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So I'm taking the time during the holidays to overhaul my commuter bike. The whole drivetrain was full of old oil, road salt and grime so I decided to take the whole thing apart starting with the back wheel. Once I got it off, I ended up with half of the rear axle in my hand and a bunch of ball bearings on the floor (I found them all eventually).

The bike has been in use for about 18 months, and there were no rust and plenty of lube inside the hub so I don't really think it's natural wear.

What could have caused the axle to break? My theory is that they pulled the nuts too tight when they built the bike.

More importantly, is this a "user serviceable part" or do I need to get it repaired/replace the wheel? Seems to me getting the ball bearings to stay in place while I insert the axle would be quite challenging.

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A standard axle can be easily replaced, so long as the hub has not been significantly damaged (which, alas, is a definite possibility, depending on how long the axle was broken). As to why it failed, it's hard to say -- could be a defective part, corrosion, or something else. Generally abuse alone will only bend an axle (with resulting bearing failure), not break it. (Axles and ball bearings are standard parts that most shops will carry.) –  Daniel R Hicks Dec 25 '13 at 18:14
    
You're going to hate this answer, but... sometimes it Just Happens. My husband and I have each broken an axle on our bikes in the last couple years, and as cyclists we are almost nothing alike. –  dsalo Dec 26 '13 at 20:56
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2 Answers

up vote 2 down vote accepted

Most of times the axle is broken when not accurately jumping with bike (the same is with obstacles on road like holes and speed bumps). Wheels with freewheel are more exposed to this because their bearings (i.e. the place where wheel touching the axle) far from the place where the bike touch the wheel, therefor it have more moment to brake the axle.
To the question about worth repair. The only available repair of this - is to put a new axle, that means it will be like a new wheel (if you do not have other problems in your wheel). If you going to replace the axle, examine the bearings cup if it have any deformation. If it have, in most cases the wheel must be replaces. Examine also the cones.

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You bring up a solid point regarding freewheels being more at risk for this, also I can remember one specific time I hit a bad pot-hole in the road. That might be the reason. Got a new axle today and will fit it later this week. –  dbr Dec 26 '13 at 16:48
    
What do you mean by 'accurately jumping'? Hitting too hard with the back wheel or at a bad angle? –  Meep Dec 26 '13 at 18:35
    
Another thing (related to freewheels given that most bikes which use freewheels are cheap, these days) is a poor quality axle on a cheap (say Wally world) bike, which can be an issue with heavier riders (as can jumps)... –  Batman Dec 26 '13 at 20:12
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@Meep all of things. If you put too much tension on the axle, it will brake. So if you jump while siting still on the saddle, all your mass multiplied by ... (my physics is really bad as well as my English) is being attached to the axle. The same when you landing with bad angle, there is more tension on the axle which can cause brake the axle. +1 to Batman. Heavier riders (or if putting items on a back rack) much more often breaking axles. –  Alexander Dec 26 '13 at 22:18
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Misaligned dropouts could be a cause. That along with a heavy rider and/or a heavily loaded touring bike. The last two caused me to break an axle 30 years ago.

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