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I've had this issue with my Kalkhoff Agattu (now out of warranty) basically since day one. The nut on the drive side of the rear dropout doesn't hold the wheel tightly enough, and it gets gradually dragged forward by the chain over 10-20km of riding (see photo).

Have taken it to a few bike shops and they've tried to swap out the nuts and non-turn washers, including I think one shop fitting a washer with the wrong angle making it cut into the dropout inside wall.

the offending nut

I was thinking I should find a shop experienced with Kalkhoff and ask them to: 1 - repair the dropout wall with a weld as best they can 2 - machine a small L-shaped piece of steel that can slip over the outside of the nut and the back of the dropout, to hold the nut in place

Do you think this will work, or is there anything else that will stop this happening?

Update 23 November:

Closer photos of the dropouts/damage are below. There is space for the wheel to move forward, but I’m sceptical the nut will hold further forward in the dropout either. Can you please recommend the right lubricant for this?

Also, this extract from the Shimano guide for the internal hub, the washers put on that I think were the wrong angle wer the 6R/6L (Silver/White). Is it correct that 7R/7L are the right washers? If so I’m hoping to give them some extra support with being held in place with a torque arm secured through the hole in the frame, to hopefully hold it in place. Or is there a better part I can use that could stop the nut sliding forward?

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  • Check bicycles.stackexchange.com/questions/83724 and bicycles.stackexchange.com/questions/63 for possible duplicates
    – Criggie
    Nov 12, 2023 at 21:07
  • To answer this we really need to see how messed up the bare dropout slot is, ideally including what the most flared out area is and whether the whole thing is opened up vs just one notched part, and also where the axle naturally wants to land when the chain is properly tensioned. We also need to see what the tire clearance becomes with the wheel all the way forward. Part of what this question is ultimately about is whether you can address the damaged dropout section by half-linking the chain. Nov 13, 2023 at 0:20

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It's important to figure out the root cause(s) in this situation, but there's not enough information here to do so.

If you don't figure out the first thing to have gone wrong, it's likely to repeat. The dropout is going to remain at least a little bit compromised here no matter what you do. The goal is to get to an outcome where it won't be compromised by a critical amount and things can function normally, but since any amount of slippage will chew up the dropout more, you have to do everything possible to make that not happen, which means figuring out why it happened in the first place. It can also make it difficult to tension the chain correctly.

Shimano IGH axle nuts absolutely need their proper lubrication and torque, especially on aluminum dropouts. (Lubrication of the threads is critical for thread preload). It's likely but not knowable with the information given that sometime early on in the bike's life, perhaps when initially built, the DS axle nut was ridden with inadequate thread preload and some movement occurred. This can in turn wallow out and erode the surface on the dropout, creating a divot, depression, or otherwise making the contact surface uneven. This can happen fairly quickly on aluminum frames. Once that's true, the hub can have trouble ever staying properly in place again.

Playing games with putting the technically wrong keyed washers on a Shimano IGH hub is an eyebrow-raising idea unless done for very specific reasons. I can imagine only one here: getting the hub all the way forward in the dropout, so that it contacts an undamaged or less-damaged surface, and then using a different pair of washers so the tabs can sit behind the axle while still positioning the cassette joint in an acceptable fashion for the cable routing. One of the potential fixes here, if in fact the root issue is damage to the dropout surface, is to half-link the chain, slightly altering the zone the locknut surfaces land in naturally, so that you can clamp to a previously unused section of dropout, which could then necessitate doing something like this with an unorthodox pair of keyed washers subbed in. It's often the case that a given bike with a given tooth count combination doesn't have one full link worth of adjustment to do such a thing, but can do it with a half-link.

For the most part, there is no such thing as welding-level dropout repairs on an aluminum frame, other than the relatively uncommon case of ones where everything is an alloy that doesn't require post-weld heat treatment. It's possible to imagine a framebuilder-level cold fix here that involves smoothing down the area and bonding on a thin steel surface that sandwiches the dropout and provides a surface for the nuts to tighten against. Some bikes do come with such a thing stock to prevent similar issues to what you've experienced, and mid-drive aluminum IGH bikes are something like the most appropriate place imaginable to do this.

Shimano IGH axle nuts can be a little finicky at the best of times and sometimes need replacing. They work, but the hardened and flatted axle design sometimes cuts into them and wallows out the threads, especially if they were under-lubricated or unlubricated. If by not being able to hold the wheel tightly enough you mean you get the sense it's on the way to stripping or otherwise doesn't feel solid when tightened, replace it.

Edit in response to pictures: It looks like the serrations on the hub locknuts are worn down to nothing. That, or they're full of aluminum from being able to twist against the frame. Either way they won't do their job properly and should be addressed, either by replacing them or digging out the former dropout material as applicable.

To lubricate the hub axle nuts, use grease or anti-seize.

This frame has what the chart refers to as "standard" dropouts. "Reverse" is backwards-opening, a.k.a. track dropouts. So depending on the angle, either the "7R/7L" or "5R/5L" combination is the correct one. What this really controls is the angle the cassette joint lands at, which in turn keeps the housing stop land at the correct place at the correct angle.

It's likely that the first thing to have gone wrong is the hub was allowed to twist in the frame from the axle nuts being undertightened, which then chewed a notch into the dropout from the anti-rotation washer bearing against it too hard for too long (easy to imagine on a mid-drive). The notched area will now likely have a hard time playing correctly with the anti-rotation washers no matter what you do. I think a good plan here would be see what chain length tricks you can employ to have the anti-rotation tab avoid landing against the notch, and also start with fresh locknuts and axle nuts, properly greased and torqued. I don't know of any other more sophisticated parts you could add to prevent rotation; there are a lot of similar bikes out there that get by without such a thing, so hopefully yours can too.

There's not an image of the inside surface of the dropouts, where the locknuts contact. Note if there's a divot or otherwise heavily mangled spot where the slippage/spin occurred (and there presumably is from looking at the locknut serrations), the hub will tend to have a hard time staying locked against that surface no matter what you do. This is a further reason to pursue chain length tricks.

To be clear, one of the likelier ways of doing this is to use a half-link to get the locknut contact position to an area that wouldn't otherwise be achievable. The other options are take away a whole link or change the gear combination. A half-link looks like this:

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  • Thanks a lot for the detailed response. I’ve added detail/more photos to the post on the points you raised.
    – user72619
    Nov 23, 2023 at 8:12
  • @user72619 I've added an update Nov 23, 2023 at 8:54
  • Thanks again. Would another option be to ask a frame builder to swap out the dropouts for vertical dropouts? Or is that infeasible?
    – user72619
    Nov 28, 2023 at 7:36
  • @user72619 Hot repairs (anything that involves welding) on aluminum frames typically require post-weld heat treatment, which in turn requires complete disassembly of the bike plus new paint. It's seldom done. There are exceptions, namely frames made of 7000-series alloys which don't require post-weld heat treatment, but those are uncommon, especially outside of racier bikes. As above, I do think it's entirely feasible that you could get a framebuilder to add some steel faces to the dropouts if there were a need to get back to a blank slate. Nov 28, 2023 at 7:53

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