back and forth. It does not try and shift up or down.
I have replaced the chain with another brand new one, replaced the rear derailleur with a new one and replaced the rear cassette with a new one. I have checked all the basics; The chain has no stiff links, it bi-directional, the gears are aligned correctly (when its on a stand, its perfect and the gears are spot on), the derailleur hanger is straight and the B tension screw is set correctly.
This happens in all gears.

Any help would be appreciated!
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Old 05-30-24, 03:43 AM
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Is the chain routed correctly through the RDER?
Look at another bike to compare.
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Old 05-30-24, 05:28 AM
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Park Tool is a site used by many. Hope this helps:
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Old 05-30-24, 06:24 AM
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Hi. Thanks for all your replies. Hopefully have got to the bottom of it. It appears to be the inner front chain ring has chain suck. I managed to get someone to ride it slowly while I watched it. Obviously can’t be certain until I swap the chain rings, but I have ordered some, so will report back in a few days! Thanks again though, I genuinely appreciate it.
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Old 05-30-24, 06:56 AM
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Originally Posted by 0055
Hi. Thanks for all your replies. Hopefully have got to the bottom of it. It appears to be the inner front chain ring has chain suck. I managed to get someone to ride it slowly while I watched it. Obviously can’t be certain until I swap the chain rings, but I have ordered some, so will report back in a few days! Thanks again though, I genuinely appreciate it.
You should try to find out why stuff is suddenly not working rather than replacing everything as a first step.
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Old 05-30-24, 07:10 AM
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Originally Posted by Kontact
You should try to find out why stuff is suddenly not working rather than replacing everything as a first step.
And I think this is what the OP just did by observing the chain run while someone else rode the bike. Andy
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Old 05-30-24, 07:48 AM
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Sorry, have to ask…. Did you cut the new chain to the correct length?

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Old 05-30-24, 12:32 PM
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Thanks for all the replies. Yes, chain is the right length. I used that formula with the chainstay length and the gear teeth.
Normally I would check it over more thoroughly before changing parts, but I need the bike soon for a multi day ride and wanted it fixed in a hurry. I have been in charge of my little girl this week and didn’t want to ignore her to much for a bike!
thanks again.
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Old 05-30-24, 01:02 PM
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Did you check the chain length to the old chain? I wouldn't completely trust a formula. I've never used them though. It's just too easy to find in the big/big combo without it being run through the RD. Or whatever the recommended way of the RD brand and model is.

I hope your planned big ride isn't too soon. Changing all that stuff right before a important to you ride isn't usually a good idea. Murphy loves to insert itself into those situations.

However I too first wondered if you have the chain routed through the RD correctly. Did you read the installation manuals for everything you put on? I know that with enough DIY experience they aren't needed. But when things like this crop up, it's always good IMO, to go back to square one and ensure all the stuff in those manuals has been covered.

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Old 05-30-24, 10:09 PM
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Originally Posted by Kontact
You should try to find out why stuff is suddenly not working rather than replacing everything as a first step.
Originally Posted by Andrew R Stewart
And I think this is what the OP just did by observing the chain run while someone else rode the bike. Andy
The OP listed a lot of parts he's thrown at it without knowing what was actually wrong.
Originally Posted by 0055
Hi. ...
I have replaced the chain with another brand new one, replaced the rear derailleur with a new one and replaced the rear cassette with a new one. ...
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Old 05-30-24, 10:47 PM
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Originally Posted by 0055
Thanks for all the replies. Yes, chain is the right length. I used that formula with the chainstay length and the gear teeth.
Normally I would check it over more thoroughly before changing parts, but I need the bike soon for a multi day ride and wanted it fixed in a hurry. I have been in charge of my little girl this week and didn’t want to ignore her to much for a bike!
thanks again.
Welcome to the forums.
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Old 05-31-24, 07:50 AM
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Originally Posted by Kontact
You should try to find out why stuff is suddenly not working rather than replacing everything as a first step.
While I would agree with your above comment had it been posted before the OP's post, #4. But the quote you choose to start your post with to me suggests that the OP is learning more about how to diagnose problems. I felt your jab at him was a bit late and not the best way to support learning.

Still, I also find myself questioning the OP's manor of what and when he's doing this service. To give the OP the credit of doubt we don't know when his multi day tour is starting. It might be the next day or a few weeks away. I see a huge difference in the "correctness" of timing service before an event/tour if the after service and start of event is a couple of days or a few weeks. Andy
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Old 05-31-24, 07:55 AM
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"Part changer" is an insult in the HVAC tech community. Diagnose the problem. Before deciding it is NOW the f ring, did you inspect it and/or check to see if it does it on the big ring?
Give me $2 on chain routing.
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Old 05-31-24, 08:08 AM
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Originally Posted by Andrew R Stewart
While I would agree with your above comment had it been posted before the OP's post, #4. But the quote you choose to start your post with to me suggests that the OP is learning more about how to diagnose problems. I felt your jab at him was a bit late and not the best way to support learning.

