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I've just bought my son a 2nd hand Orbea MX 27 (thought it was an MX40 but the groupset is consistent with the tech specs of the 2020 27). It was hardly used so doesn't appear to be age / mileage related and I have the same problem. It looks like the smallest crank and the highest gear at the back are incompatible with the bike. A previous post on something similar seems to have also highlighted that the chain length might be wrong. This might be the same for this bike. Specs of the bike here: https://www.nrgcycles.co.uk/kids-bikes/kids-mountain-bikes/2020-orbea-mx-27-xs-dirt__73958 enter image description here

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    It appears the chain has a sticky link, which might account for the lock up problem mentioned. Otherwise with a chainring that small, there's not a lot that can be done short of a radically configured chainstay.
    – Dan Gao
    Aug 23, 2023 at 14:28

4 Answers 4

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Based on the photo, the front derailleur is a long way from the chainring.

I originally thought the chainring was rather small given the height to the front derailleur. So I guessed that someone had changed crankset or chainrings to smaller ones. Also I knew the older versions of this had a triple crankset, but if this is 2020 year or newer, it did come with a double. Apologies. but my initial idea doesn't apply.

You could change to larger chainrings to minimise this scraping. Here's a 2019 Orbea with larger chainrings - notice the space under the chain but that same downslope to the chainstay.

enter image description here

The other cheaper option is don't ride in small:small combo. Tell the rider to stay on the large chainring unless there's a long uphill climb coming and only then drop to the smaller chainring. Small-big abd big-small combos shouldn't have the same ability to rub.

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    Cheers Criggie. Yes i originally thought that the original supplier of the bike had put lesser components on it as most of the searches on the internet show the bike shipped with 8 speed groupset, however the link i posted with the bike does show it with the Tourney rear / Alivo front derailleur setup. I have to say in the big crank the front trim is required between the high half / low half gearing because of inner / outer chain rub. Seems like a fudge for the components used. Great frame, but poor choice of groupset. I've tried educating my 12 year old adsd kid in the art of gear selection!
    – Robert W
    Aug 24, 2023 at 6:45
  • @RobertW Somethig like "prefer to stay in the big ring unless you're going up a hill AND run out of low gears" might work
    – Criggie
    Aug 24, 2023 at 20:08
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As a quick answer for now, I don't think it's a problem because you wouldn't normally use this combination of gears.

In fact I was taught as a kid to never even try this because of the strain it puts on the chain and derailleur. This may be apparent if you look at the chain "in the plane of the chain", i.e. top down or from behind the bike.

As a rule of thumb:

The low gear at the front (small chain wheel) should only be used with the lower gears at the back (that half of them with big cogs).

The high gear at the front (big chain wheel) should only be used with the higher gears at the back (that half of them with small cogs.

That's not to say that your chain is or isn't the correct length, that's something that would need to be checked separately.

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    I agree with that however we are talking about my 12 year old son who doesn't have that kind of sense! :-) However I have never seen a bike where the chain practically lies on top of the chainstay before
    – Robert W
    Aug 23, 2023 at 10:54
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Assuming the bike is in original spec you might actually be right that this gear combination is incompatible by design and that has just been accepted as limitation when building it because nobody rides like that.

In essence, the small (22t) chainring is too small for the chain to (reliably) run above the chainstay, you could only opt for larger chainrings to mitigate that, depending on groupset compatibility and required gearing for your son's typical rides.

I don't think the problem here is chain length here because the top of the chain is under tension (especially under load), the slack would typically appear on the bottom if the rear derailleur can't extend, anymore. It doesn't look like there is too much slack in the chain, if it shifts and runs well in "usual" gear combinations, that's probably OK.

In a practical sense, I think you have two main options

  • Tell your son to avoid this gear combination, the rubbing will give him a good acoustic cue :)
  • If he is only riding in the flat and the range of the cassette alone is sufficient, somehow blocking the front derailleur for a "false 1x build" could be an option to safe the frame. I'm not sure what the best approach would be, perhaps turning the inner limit screw in to the extreme, experimenting with cable tension so that he can't shift down. A full 1x conversion would be a much cleaner solution but probably too expensive, maybe taking off the derailleur and adding a chain-catcher? It probably depends how creative you want to get to avoid him destroying the frame unintentionally.

I wouldn't be as strict regarding "cross-chaining" as the other reply, it increases chain wear but doing it for short period of time might be legit depending on the riding situation. Not even road cycling pros always follow the small ring - large cogs, big ring, small cogs rule all the time. :)

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Add a chainstay protector.

You'll have to find one that will work around the cable guides on the underside of the chainstay, but a decent chaninstay protector will protect the frame.

Buy several, check its condition periodically, and replace it if it starts getting worn through.

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  • Trouble is he's said it locked up on him last time he was out on it which suggests its dangerous. Appreciate all the comments where you shouldn't cross chain...totally understand that, and yes, you can visibly hear it but if he's riding along and in the highest gear and accidentally drops the crank he could end up going over the handlebars. I've personally never had a bike that had the potential to throw him out the saddle in such a way
    – Robert W
    Aug 23, 2023 at 12:12
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    Locking up and chain rub can be different problems, your question (as it stands) doesn't mention it locked up.
    – pateksan
    Aug 23, 2023 at 13:45

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