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I'm getting some chain rub on the hardest gear in the front and 2 hardest gears in the back where the chain hits the front derailer cage outer side. When I spin the pedal it seems like the chain is not moving straight e.g. in certain points of the revolution the chain comes closer to the front derailer cage to the point of barely touching. I have tried adjusting the derailer at the LBS but still have this issue of rubbing the derailer cage. Can I go for a 9 speed chain on my 8 speed Claris since the chain is thinner and reduce this issue?

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    I'm going to say "probably" as a direct answer to your question. But I would also suggest you shouldn't have to be asking it. If your drive train is properly setup and aligned there should be no rub, maybe back to the LBS for another crack at it. Would also think that the difference in width between an 8 and 9 speed to be so nominal unlikely there being any real improvement.
    – Hursey
    Jul 18, 2023 at 3:49

3 Answers 3

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When I spin the pedal it seems like the chain is not moving straight e.g. in certain points of the revolution the chain comes closer to the front derailer cage to the point of barely touching.

You have to fix that first. The front rings are bent and need to be trued so that they track straight. The lateral motion from a bent ring is more significant than the differences in chain width.

No matter how true you have your front rings, under heavy pedaling, like during hill climbing, there is frame flex, and derailleur rub may show up which is absent under light pedaling. If you lower your expectations to accept this, you will have an easier time.

Make sure you understand the limit screws. Front and rear derailleurs have them. The limit being set too tight on the front derailleur can promote rubbing. To loose a limit can result in the chain coming off. You have to fine tune it.

Check the rotation of the front derailleur around its mounting axis (the seat tube). The cage should be "aimed" toward the center sprockets of the cassette. When the derailleur is rotated around the axis, because it has a curved profile to match the rings, under rotation, its aperture will appear slanted. That creates an offset between how it tracks the smaller and larger rings. Even if it looks well positioned, it's one more parameter at your disposal; it may be worth playing with to optimize it.

The height of the derailleur (up and down position on the seat tube) is another parameter to tweak that can interact with the rubbing issue.

I can personally attest that swapping a 6-7-8 KMC chain for a 8-9-10 speed SRAM on an 8-speed bike stopped front derailleur rub in the extreme positions. (This was in a bike where the rear derailleur is a Shimano Sora, which is rated for 9 speeds, so at least that component specifically "likes" the thinner chain.) The bike's rings were true, and everything was adjusted as well as could possibly be. In the end, the narrower chain made final difference in eliminating the remaining faint rub.

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  • +1. Funnily enough on a new/old cheap bike I've just bought, I've got the same sort of problem - bad rhythmic chain rub in the big ring, and at a low torque part of the pedal stroke (right foot 12 o'clock to 1 o'clock). The big ring is clearly bent if I look down while pedalling. What's odd is that the plastic chainring guard is fine. But that provides a point of reference. It takes a clear road and a certain level of confidence to watch the chainring through a couple of strokes at a time, but if the ring is bent it should show up turning the pedals on a stand.
    – Chris H
    Jul 18, 2023 at 13:21
  • is it the rings or the say axis on the crank not inserted properly? this can come from multiple culprite or is it the rings for sure?
    – Lightsout
    Jul 19, 2023 at 6:13
  • @Lightsout Cranks on an entry-level 8 speed system typically don't have rings intended for replacement; they are permanently mounted to the crank. It's unlikely that out-of-true wobble is caused by some mounting problem; it's much more likely that the ring is bent. Bent rings can be bent back. It's not easy to get it perfect, but it can be improved a lot.
    – Kaz
    Jul 19, 2023 at 6:23
  • its a fsa tempo crank so I can buy a new ring but will run $40 at least
    – Lightsout
    Jul 20, 2023 at 5:51
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This might work, but it will likely be trading one problem for another.

9-speed chains do shift on 8-speed cassettes, but there tends to be noticeable lag to complete the shift compared to using the correct chain.

In front, the mismatch between chain width and cage inner width can cause issues where, for example, by the time the starting position of the inner cage is close enough to the chain to shift it smoothly from the small to large ring (or middle on a triple), the gap with the outer cage when in the high gear is too large, which can result in derailing or poor shifting in the other direction.

This issue is usually the result of a front derailleur that's either tweaked or needs re-positioning.

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  • so would you say I need to adjust the limit screw outer and not the cable tension?
    – Lightsout
    Jul 18, 2023 at 5:21
  • @Lightsout I wouldn't want to speculate without seeing it, but what I will say is that a very large portion of "unsolvable" front derailleur situations I see, where it rubs somewhere no matter what, are the result of it being mangled and in need of replacement. Jul 18, 2023 at 5:32
  • @Lightsout I guess it could be both. The limit is a hard limit beyond which the FD can't go, no matter the cable tension. However, it needs a certain amount of tension so that it gets to the outer position. If you can manually move the FD so that it has enough free room but it doesn't go there through the STI, probably more tension, otherwise, give the limit screw a twist as well. That's the parameters, adjustment is always a bit of try & error. ;)
    – DoNuT
    Jul 18, 2023 at 6:57
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This feels like patching the symptom and not the root cause.

Yes you could use 9 speed chain on an 8 speed bike, but shifting will be a little sloppier. You'll be moving from 7.1mm wide chain to 6.7mm wide chain, saving 0.4mm ( 1/64th of an inch or 15 thousandths of an inch) which is not a lot. Personally I wouldn't bother.


Does your front derailleur support Trim? That is more stop positions than your crank's total number of chainrings, and you an access them with smaller/shorter pushes of the shifter.

Another option is to try adusting the "low" limiter on the front derailleur to get a little more clearance.

Check the inside of the front derailleur cage too - it may have some dings and burrs which are artificially narrowing the entry. Use a ruler to check if the cage is bent - it may simply need straightening.

Front derailleurs are fiddly and can still function even with minor damage like a bend.

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  • Personally I tend to accept that if a bike has to have chain rub, its better in the lowest gear combinations, so small chainring and big cog. Mostly because I'm going slow here, whereas if I'm in a top gear I want to go as fast as possible.
    – Criggie
    Jul 18, 2023 at 4:22
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    If the current chain is actually a "6-7-8" speed chain, it could be about 7.3 mm wide.
    – Kaz
    Jul 18, 2023 at 7:01
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    Just to check my own understanding, but wouldn't the effective difference actually only be 0.2mm relative to the derailleur? Assuming the chain is centered on the chain ring it's 0.2mm on each side +/- a little manufacturing tolerance
    – Hursey
    Jul 21, 2023 at 3:38

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