Still, I also find myself questioning the OP's manor of what and when he's doing this service. To give the OP the credit of doubt we don't know when his multi day tour is starting. It might be the next day or a few weeks away. I see a huge difference in the "correctness" of timing service before an event/tour if the after service and start of event is a couple of days or a few weeks. Andy
I was suggesting the OP change his general approach to diagnosis moving forward.


You've chastised me twice now for a single post. I get it. You can stop.
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Old 05-31-24, 08:47 AM
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Originally Posted by Iride01

I hope your planned big ride isn't too soon. Changing all that stuff right before a important to you ride isn't usually a good idea. Murphy loves to insert itself into those situations.
Too true. I had a solo century planned for last weekend so I put a new chain on because it was time and a sensible thing to do, then the weather decided to wet itself all over me. Not a big deal usually, but my ride included a mountain ascent, and it was already 37 before the climb started. Called it at 42 miles, I should've left the old chain on.😭
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Old 05-31-24, 09:19 AM
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In your thread title you say "rear derailleur racing" what do you mean by that?

And does this seem to happen in all rear cogs or just a few? And it's only when in the small ring?
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Old 05-31-24, 10:47 AM
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Using a chain until it breaks usually means chainrings and rear cogs are worn out, too. Check this, or have it checked.
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Old 05-31-24, 11:08 AM
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I’m also curious as to what is meant by “racing”. But with the given info, I’d suspect chain routing or very worn chainring.
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Old 05-31-24, 11:44 AM
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Originally Posted by 0055
I have a 2020 Specialized diverge. The other week, the chain snapped. I replaced the chain, but when I peddled the rear derailleur 'rocks' back and forth.
A picture of your derailleur might help us figure out what this means.
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Old 05-31-24, 01:09 PM
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Originally Posted by bboy314
I’m also curious as to what is meant by “racing”. But with the given info, I’d suspect chain routing or very worn chainring.
Given the language in the first post, I suspect it's a typo that was meant to be "rocking."
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Old 05-31-24, 06:06 PM
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Does anyone acxtually "calculate" chain length rather than just fitting it according to standard methods and/or the derailleur instructions?

If so, Why? Wouldn't you ultimately have to verify the length in actual practice to make sure it's long enough for the big/big combination and that it's not too long - that the cage can take up the slack on the small/small combo

Is it an engineer thing?
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Old 05-31-24, 06:10 PM
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Originally Posted by Kontact
I was suggesting the OP change his general approach to diagnosis moving forward.
While there is definitely merit in this, at this point the RD is probably the only excessive purchase. Having an extra chain is not a problem.

If the OP can afford it, replace “both” rings and maybe the cables and housing. The drivetrain will be set for quite a while.

John
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Old 05-31-24, 08:37 PM
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Originally Posted by Camilo
Does anyone acxtually "calculate" chain length rather than just fitting it according to standard methods and/or the derailleur instructions?

If so, Why? Wouldn't you ultimately have to verify the length in actual practice to make sure it's long enough for the big/big combination and that it's not too long - that the cage can take up the slack on the small/small combo

Is it an engineer thing?
I know of a few formulas to determine chain length but usually let the system tell me what length works best. I'll start with a long chain and see how the various aspects work (shifting response in various combos, big/big ability, chain wrap abilities, pulley knock) before removing a link pair to shorten the chain. Sometimes the "might be on the long side" chain length passes my standards. Sometimes not and I shorten the chain and retest.

I think that many people don't have the experience and understanding of the nuances of drivetrain performance (usually meant as shifting) to be able to trust or even determine this stuff and the companies have found a cookie cutter style of instructions is easier to follow and result in a minimum level of performance. In the modern era of internet smarts the ability to be "book smart" is so easy, and the willingness to post negative comments so quickly is great, makes any other instructional method hard to consider. Andy
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Old 06-01-24, 01:50 AM
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Originally Posted by Camilo
Does anyone acxtually "calculate" chain length rather than just fitting it according to standard methods and/or the derailleur instructions?

If so, Why? Wouldn't you ultimately have to verify the length in actual practice to make sure it's long enough for the big/big combination and that it's not too long - that the cage can take up the slack on the small/small combo

Is it an engineer thing?
That's my thoughts too. It's one thing to calculate the needed rear derailleur capacity, that's pretty much the rule before experimenting on the bike. But total chain length has SO many variables, not just chainstay length (assuming measure from BB center to rear axle center), and big cogs and chainrings, but also RD cage length and pulley size and positions, and in relation to RD B-pivot, plus, even RD hanger position, it can vary a bit.

That said, chain length I think does not explain the RD "rocking" (laterally?), unless chain was so short it pulled the cage WAY forward, and could either a) unscrew the derailleur at its mount (definite lateral rock), or b) cage and linkage so far forward that the pantograph is out of position and moving wrong with respect to the rear cogs.

TBD.
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