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Chain goes slack when I stop pedaling in smallest cog

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Chain goes slack when I stop pedaling in smallest cog

Old 11-18-23, 08:56 PM
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Chain goes slack when I stop pedaling in smallest cog

Hey everyone -

For additional detail: I'm building a bike for a friend of mine - he gave me a GRX-812 RD along with 105 11s shifters. GRX 1x 40t crankset, 11 speed cassette, 11-34. Everything shifts well going up and down, but when I took the bike for a test ride, I noticed that the chain goes slack and usually drops between the smallest cog and the frame if I stop pedaling. This happened to me a few times when coasting before a turn and trying to clock my feet properly.

If I put the clutch to the "on" position on the RD, it does not do this, but if I turn the clutch "off," it does it every time. Limit screws are set properly, I sized the new chain using the big-big +2 link method, what am I missing?

If it were my bike, I'd just not coast in this smallest gear, but this isn't mine, so I don't want to create any sort of dangerous possibility for my friend, and it really should not be doing this with the clutch off.

Any ideas?
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Old 11-18-23, 09:13 PM
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In your video I only see backpedaling - no coasting.

Two thoughts: If your freehub mechanism is sticky, the freehub won't freewheel easily. Being a very small cog, the high gear has the least amount of leverage to counter this effect. So the cassette carries forward with the wheel, creating slack.

Is your chainline correct? If the chainring is too far inboard, when you backpedal like you do in the video, the angle of the chain is going to cause it to snag the next cog to the left of the little cog. This may happen even with a correct chainline, but I would make sure your crank and rear spacing choices make sense.
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Old 11-18-23, 09:58 PM
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[QUOTE=speedyspaghetti;23076641]
If I put the clutch to the "on" position on the RD, it does not do this, but if I turn the clutch "off," it does it every time. Limit screws are set properly, I sized the new chain using the big-big +2 link method, what am I missing?/QUOTE]

You answered your own question. The RD is designed to operate normally with the clutch engaged. Disengaging the clutch should only be done when servicing the bike or removing the rear wheel. Leave the clutch on.
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Old 11-19-23, 08:56 AM
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The der cage clutch is doing exactly what it is designed to do, keep the untensioned chain from bouncing about.

No details about the rear hub and freehub body condition. Of my over half dozen Phil cassette rear hubs about half have this coasting chain droop condition when in the small rear cogs. My DT hubbed bike doesn't have any. Andy
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Old 11-19-23, 12:43 PM
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Some freehubs are incipiently draggy and others get that way because of lack of maintenance. Freewheeling at high speed on the smallest cog brings it out.
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Old 11-19-23, 02:14 PM
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That doesnít look like a pedalling action that would happen in real life but I get youíre trying to illustrate a problem that does. If you spin it up to a normal speed, like 90rpm, and let go of the pedal does it get pushed round by the chain?
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Old 11-19-23, 02:45 PM
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this is 100% normal and generally not of concern.

Freewheels (overrunning clutches) have a bit of friction, so tend to push the chain forward. This is resisted by the RD cage maintaining tension in the lower loop. The reason you see it with the smallest sprocket and less with larger ones is a simple issue of leverage. Whatever torque the F/Ws internal friction causes, has to be resisted by the chain, which has less leverage with smaller sprockets.

Also, since you're backpedaling, inertia comes into play. You're trying to suddenly push a heavy chain backward, and shouldn't be surprised that you get sag when doing so.

If this bothers you, you MIGHT be able to increase RD cage spring tension, However, this will increase drivetrain drag, so is a poor tradeoff to solve a non-problem.

The right solution is to accept that this is how things work, and protect the top of the chainstay.
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Old 11-19-23, 03:08 PM
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It seems to me that a 1x drivetrain on an off road bike needs to be able to backpedal in every gear, and if the chain falls off into the dropout that is not acceptable as you move the pedals to avoid obstacles. But the clutch solves the problem.
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Old 11-19-23, 03:54 PM
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Originally Posted by Kontact
It seems to me that a 1x drivetrain on an off road bike needs to be able to backpedal in every gear
....
Points well taken, however, gravel riding isn't like technical mtn biking, so I doubt he'd ever need to back pedal to clear 4"+ obstacles.

