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To start off, I have snapped my rear axle on my current bicycle four times already. Usually the wheel never "dislocated", so I was fine, but the last time I almost drove off a 10m drop because I almost lost the rear wheel. (It got stuck between the seat pole thing and was held on by the chain.)

I have no clue about bicycles though. I just bought what the dealership recommended to me at the time of purchase. (I would gladly read up & learn about that stuff, but I have the case of mountain stupid, where I do not even know what terms to look up...)

I have been riding every day on a relatively steep decline / incline (~20%) which is paved with beton (concrete) blocks, and I am quite heavy (120kg with clothes). That weight is still under the "rated for" weight of my current bicycle, but I'm guessing the axle snapping is caused by my weight.

The bicycle originally had a "tube" axle which you could tighten with a lever. Originally I replaced it with an identical one, which also snapped so I tried replacing it with a solid one, hoping that would be better but that snapped too. Twice.

I am really unsure what to do about it / what to get now, and was hoping I could get some advice of what to look for. I live in Europe, so if you are going to recommend a product it would be great if I wouldn't have to import it.

My current bicycle is from a company called "Bulls - The Ride You Want", but I couldn't find anything about them online except for a really crappy advert on YouTube.

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    Bulls is a rather large German company. bulls-bikes.com/gb-en They are one of the brand, they are part of a larger company. de.wikipedia.org/wiki/Zweirad-Einkaufs-Genossenschaft – EarlGrey Mar 18 at 14:51
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    "beton"="concrete", but that doesn't tell is what it's like to ride: it would be good to know the surface quality as well. If you're feeling the edge of every block as you cross it, so is the axle, and you can reduce the loading on it by taking your weight on your feet instead of your saddle – Chris H Mar 18 at 14:56
  • @ChrisH It is almost impossible on these concrete panel roads I am familiar with. The blocks are too close to each other - typically 3 m - and there are often small gaps between them. It is one of the worst surfaces I know. One is constantly bouncing at the gaps and hits random step-ups or step-downs unpredictably. You typically have to ride the whole panel road section out of the saddle, it is almost impossible to just unload the bike at each gap. Like this mapy.cz/… – Vladimir F Mar 19 at 8:45
  • @VladimirF I've ridden very similar stuff on my tourer, so I know what you mean. But it's possible seated, even if it's bad for the bike and uncomfortable I tend to stay out of the saddle, and not pedal all the time, at least if it's flat. I've ridden other types of concrete block roads that are much better, and some that are worse (some of the perforated ones to allow drainage). But I think we're in agreement that smashing through seated isn't going to help, for a likely range of concrete-paved surfaces – Chris H Mar 19 at 9:16
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You almost certainly have a 7 or 8 speed freewheel hub. They can do this, especially 8. Google those terms along with broken axles.

8 speed freewheel hubs had some years of prominence circa 2000-2001 and then were rejected by the industry because of these problems. They have now made a return due to manufacturers answering the pressure to fit so many other name features into a low price point. They're cynical trash.

If the above surmising is true, get a compatible cassette hub wheel and a cassette, plus a new chain.

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    To elaborate on this: On freewheel hubs, the drive-side bearing (between the hub and axle) is inboard of the freewheel. This means that the part of the axle under the freewheel, extending to the dropout, is unsupported. With 7 or 8 speed freewheels, that's a long unsupported span. Cassette hubs move the bearing outboard, so there is much less unsupported axle. – Adam Rice Mar 18 at 15:39
  • I used to have a cheap non-Shimano 8 speed cassette hub that didn't have the bearings near the side in the manner Shimano hubs had in 2008. I assume Shimano had a patent on the outboard bearing and other manufacturers wanted compatibility with cassettes without infringing on the Shimano patent. Needless to say, the axle failed. – juhist Mar 18 at 16:27
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    @twiggeh Just to avoid confusion, a "freewheel hub" is different from a "freehub" or cassette hub. – Armand Mar 19 at 2:39
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There is one other possibility that bears mentioning:

If the hub is somehow deformed, such that the bearing cups are not "square" to the axle, then the rotation of the hub can tend to "grab" the cones and turn them on the axle. This can increasingly tighten the cone and increase the tension on the axle, causing it to snap.

I had a front wheel with this problem -- broke two axles before I replaced it. After replacement I examined the hub closely and it was indeed slightly deformed -- slightly bent sideways. I suspect that this was due to some accident during manufacture or initial assembly.

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