New chain skipping

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esuhl
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Joined: 22 Mar 2017, 3:20am

New chain skipping

Postby esuhl » 5 May 2019, 7:59pm

To improve gear shifting slightly, I re-aligned my rear mech hanger and replaced a worn chain. The cassette scrubbed up like new without any noticeable wear. However...

The new chain skips every 1/2 rotation of the cranks/pedals on the smallest 3 or 4 gears. The smaller the gear combination, the less force required for the chain to skip.

I put the bike in a stand, applied the brake and rotated the cranks under gentle force. I can actually see the chain rising over the teeth of the rear gear when it skips. I can't understand how that is possible with the tension on the chain... :?

Also, the new chain is slack enough to rest on the chainstay when in the small/small pair of gears. Admittedly I won't be using that gear combination, but it seems a bit odd.

The new chain has the same number of links as the one it replaced, and I used the equation in the following link to confirm that it was the correct length: https://www.parktool.com/blog/repair-he ... -section-6

I'm completely baffled. Can anyone suggest anything I could try to identify/fix the problem...?

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honesty
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Re: New chain skipping

Postby honesty » 5 May 2019, 8:33pm

I had something similar after replacing the chain and cassette once, and I had to in the end change the middle (most used) cog on the chainset. Then it stopped.

rotavator
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Re: New chain skipping

Postby rotavator » 5 May 2019, 9:07pm

I have had a similar problem when

1. I used the wrong quick link for the new chain, may be a 9sp link with a 10 sp chain, it took me ages to figure that out :oops: .
2. When I fitted a new RD and new cassette, both 9 sp, I could get the indexing right at one end of the cassette or the other but not both at the same time. In the end I bought a thumb shifter with option of friction shifting so no more indexing woes.

I assume that you have checked that the jockey wheels now line up correctly with the sprockets after the realignment.

The fact that it is the smallest sprockets associated with the skipping suggests to me that they may be worn even it is not obvious, the small ones tend to wear out sooner than the big ones on my bikes therefore I would also try a brand new cassette just to see if that fixes the problem. Even if it doesn't, you have a replacement ready when the existing one wears out.

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NUKe
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Re: New chain skipping

Postby NUKe » 5 May 2019, 9:22pm

It Will be one of three things in the following order
1 sprockets are worn
2 stiff link
3 chain rings
My money is on the first, if the old chain was worn too Far, I.e. past an 1/8th of inch over 12 inch, then the new chain will slip on the sprockets. Even a worn set of sprockets don’t look that worn.
NUKe
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Brucey
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Re: New chain skipping

Postby Brucey » 5 May 2019, 10:34pm

how worn was the old chain? My guess is that it (and the cassette) was too worn and that this is why you have the skipping; it is very common if you change the chain only but leave it too late. However you should check this again with some tension in the chain before condemning the sprockets.

That the chain isn't tensioned in some gears may mean that the chain is simply too long but it may also mean that the derailleur isn't able to take up the slack because it is either faulty or the wrong specification for your gearset.

cheers
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~~~~~~~~~~~~~~~~~~~~~~Brucey~~~~~~~~~~~~~~~~~~~~~~~~

esuhl
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Re: New chain skipping

Postby esuhl » 5 May 2019, 11:33pm

honesty wrote:I had something similar after replacing the chain and cassette once, and I had to in the end change the middle (most used) cog on the chainset. Then it stopped.


The front chain rings are a little worn, but I can see the chain jumping over the REAR gears. Also, the skipping occurs on the smaller two (of three) chain rings, and only on the smaller rear gears. I'm fairly sure the chain rings aren't an issue.

I said on the small/small gears the chain rests slack on the chainstay. But I later noticed that even with force on the cranks to take up the slack at the top, the lower loop of the chain hangs slack towards the ground. This slackness means it's not pulled firmly against the jockey wheels and cassette gears, causing it to "bounce over" the teeth of the cassette when force is applied.

The slackness of the chain definitely looks wrong to me. :(

esuhl
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Re: New chain skipping

Postby esuhl » 5 May 2019, 11:35pm

rotavator wrote:I have had a similar problem when

1. I used the wrong quick link for the new chain, may be a 9sp link with a 10 sp chain, it took me ages to figure that out :oops: .


The new chain is an identical model to the old one, and I'm using the quick link it came with.

rotavator wrote:2. When I fitted a new RD and new cassette, both 9 sp, I could get the indexing right at one end of the cassette or the other but not both at the same time. In the end I bought a thumb shifter with option of friction shifting so no more indexing woes.

I assume that you have checked that the jockey wheels now line up correctly with the sprockets after the realignment.