Doubly so, since the problem only manifests in higher gears, which he won't be using on surfaces where pedal clearance is problematic.

So yes, buying a clutch RD might help, but accepting and adapting to this would be FREE.
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Old 11-19-23, 05:03 PM
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Originally Posted by FBinNY
Points well taken, however, gravel riding isn't like technical mtn biking, so I doubt he'd ever need to back pedal to clear 4"+ obstacles.

Doubly so, since the problem only manifests in higher gears, which he won't be using on surfaces where pedal clearance is problematic.

So yes, buying a clutch RD might help, but accepting and adapting to this would be FREE.
I know that I backpedal to bunny hop, and that happens at any speed. It just isn't acceptable for backpedaling to put the chain into the dropout. I would accept the chain going into the wrong gear, but it should never leave the cassette.
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Old 11-19-23, 05:20 PM
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Originally Posted by Kontact
I know that I backpedal to bunny hop, and that happens at any speed. It just isn't acceptable for backpedaling to put the chain into the dropout. I would accept the chain going into the wrong gear, but it should never leave the cassette.
I guess that decades of fixed gear riding has me able to bunny hop in any crank position. (or even needing to coast)

However. I agree that the chain hopping off to the outside and jamming is unacceptable. Proper chain line would have the chain wanting to move toward the center of the cassette, so I'd doublecheck that.

I'd also confirm that the bike is vertical when checking the issue, to eliminate gravity as a consideration.

If no cause can be found and fixed, the OP may approach it from another direction. First of all, there simply shouldn't be enough room between the sprocket and frame, so is the axle too long? Or can the OP fit a spacer behind the cassette?
As a last resort, a small spacer or block can be glued to the inside of the dropout to prevent the chain from coming off the end.
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Old 11-19-23, 05:27 PM
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Originally Posted by FBinNY
Points well taken, however, gravel riding isn't like technical mtn biking, so I doubt he'd ever need to back pedal to clear 4"+ obstacles.

Doubly so, since the problem only manifests in higher gears, which he won't be using on surfaces where pedal clearance is problematic.

So yes, buying a clutch RD might help, but accepting and adapting to this would be FREE.
backpedalling to lift the inside pedal for cornering is quite common. i also backpedal to flatten the pedal position when standing to lessen bumps/potholes/curb lips at driveways, etc.
some backpedal for no good reason, much like blipping the throttle of their M/C for no reason at all when idling...

Things that can and do cause excess SEAL or FREEHUB DRAG are: an improperly installed Seal.. an improperly installed Axle Cone that folds the Seal over or rubs a plain metal Dust cover, A BENT DUST COVER OR SEAL, Too thick a grade of Grease used during a rebuild, or Debris/hair/twine.fishing line getting into the interface between the Freehub and axle.

other possible causes? A TWISTED Derailleur/der, cage, A Rusted or Damaged Chain(only happens at a specific portion of the chain), or excessive Drag at the Derailleur ROLLERS (hair/grass/fishing line, or RUST can do this).

And.. i've had this happen when an IMPROPER CHAIN was installed on a higher gear count Cassette...

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Old 11-19-23, 05:31 PM
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Originally Posted by FBinNY
I guess that decades of fixed gear riding has me able to bunny hop in any crank position. (or even needing to coast)

However. I agree that the chain hopping off to the outside and jamming is unacceptable. Proper chain line would have the chain wanting to move toward the center of the cassette, so I'd doublecheck that.

I'd also confirm that the bike is vertical when checking the issue, to eliminate gravity as a consideration.

If no cause can be found and fixed, the OP may approach it from another direction. First of all, there simply shouldn't be enough room between the sprocket and frame, so is the axle too long? Or can the OP fit a spacer behind the cassette?
As a last resort, a small spacer or block can be glued to the inside of the dropout to prevent the chain from coming off the end.
In this case, the OP has the problem solved by using the clutch. I was speaking more generally, because 1x road bikes exist.
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Old 11-19-23, 05:58 PM
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Originally Posted by Kontact
It seems to me that a 1x drivetrain on an off road bike needs to be able to backpedal in every gear, and if the chain falls off into the dropout that is not acceptable as you move the pedals to avoid obstacles. But the clutch solves the problem.
The clutch is the key. That's why most mid to high end derailleurs have clutches for their off road derailleurs. To prevent that from happening, or to keep it to a minimum.

