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How do I tighten the loose chain on my son's bicycle given an enclosed Nexus rear hub (3 gears but no visible external cogs) ?

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    Generally you slide the rear axle back. It may be that the chain has "stretched", however, such that no further adjustment is possible. (A photo would help.) Feb 12, 2018 at 4:18
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    If the chain is elongated to the point the rear wheel is too far back, its time for a new chain. Don't be tempted to keep removing links to shorten the chain cos that will accelerate wear on the chainring and cog.
    – Criggie
    Feb 12, 2018 at 6:28
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    How old is the chain, how many miles has it done? It may be time for a replacement. There are rather cheap and useful gauges to measure chain lengthening effectively. By moving the wheel further back you will just remove the symptom but not the cause and you might damage cog and chainring as @Criggie writes.
    – Carel
    Feb 12, 2018 at 16:56

2 Answers 2

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There are two mechanisms for chain adjustment on non-derailleur bikes, either: the frame should have horizontal slots that the wheel axle fits into, and is secured with large nuts on the threaded axle ends. Chain tension is adjusted by moving the whole wheel within these slots; or the bottom bracket is mounted in a eccentric cylinder in a oversize bottom bracket shell.

Park Tool has a video on single speed chain installation and adjustment that you may find helpful to adjust the chain tension correctly.

As noted in comments, you should check that the chain is not worn out. (The pins and bushings wear and become slightly looser which causes the chain to elongate). A worn chain accelerates wear on the sprockets.

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    Given this is specific for Nexus 3 speed, do you have the time to add the info on how to adjust cable length and tension for Nexus hubs? Typically there's a window where the mechanic needs to line up two tick marks by adjusting cable length.
    – BEVR1337
    Feb 13, 2018 at 8:31
  • @BEVR1337 Unfortunately I have no knowledge about that, and it would not be relevant to the original question here. You should just post a new question. Feb 13, 2018 at 12:23
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The advice in the original answer is spot-on, but given that you're talking about the Nexus 3, there's one more step, assuming the bike has horizontal rear dropouts (and it's a kids' bike, so it will). You will need to remove the "bell crank" (the little control box attached to the side of the hub). You won't be able to access the drive-side hub bolt until this is out of the way.

Just loosen the black fixing bolt on the end of the box ("A" in the picture below) using a hex (Allen) key, then you can pull the box right off. See the picture below for details (from the dealer manual). The picture is about attaching the bell crank, but it's the same idea.

bell crank attachment

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