When I pedal gently (or in the work stand), the indexing works well across all gears. The wheel rims are true, the gauge shows the hanger is accurately aligned, and the jockey wheels visibly appear well-aligned too.

I've never managed to get the top jockey wheel to perfectly align with both large and small gears -- there's always a minor change in angle at the extreme positions, but it's always worked well before now, so... I don't think gear/hanger/mech alignment is the problem.

Brucey
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Re: New chain skipping

Postby Brucey » 6 May 2019, 12:06am

esuhl wrote: …..This slackness means it's not pulled firmly against the jockey wheels and cassette gears, causing it to "bounce over" the teeth of the cassette when force is applied.....


if the sprockets are slightly worn then they will be slightly hook-shaped, and under load the rollers will 'climb' over the sprocket instead of meshing with the sprocket. The difference between a sprocket that will work with a new chain and one that won't may only be a few microns of metal, you can't always tell by looking at the sprocket.

The chain would skip in most of the sprockets if there was a stiff link and it wouldn't happen every half turn of the cranks. The symptoms you describe are exactly those you would expect with worn sprockets; the skipping only happens under load.

It isn't a bad idea to keep a spare (new) cassette on hand. If, under test, the chain skips on one cassette and not on the other, you can bet that the first cassette is worn.

cheers
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~~~~~~~~~~~~~~~~~~~~~~Brucey~~~~~~~~~~~~~~~~~~~~~~~~

esuhl
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Re: New chain skipping

Postby esuhl » 6 May 2019, 12:20am

rotavator wrote:The fact that it is the smallest sprockets associated with the skipping suggests to me that they may be worn even it is not obvious, the small ones tend to wear out sooner than the big ones on my bikes therefore I would also try a brand new cassette just to see if that fixes the problem. Even if it doesn't, you have a replacement ready when the existing one wears out.


Brucey wrote:how worn was the old chain? My guess is that it (and the cassette) was too worn and that this is why you have the skipping; it is very common if you change the chain only but leave it too late.


When I wore out my first chain, I'd left it way too late and replaced the chain, cassette and chain rings at the same time. I'd been measuring the chain for wear every few months, determined to not have to replace the whole drive-train next time. But the wear went from less than 0.75% to way over 1% in no time!

Last month I did something really stupid. I mis-threaded my chain through the jockey wheels so it was running over a metal tab between the jockey wheels. I couldn't work out why the chain was clicking/humming and rode about 8 miles before fixing it. :oops: Since then I've done maybe 150 miles tops, without any problems until I tried to adjust the mech hanger...

So... that explains why the chain is suddenly so worn. :oops:

Brucey wrote: However you should check this again with some tension in the chain before condemning the sprockets.


Yes -- the chain tension looks very wrong. I just can't figure out why it's happening... :?

Brucey wrote:That the chain isn't tensioned in some gears may mean that the chain is simply too long but it may also mean that the derailleur isn't able to take up the slack because it is either faulty or the wrong specification for your gearset.


I put the chain on the big/big gears and tried pulling the chain so a link would overlap (as if I'd removed a link), but the chain wouldn't stretch that far. Yet the current slack on the smaller gears looks insane.

The bike came with an 9-speed SRAM X-7, and I replaced it with a 9-speed X-9. It's been working fine for a year or two, so I'm sure it's compatible... And it certainly looks okay... I've never noticed such an obviously slack chain before. But I think it being faulty is just about the only explanation that makes sense...

I'll have another look when I get a chance. Cheers all! :D

esuhl
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Joined: 22 Mar 2017, 3:20am

Re: New chain skipping

Postby esuhl » 6 May 2019, 12:24am

Brucey wrote:
esuhl wrote: …..This slackness means it's not pulled firmly against the jockey wheels and cassette gears, causing it to "bounce over" the teeth of the cassette when force is applied.....


if the sprockets are slightly worn then they will be slightly hook-shaped, and under load the rollers will 'climb' over the sprocket instead of meshing with the sprocket. The difference between a sprocket that will work with a new chain and one that won't may only be a few microns of metal, you can't always tell by looking at the sprocket.

The chain would skip in most of the sprockets if there was a stiff link and it wouldn't happen every half turn of the cranks. The symptoms you describe are exactly those you would expect with worn sprockets; the skipping only happens under load.