Last edited by frdfandc; 11-19-23 at 06:01 PM.
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Old 11-19-23, 06:37 PM
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Originally Posted by frdfandc
The clutch is the key. That's why most mid to high end derailleurs have clutches for their off road derailleurs. To prevent that from happening, or to keep it to a minimum.
I thought it was to reduce chain slap?
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Old 11-19-23, 07:08 PM
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Originally Posted by choddo
I thought it was to reduce chain slap?
Yes. That's a form of chain slap.
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Old 11-19-23, 09:05 PM
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You're missing or ignoring the fact that the OP has 2 separate and distinct problems.

The first is the temporary upper loop slack, which might (or not) be caused by excess freewheel drag and/or mitigated with a clutched RD. Or simply lived with EXCEPT for the second problem.

The second, and more important issue is that the chain is coming off and jamming between hub and frame when he backpedals. Clearly, there's excess clearance between the cassette and frame. If that's fixed as I described earlier in post #11, the chain won't jam, and the OP can then decide whether or not he wants to try reducing the chain slap. OTOH, if the OP opts to fix only the chain slap issue, the chain may still come off when he backpedals.
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Last edited by FBinNY; 11-19-23 at 09:27 PM.
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Old 11-19-23, 11:29 PM
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Originally Posted by FBinNY
You're missing or ignoring the fact that the OP has 2 separate and distinct problems.

The first is the temporary upper loop slack, which might (or not) be caused by excess freewheel drag and/or mitigated with a clutched RD. Or simply lived with EXCEPT for the second problem.

The second, and more important issue is that the chain is coming off and jamming between hub and frame when he backpedals. Clearly, there's excess clearance between the cassette and frame. If that's fixed as I described earlier in post #11, the chain won't jam, and the OP can then decide whether or not he wants to try reducing the chain slap. OTOH, if the OP opts to fix only the chain slap issue, the chain may still come off when he backpedals.
No, the OP says the second issue goes with the first, and both don't happen with the clutch on.
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Old 11-20-23, 02:28 AM
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Originally Posted by frdfandc
Yes. That's a form of chain slap.
Getting the chain falling between the cassette and dropout? Ok fair enough.
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Old 11-20-23, 07:42 AM
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Originally Posted by choddo
Getting the chain falling between the cassette and dropout? Ok fair enough.
It might be more precise to say that chain slap is one of several symptoms of chain slack from the freehub turning despite the tension of the jockey wheels.
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Old 11-22-23, 05:38 PM
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Originally Posted by choddo
Getting the chain falling between the cassette and dropout? Ok fair enough.
It's a symptom of chain slap. If the chain is not in tension then the chain can fall between the cassette and the frame. Hence keep the clutch on when operating.
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Old 11-23-23, 01:34 AM
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Originally Posted by frdfandc
It's a symptom of chain slap. If the chain is not in tension then the chain can fall between the cassette and the frame. Hence keep the clutch on when operating.
OK but isnít that normally when the bike is getting thrown around? Not just from chain sag when you coast or backpedal? At least on road I thought it was ok to leave the clutch off for easier shifting.

Itís like thatís a symptom of the problem, the clutch isnít on to help maintain tension when the upper chain gets pushed forward by the cassette for some reason, not the cause?
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Old 11-23-23, 11:23 AM
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Originally Posted by choddo
OK but isnít that normally when the bike is getting thrown around? Not just from chain sag when you coast or backpedal? At least on road I thought it was ok to leave the clutch off for easier shifting.

Itís like thatís a symptom of the problem, the clutch isnít on to help maintain tension when the upper chain gets pushed forward by the cassette for some reason, not the cause?
The "cause" could be a very small sprocket (10t), a heavily sealed freehub designed for mud, short chainstays and extreme crossover.

We take it for granted that this shouldn't happen, but the basic bike derailleur set up was with a 13t cog on a 5 speed freewheel. We have since expanded the width of rear cogset from 24mm to 42mm (13 speed Ekar). Somewhere in there a compromise was made between capability and function.
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