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My rear hub keeps coming loose which causes the rear wheel to wobble.

I'm pretty sure that I know how to tighten the cones properly and I've also had the bike serviced in my local shop. Every time the wheel will be fine for about 100km and then it will start to wobble again.

What could have happened to cause this, and how can I fix it? Could replacing the axle, cone nuts and bearings help?

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    Usually, if you adequately tighten the lock nuts against the cones then this is not a problem. The only other cause that I know of would be a defective hub housing that is causing the bearings to grab at intervals, the force of which can be sufficient to force the cones looser/tighter regardless of how well the lock nuts are set. (Hint: Examine the cones and cups very carefully for signs of "spalling". Also examine the axle for signs that a section of it has been stretched.) – Daniel R Hicks Dec 24 '15 at 21:04
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    Did it start suddenly - like after a crash or impact? Or was it a more gradual change ? Next time the axle is out, eyeball the thread and see if its damaged at all. Also look for a bend in the axle, this can unwind things as you roll. Hitting the problem with your wallet (ie replacing the axle and cones and bearings) may fix it, or it may not. – Criggie Dec 24 '15 at 21:14
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    (Usually, replacing cones, bearings, and axle is relatively simple and inexpensive and is worth a shot if you can't find any specific cause for the problem.) – Daniel R Hicks Dec 25 '15 at 4:32
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Either your cones are coming loose, or the bearings are absolutely fried.

I had this same thing happen to me when I was first getting into bikes.

The hub was loose, I tightened it, I went for a ride, the hub was loose again.

I finally got up the nerve to take the hub apart (yeah, I was pretty n00b back then). The cones and cups were damaged unbelievably. And a stray bearing was well on its way to eating through the axle.

You've had the bike "serviced"... did they take the hub apart?

If the cones are coming loose, you need to lock them in tighter and make sure that you're adjusting correctly (just loose enough that there's no binding. A little play is okay).

In the case that the cups and cones are completely fried (at the rate that the wheel is coming loose, they would be completely fried), you need a new hub.

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In addition to Daniels comment.
The bearings cup when damaged may stretch inside making more room for bearings. Try to see whether your wheel is closer to the frame than it was earlier.
In such case you should replace the wheel.

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    If the cups inside the wheel's hub were allowing more room for bearings, then the extra space should be taken up by tightening the cones correctly ? For it to be okay on fitting then to loosen off implies something is moving when it shouldn't. Your suggestion would fit the symptoms if the axle couldn't be set right at all ? – Criggie Dec 25 '15 at 4:51
  • If it's damaged, it doesn't stretch. It gets ground down more. – BSO rider Dec 25 '15 at 14:08
  • When bearings damaging the cup, the cup is cracks. Then and until the cup is separated it's walls from it's bottom, it continuously entering the hub. – Alexander Dec 26 '15 at 21:49
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Same problem on an old Shimano freehub. I could not get it to adjust properly over the long run. It had a lot of mileage. The eventual problem is that the cone/axle assembly had threads that were slightly worn from years of use. There are two options you can try.

Cheap Option: Use threadlocker (Loctite Blue) to ensure the cones are secured where you want them. This also has the added benefit of taking up extra space between the threads, hampering the wandering cone phenomena. But it will come undone over time.

What will absolutely work: Go on eBay and look for a hub rebuilding kit. It will include a new axle, cones, locknuts, washers, etc. This will eliminate the thread play and keep things nice and tight. It's what I did to solve my problem. That particular hub had 10 years on it, and the new axle/cone solution worked like a charm. I still use that wheelset for bad weather.

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