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How to solve chain derailment on BBSHD and BBS02

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    How to solve chain derailment on BBSHD and BBS02

    Assuming the bike was running fine and you just installed the kit, derailment is probably a chainline issue.
    For non-chainline related issues please see the section further below.

    You have 4 different approaches if it is a chainline issue.
    1. Use a different chainring that wraps around the motor housing
    2. Rework the cassette
    3. Switch to IGH
    4. Adjust limit screw on derailleur to keep it out of these gears (and compensate for this by using a larger cassette)
    The chainline is how straight the chain is going from the rear to the chainring. Typically this problem presents itself when in the largest rear gear (low gear), if the chain is too angled in this gear the chain may be prone to falling off. Since each bike is slightly different this is common for a BBS build until the user finds what works best for them. A good first step to addressing the issue is a chainring with good offset, moving the teeth as far inbound as possible. Chainrings all have different offset, see this link for a comparison of the offset.

    Stock steel chainring has decent offset, as do rings like the luna eclipse and lekkie. However you want to make sure not to move it so far over that it is touching the chainstay. The Eclipse can do this as it moves the chain further towards the bike than any other chainring, though you can use a chainring spacer or two to move it further enough back that it will clear the chainstay if need be. So long as you have enough chainring spacers the Eclipse is the best option. Click here for a visual comparison of chainrings, and here for a video (note in video the lekkie measurement is slightly off, it does have better offset than stock).

    Any chainring that is larger than 42t can wrap around the secondary reduction housing to put the teeth further inbound. However just because it CAN, does not mean that all the >42t chainrings actually do. The most affordable chainrings that are 42t do not provide as much offset as the expensive ones. The chainring with the most offset is the Eclipse, followed by the Lekkie. Another great thing about the aftermarket chainrings is that unlike stock they are "Narrow Wide", meaning each tooth alternates between narrow and wide for a more exacting fit on the chain, this helps keep it on.

    Much smaller chainrings (30 tooth) will be more problematic since it moves the chainline much further to the right, altering the offset in the wrong direction. This is unavoidable since the rings are so small they cannot wrap around the motor housing like the larger ones. Often the smallest chainrings may present some chainline problems unless this is addressed as described below.

    Here's a visual aid for chainline differences:
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    Example of a Mighty Mini 30T chainring with no offset and resulting poor chainline.
    The Luna Mighty Mini 30T is a popular chainring but can be problematic without additional modifications due to completely flat design that does not offset the chain
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    Alternately you can rework your cassette, this is where you rearrange the sprockets on a cassette so you keep the gear range but you use less gears to do so. You may need to buy a cheap cassette to mod it, this link goes over options on that. It's a lot easier than you might think, and it saves wear on drivetrain since you shift less.

    You can switch to an IGH. With this your chainline never changes no matter what gear you have it in. (Nexus 3, Alfine 8 and Sturmey Archer 3 can take BBSHD power)

    Generally speaking, adjusting the derailer limit screw is not ideal as it limits your gear range, but it is an easy option.

    Non-chainline related possibilities:
    • If you are using the stock chainring this actually has pretty good offset but it is not a narrow/wide chainring so it does not have a good tooth profile for gripping the links. You may want to consider upgrading to a different ring.
    • You also want to check the chain itself:
      • Whether it is stretched and worn out (there are chain wear tools you can buy the check the chain).
      • Whether it is properly lubed
      • What speed it is: Ideally you want to use 10spd or lower. Higher speed chains are thinner and more problematic to sit well on the chainring at all angles when shifting gears
      • A chain may skip or derail if a link within that chain is binding up. You can put the chain on the stand and run it through the gears by hand or using the walk function on the display, if you see a particular link causing it you may need a new chain.
      • Whether it is properly sized
    • You also want to check the chainring itself, see if there are any notches, bends, debris or anything else that might cause the chain to bind up on the ring or not properly mesh with the ring teeth.
    • If you are only seeing problems on the rear cassette you probably need to tune the derailer.
    • A lot of this is a common part of bike maintenance but is especially more important when you are putting a lot of power through the drivetrain. The higher the power you are putting through it the more it is going to magnify any inconsistencies or improperly maintained parts. If still seeing issue, a local bike shop may be able to provide further insight on the cause.
    Examples of damaged chainring teeth causing drivetrain issues:
    Click image for larger version

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    Assuming the chain is new and the chainring and cassette look visually good you can put the bike on a stand and use the walk function or even rotate the cranks by hand to slowly run through the entire length of the chain and stop it at whatever point while it is in the middle of derailing.

    Once you see it happening, stop so you can closely inspect it at that point. See if this is happening on the front, on the rear, and where. Maybe a link is binding up, maybe the chain is not centered on the tooth (in other words the chainline is not good), maybe there is damage on a particular tooth causing it to stick, etc. Once you see where the issue is originating from it will be easier to fix.

    Assuming everything looks fine both on the chainline and the non-chainline possibilities, you may want to consider a chain guide. A chain guide might be as simple as leaving your front derailleur on, or using an N-Gear jump stop... Or something more complex.

    I myself have a hard time determining if the chainline is straight or not. Concerning the 2 photos it looks like if they dropped to the smaller cogs the fat bike would be good and the 700c crooked.


      It's just the way the picture was taken, at an angle so I could get the most detail into the pic. In the lower pic it is 100% parallel with the middle cog on the cassette, ideally this is what you're aiming for.

      But yes the fatbike would only be good in the gears furthest from the frame, that is a certainty. That bike would be a good candidate for reworking the cassette if sticking with the mighty mini chainring.


        i run a standard steel sprocket on front then on rear i have a nexus 8 speed hub with a 22 tooth sprocket the other way round with the dish outwards, can run chain slack and never comes off, even down rough footpaths