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I'm looking for possible/likely causes of an issue with my front chain-rings on my bike. It's a 3x8 setup and the problem is worst on the smallest front chain-ring, and worse again when combining smallest front chainring with smaller rear gears (so when chain tension is lowest).

What happens is below the chain-ring (the 'loose' side of the chain which is kept under tension by the derailler spring) the chain stays stuck to the chain ring (beyond the lowest 6-Oclock point where it would normally head off toward the derailler) - it then rides up and gets jammed into the frame. I commute on it, so its out in all weather and has a fine paste of road-muck all over it.

A few thoughts:

  1. Maybe it's the spring in the derailler being too soft / gummed up not providing enough back tension (if I hold the derailler to act like a stiffer spring it fixes problem on the stand)

  2. Maybe front chain rings so gummed up they're sticking to the chain and so holding onto it.

  3. I have just put a new chain on it (cut to same number of links as the old one) - I normally get a shimano one but got a cheaper brand this time. Also it's a 9-speed chain on a 8-speed cassette which I know isn't ideal so maybe that's it (it happened once or twice before chain change but now happens all the time).

I can give it a good clean which might help if 1 or 2 is to blame - but any other thoughts?

Photos of 1st and 2nd chainrings linked below. The small chainring didn't look to bad to me (clearly I'm no expert) - the middle one looks worse yet has less chainsuck. Will try a thorough clean but if that chainring looks a lost cause let me know!

Small chainring

small chainring

Middle chainring

middle chainring

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    You put a new chain on - did you trim it to be the same number of links as the old chain? New Chains are normally a few links longer than you need.
    – Criggie
    Oct 22, 2022 at 20:54
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    @Criggie thanks! Yes I cut new chain to same number of links as the one I took off - but good to check 👍
    – kabdulla
    Oct 23, 2022 at 9:53
  • The detailed description of the problem makes this a good question. I've added chainsuck to the title so that this may be easier to find in the future.
    – Weiwen Ng
    Oct 23, 2022 at 10:52

2 Answers 2

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This is called "chainsuck" and you've identified some contributing factors.

The RD cage tension spring can play a role as you've found, but unless it's more malfunctioning somehow (RD tweaked or broken such that the cage isn't tensioning the chain reliably), it should not be regarded as the key factor in preventing chainsuck.

If the chainring is dirty or worn (sharktoothed), that can be a direct cause. All chainrings will begin to produce this effect eventually once sufficiently worn and then paired with a new chain.

Some chains or chain and chainring combinations can have more or less tendency to "just do" this, usually by way of how their finishing or quality levels are interacting. I've tended to see cheap chains that might have rougher finishing do it more.

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    Many thanks. Great to know what it's called (also satisfyingly descriptive name!). I've worn out a few rear gears and had the resulting skipping - but chainsuck seems a less intuitive consequence of worn gears (the limit of a worn gear being no teeth at all - skipping makes sense to me).
    – kabdulla
    Oct 23, 2022 at 10:16
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It is usual for transmission problems to appear if you change only the chain on a system that is already too worn up. If a chain is used for too long, it damages the cogs and chain rings.

As gears and chains wear out, gear teeth begin to lose their shape. Most teeth should be symmetrical, that is, they have the same profile "front and back". With use, the profile that actually pulls or is pulled by the chain, is filed down, and in some cases, the tip of the teeth is also bent backwards due to the pulling force of the chain. That means a very worn tooth looks like a shark fin or a small hook. This indeed can "hook" the chain upwards. Naturally, if the chain is under low tension, this hook has more of an easy job pulling the chain up.

Here is my attempt to illustrate a worn tooth (red) as opposed to a new one (black): enter image description here

Another factor: have you seen a chisel "mushroomed" out for being hammered? That is also happening to the border of your chainring teeth, forming a "ridge". Those deformations make part of the teeth wider and thus prone to jam inside the chain, making difficult for it to "leave" the gear.

enter image description here

In some cases, the "hook" can be reduced with a file or a rotary tool and a small grinding stone (its diameter should be approx. the same as a chain roller). The "ridges" can also be removed with the same tools. I did it once but on one worn cog, rather than a chain ring, and did it just to prove a point, it's not really worth the hassle or the time, It's like sculpting the teeth back into shape.

In the case of the crankset in the pictures, I'd quickly file down the ridges on the side of the teeth on the middle ring and closely inspect the small ring for the same, just enough to be able to ride the bike while a replacement arrives on the mail or ride to the shop and replace the crankset (Only worth if a suitable replacement is not easy to find or if you want to take some time to shop for a slightly better crank). This crankset seems to be riveted, that is, it has no removable chain rings, so, it is more difficult to file the under side of the teeth.

Also, increase chain tension using the B screw in the rear derailleur. Of course, all transmission components should be clean and free of gunk. Only a tiny amount of lubricant on chain rollers and derailleur pivots, everything else should be clean.

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