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Some days ago, I was riding my bike towing a child transporter. I was going steep uphill and put quite some force on the pedal. Suddenly, I heard a loud snapping sound and the back wheel was blocked. After dismounting the bike, I found that the back wheel actually had jumped out of the rear dropout. I have a Shimano Alfine with a solid axle and had no tools with me to tighten the axle nuts.

Luckily, nothing worse has happened than that I was standing in the middle of a forest below 0 °C with my two year old daughter in the child transporter and a bike with a loose back wheel. The way back to civilization was not too far, so we managed all right, but I would like to prevent this from happening again.

I have been using the child transporter on a regular basis for a year now, and nothing similar happened before. I recently (two weeks ago) switched to the Alfine hub from a derailleur setup which had a quick release (might be relevant for this case). I am very sure that I had tightened the axle nuts enough when mounting the back wheel.

What I suspect is that the force exerted by the child transporter in this steep uphill passage was big enough to rotate the bracket holding the shaft of the child transporter counterclockwise, thus partly loosening the axle nut and releasing the back wheel.

Now my questions are: Do you think that this explanation is correct? Have you made a similar experience? And most importantly: What can I do that this will not happen again? Thank you for your help.

Update after comments and Chris H´s answer below: I had a closer look at the bracket as well as the green washer on the left side, see photos below. The bracket has some grooves on the inside which are probably meant to increase friction when mounted directly on the dropout without washer, like when using a quick release. The green color from the washer is rubbed off where it was in contact with the bracket. So there is evidence that there was some movement going on and that this is probably the critical spot.

Update after doing some further research: The way I mounted the bracket on the axle of the back wheel together with an internally geared hub is not how it is recommended by the child transporter manufacturer and should not be reproduced by anybody. Instead, they have an adapter which replaces the axle nut on the left side for this purpose (https://www.thule.com/en-gb/accessories/thule-internal-hub-hitch-adapter-shimano-_-20100797). This is the way I will go, and this should be considered when changing from a derailleur setup where there are no washers to an internally geared hub which needs the non-turn washers.

Now some photos in order to support the descriptions above: To give a detailed impression of how the child transporter is attached to the bike, here are some photos. First an overview of the situation:

Overview of bike with child transporter

Close-up of the bracket holding the shaft of the child transporter:

enter image description here

How the bracket is mounted on the left side:

enter image description here

How the back wheel is fixed on the right side:

enter image description here

Inside of the bracket holding the child transporter shaft: enter image description here

Green washer after having been used in contact with the bracket: enter image description here

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    Rule 1: Carry some tools. – Daniel R Hicks Dec 7 '19 at 22:35
  • What are the bright green and blue parts? I'm especially concerned with the green one in between the trailer bracket and the frame. Certainly my trailer bracket shouldn't have anything between it and the frame. – Chris H Dec 8 '19 at 7:51
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    @ChrisH Those are anti-rotation washers for the internal hub. Unless the bracket has a feature that is supposed to engage the frame for its own anti-rotation purposes, the washers shouldn’t cause any problems. – RLH Dec 8 '19 at 8:10
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    I suspect there's not enough friction between the washer and the bracket, so torque on the bracket can loosen the nut. – Chris H Dec 8 '19 at 9:28
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    Being able to take a bike on good public transport is so rare that many of us have never been able to - riding home is often the only way – Chris H Dec 8 '19 at 13:43
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The green anti-rotation washer is at least part of the problem. Its purpose is to prevent the nut rotating easily , but with the bracket in between, it can't, and any torque on the bracket is transferred to the nut.

I expect you'll have better results if you put the washer outside the bracket; this works on mine with QR skewers. Even omitting the washer might be better than what you've got now. With an extra part in there you may need to do things up a little tighter than normal in addition.

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  • You are probably right that the green washer is important, see the information I added to the question. When installing the washer, I followed the order proposed by Shimano (see page 17 in manual: si.shimano.com/pdfs/dm/DM-SG0004-04-ENG.pdf). It is stated there that "when installing a part such as a mudguard stay to the hub axle", it should go between non-turn washer and nut. It does not specify anything about my case, but I treated it similarly. – Snijderfrey Dec 8 '19 at 12:45
  • To my understanding, the non-turn washers on both sides should also prevent the axle from rotating because otherwise you will lose the adjustments for the shifting. I guess I will just have to experiment a bit with the setup. – Snijderfrey Dec 8 '19 at 12:46
  • A mudguard stay shouldn't ever exert a torque,so that is different, but it wouldn't surprise me if Shimano said don't pull a trailer with it, which is silly. Do the washers have tabs that engage with a groove in the axle? It would be hard for them to stop it rotating otherwise – Chris H Dec 8 '19 at 13:04
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    You might be able to guess from the last photo above that the cross-section of the axle is not circular but flattened on two sides. The washer has exactly this shape and additionally protrudes into the dropout, so the axle cannot rotate. Therefore I am not sure if it is a good idea to swap washer and bracket, but I might give it a try anyway. I now have a 'reference hill' which made the construction fail once, so I can test other ways of mounting the wheel there. – Snijderfrey Dec 8 '19 at 13:29
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    Your answer brought me on the right track, so I´ll mark it as accepted answer. I found an adapter by the child transporter manufacturer which should solve the issue (see edit in the question or thule.com/en-gb/accessories/…). – Snijderfrey Dec 8 '19 at 16:23

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