It isn't a bad idea to keep a spare (new) cassette on hand. If, under test, the chain skips on one cassette and not on the other, you can bet that the first cassette is worn.

cheers


Ah, thanks again! It sounds like it's either the rear mech or the cassette, then. I'll investigate further! :D

gregoryoftours
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Re: New chain skipping

Postby gregoryoftours » 6 May 2019, 3:43am

Check for binding and/or poor spring tension in the jockey cage pivot of the rear derailleur. The pivot could have stiffened up with corrosion (SRAM mechs are more prone to this). If the gears were run with a bit too short chain the jockey cage spring could have been over stretched and damaged. Also check that the b-tension screw is set the same as it was before you took the derailleur off, and that it's not been squashed against the tab on the dropout when the derailleur was refitted, or the thread stripped in the body of the derailleur. Any of these could cause the slack chain, or could it possibly be that at some point in the past you increased your big sprocket size making a longer chain necessary? If your rear mech didn't have a long enough cage to take up the extra slack you might not have noticed because small and small is a combination one usually avoids. It does sound as if your cassette is worn, it would tend to slip on the smaller sprockets as they often wear first, and more when in middle and small rings on the front because the pulling force is greater when pushing on these rings.

peetee
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Re: New chain skipping

Postby peetee » 6 May 2019, 11:24pm

If your gear hanger was bent is it also possible that your rear mech is too?
It doesn't have to be out far for the possibility of the chain catching on the jockey cage side plates.
Also agree with the stiff link option and add that the quick link might not be properly engaged.
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esuhl
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Re: New chain skipping

Postby esuhl » 7 May 2019, 2:15am

gregoryoftours wrote:Check for binding and/or poor spring tension in the jockey cage pivot of the rear derailleur. The pivot could have stiffened up with corrosion (SRAM mechs are more prone to this).


The pivot moves really smoothly, and the tension seems okay -- hard to tell. But a spring/tension failure there would totally make sense. Definitely worth a closer look... (That must be it, surely!?!)

gregoryoftours wrote:If the gears were run with a bit too short chain the jockey cage spring could have been over stretched and damaged.


The chain size has never changed, but it's possible the chain may have jammed and I overstretched the jockey cage at some point... :-/

gregoryoftours wrote:Also check that the b-tension screw is set the same as it was before you took the derailleur off, and that it's not been squashed against the tab on the dropout when the derailleur was refitted, or the thread stripped in the body of the derailleur.


I played around with the B-screw, and can screw it in further to take up the visible slack, but the chain still doesn't seem like it's under the right tension at all... Everything looks in good condition, no stripped threads or visible damage.

gregoryoftours wrote:...or could it possibly be that at some point in the past you increased your big sprocket size making a longer chain necessary? If your rear mech didn't have a long enough cage to take up the extra slack you might not have noticed because small and small is a combination one usually avoids.


Nope -- the gears/drivetrain are all of the same dimensions as when the bike was new. And the bike has been working perfectly well with this setup for months. :?

gregoryoftours wrote:It does sound as if your cassette is worn, it would tend to slip on the smaller sprockets as they often wear first, and more when in middle and small rings on the front because the pulling force is greater when pushing on these rings.


Well, I just ordered a new cassette, so I'll give that a go.

peetee wrote:If your gear hanger was bent is it also possible that your rear mech is too?
It doesn't have to be out far for the possibility of the chain catching on the jockey cage side plates.

The rear mech looks pretty straight to me. The chain runs through with good, even spacing on either side, so it's not rubbing against the side-plates.

It's the sudden slackness of the chain (any chain!) that seems so odd.

peetee wrote:Also agree with the stiff link option and add that the quick link might not be properly engaged.


Good call, but it's definitely engaged.

-------------

I think I've got an old X7 mech somewhere. I'll have a play with that and the new cassette when it arrives.

Cheers, all! :D

gregoryoftours
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Re: New chain skipping

Postby gregoryoftours » 7 May 2019, 11:24pm

Does the chain sag when the bike is stationary? If when freewheeling may be sticky freehub - the sag would be more pronounced on the smaller sprockets when coasting. Also check jockey wheels turn freely and don't have side to side slop, although that's not really likely to be related unless back pedaling slightly. How does it behave pedaling steadily backwards in a fairly straight chainline?

esuhl
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Re: New chain skipping

Postby esuhl » 10 May 2019, 9:31pm

gregoryoftours wrote:Does the chain sag when the bike is stationary? If when freewheeling may be sticky freehub - the sag would be more pronounced on the smaller sprockets when coasting.


I had a sticky freehub last year, which I cleaned and re-greased, so I know (from experience of what it feels like) that it's not that. And yes -- when stationary, the chain touches the chainstay when on the small/small gears. I can use the B-screw to remove the visible slack, but clearly the chain is under insufficient tension.

gregoryoftours wrote:Also check jockey wheels turn freely and don't have side to side slop, although that's not really likely to be related unless back pedaling slightly. How does it behave pedaling steadily backwards in a fairly straight chainline?


The jockey wheels glide round smoothly without any wobble. Back-pedalling works fine -- no chain jumping, grinding or skipping at all.