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  1. #1
    Join Date
    Oct 2003
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    slc
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    10,421

    Chain drop issue

    I could use a little help diagnosing the cause of chain drops that are happening on my son's bike. The chain is occasionally dropping off the chainring during shifts. It's a 1x10 GX drivetrain with a 32t SS non-NW chainring. It's happening while going uphill so it's not the chain being bounced off. The first time it happened he was shifted all the way up the cassette so I thought it was a chainline issue at first (it's a 20" bike with 330 mm chainstays so the chainline is pretty severe), but it's happened several times now while in the middle cogs when the chain was dead straight. If he way up the cassette it drops to the inside, but otherwise drops to the outside. When they've happened the chain tension was probably higher than ideal, but he didn't appear to be mashing on the pedals particularly hard and at 45 lbs he can't put that much pressure on them anyway. I've never seen similar drops before, it's really puzzling.

    The obvious solution is to put on a NW ring, but I want to be sure I'm not missing anything before I spend the money. It's all brand new stuff, initial cable stretch has been taken out, chain length looks good, and it shifts perfect in the stand.

  2. #2
    Join Date
    Oct 2003
    Location
    Banff
    Posts
    20,127
    NW and a derailer with a clutch or higher tension.

    Kids 20" wheels are inflated hard, for a 45p kid, so the bike still bounces a lot, even on ups, Also any suspesion is not really set up for someone that small. And 20" wheels are deflected more then a bigger wheel.

    All this makes is easy for a chain derailment.

    Also adding a F derailer/chain guide can help too.


  3. #3
    Join Date
    Jan 2006
    Location
    Carbondale
    Posts
    10,345
    Throw a NW and/or chainguide?

    It's not designed for that geometry is probably what it boils down to.. like you said, it's exaggerating the chainline.

  4. #4
    Join Date
    Dec 2007
    Location
    Hell Track
    Posts
    8,402
    ^^ what those guys said. There's a reason that lots of single ring kids bikes come with those "chainguide" plates on either side of the chainring.

  5. #5
    Join Date
    Oct 2003
    Location
    slc
    Posts
    10,421
    It's a clutched derailleur (GX 2.1), tires are inflated to a reasonable pressure for a human that size (15 psi), and the fork is a custom kids fork that actually works. He uses the full 80 mm of travel on bigger hits, though it is a hardtail. It's happened in some pretty smooth spots at low speeds. So far it has only happened when shifting, and never while descending rough stuff at speed. The chainline is severe when shifted all the way up the cassette, but most of drops have happened in the middle of the cassette when the chain is straight.

    Like I said, buying a NW will probably solve it but I want to make sure I'm not missing something obvious first. Thanks.

  6. #6
    Join Date
    Feb 2005
    Location
    The Pacific Northwet
    Posts
    221
    Bent tooth? A small twist can cause this intermittently. it will only happen when that tooth is in the right position when the shift occurs.
    Quote Originally Posted by hortence View Post
    When I did twice the work for half the control, I was a whiny little bitch

  7. #7
    Join Date
    Aug 2016
    Posts
    355
    How's the drivetrain wear? Something sounds fishy. The chain drop shouldn't be happening from normal use.

  8. #8
    Join Date
    Aug 2016
    Posts
    355
    Edit: somehow missed the "all brand new parts" duh.

  9. #9
    Join Date
    Apr 2007
    Posts
    2,146
    My kid is on a spawn with suspension. The Bike is crazy expensive but my folks got it for him so WTH. This same thing is happening. I looked at BB spindle length, the shop measured it at 110mm. They said they could get 103mm. The chain line is so bad, even from the 5th cog up. Seems silly to replace a new BB with a new BB but the crashes resulting from him trying to pedal after the chain is dropped are some of the worst he's taken.

  10. #10
    Join Date
    Apr 2004
    Location
    Southeast New York
    Posts
    6,602
    It's not silly if it's necessary.

  11. #11
    Join Date
    Feb 2009
    Location
    Eastern WA
    Posts
    487
    A loose bottom bracket? With just a little side to side play that is the same as a shift or dropped chain.

  12. #12
    Join Date
    Mar 2006
    Location
    Missoula, MT
    Posts
    19,973
    ^^^square taper crank?
    No longer stuck.

    Quote Originally Posted by stuckathuntermtn View Post
    Just an uneducated guess.

  13. #13
    Join Date
    Oct 2003
    Location
    slc
    Posts
    10,421
    Quote Originally Posted by DaveVt View Post
    My kid is on a spawn with suspension. The Bike is crazy expensive but my folks got it for him so WTH. This same thing is happening. I looked at BB spindle length, the shop measured it at 110mm. They said they could get 103mm. The chain line is so bad, even from the 5th cog up. Seems silly to replace a new BB with a new BB but the crashes resulting from him trying to pedal after the chain is dropped are some of the worst he's taken.
    Interesting, which Spawn? I assume that is without a NW chainring?

    Quote Originally Posted by farmguy View Post
    A loose bottom bracket? With just a little side to side play that is the same as a shift or dropped chain.
    BB is rock solid. It's an external BB with 2-piece cranks. I'm not sure what the shell width is.

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