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My friend's bike broke and this thing came out... From various sources I found that it's probably a cog from deraileur. He went to bike shops but none could find a replacement that's for this exact brand (and no one told him what brand it is even when he asked...). Therefore it would be great if anyone could give some pointers of what brand/size/etc so we could buy it off the internet (if you know where that would be really helpful too!)

I could also update with more info if needed, e.g. if you need the brand of the bike

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    It is definitely a cog from a derailleur, however the importance of replacing it with the exact same thing is negligible. It will be nearly impossible to identify it by that alone. More importantly how many speeds are on the cassette/freewheel? (How many gears are on the back) A picture or brand name of the actual derailleur that that broke off of would be helpful as well. – Nate W Feb 24 '16 at 16:28
  • That looks like a Jockey wheel from a rear derailleur. Could you take a picture of the derailleur and/or tell us what kind it is? Judging by the center nubbin I'm guessing it'll be a fairly old model of rear derailleur. – MPoirier Feb 24 '16 at 16:30
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    That's a jockey wheel. Your friend needs a new rear derailleur (and possibly some other things, depending on what caused the derailleur to fail). – Batman Feb 24 '16 at 16:30
  • Is that the top one or the bottom. If that is the bottom and only part that came out it might work with that wheel. Not that it would work very well but just enough to figure out if the rest of the derailleur is working. You may need a new derailleur. – paparazzo Feb 24 '16 at 19:43
  • If you don't have the rest of the rear derailleur, it may be possible to get an idea of what brand it's from by looking at the branding on the front derailleur and/or the shifters. That, combined with how many sprockets are on the cassette, should help to at least point in the right direction. – Altom Feb 24 '16 at 19:48
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It is a derailleur pulley, and fairly worn out one at that. The easiest way to find a fitting replacement is to buy a set from an aftermarket manufacturer that comes with adapter shims for different derailleurs. You can simply try different shims until you find the one that fits.

They do come in different teeth counts. From the picture it looks like this one used to have 11 teeth.

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  • Actually, it's hardly worn at all. – Daniel R Hicks Feb 25 '16 at 13:35
  • One half is hardly worn, the other half looks missing. – Emyr Feb 27 '16 at 7:57
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As the others have said, it's the "jockey wheel" from a rear derailer. If you look at the derailer there will be another, nearly-identical wheel. If you can find a replacement which fits in terms of the bolt holding it in place then you're probably fine -- it's ideal if it's the same diameter, but that's not absolutely necessary, and it's certainly not necessary to find the same brand. Likely something can be scavenged off an old bike, and often bike shops will have a stock of the wheels.

However, if the jockey wheel is in that condition the derailer itself may have been damaged and need replacement.

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