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I have a Specialized Allez Elite road bike, purchased brand new in June this year. So far I've covered around 300 miles in total on it.

During my ride yesterday, as I was going up a slight hill, I switched to the smaller front chain ring. As I did this, I heard a sickening crunch, pedals jammed, looked down to find one of the gear changing parts is jammed into the back chain rings and chain has come off. Luckily I managed to get onto the footpath and dismount without any problems.

I don't know much about bikes so have provided an image of the damage: enter image description here

Can anyone advise on what parts will need replacing? Also, how can I prevent this from happening again?

Thanks

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  • You have the tag correct. That is a rear derailleur. If you don't know much about bikes I suggest you take the bike to a bike shop for repair.
    – paparazzo
    Oct 26 '14 at 12:38
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    Yeah, the derailleur disassembled itself. Most likely something jammed, and my first guess would be that the derailleur was misadjusted and got caught in the spokes. (If so you will probably see some scratches/dents on a couple of spokes.) The spoke guard you have is supposed to prevent this, but it appears to be a hair undersized. Otherwise, sometimes a chain sort of gets in a knot, especially when shifting to a smaller front. Oct 26 '14 at 13:01
  • I think you can see a slightly gouged spoke on the left, but I'm not sure so I didn't want to put it in the answer. In any case, these things happen.
    – Batman
    Oct 26 '14 at 13:06
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    I would be taking back to the shop. It's done less than 500km, and no matter what you think of Shimano, Tiagra is meant to be fairly good quality. If you didn't hit anything, then it's failed due to poor setup. Did you take it in for a "tune up" after a few rides? Did the shop suggest that?
    – andy256
    Oct 26 '14 at 13:27
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    @Batman yeah, the spokes at 12 o'clock have that "just eaten a derailleur" look about them.
    – andy256
    Oct 26 '14 at 13:32
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The derailleur is not a particularly strong part, and in fact when it/its hanger breaks it can often save your frame.

Some common reasons for it breaking are:

  • Hitting an obstacle (e.g. tree branch)

  • Going into the spokes (badly set low limit screw or the derailleur was wobbling enough for some reason to get caught in the spokes)

  • Chain too short.

What you need to do is buy a new compatible derailleur [e.g. whatever was spec'd on it before] (and possibly a derailleur hanger as well). Then you need to get the hanger alignment checked out (using something like the Park Tool DAG-2). Then, get the spokes checked to see if they have taken any damage and have adequate tension (you will likely want to ask a bike shop about this).

It looks like the protector disc (plastic) is still in tact without any damage.

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    I think a crash I had a few weeks ago may have dislodged the hanger/derailleur. It seemed fine after the crash and also did one or rides after that without any problems.
    – Mr B
    Oct 26 '14 at 17:17

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