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I have replaced the derailleur on my daughters Dawes Academy 26, but I think it looks wrong.

On the lowest cog (gear 8) the derailleur looks like this: enter image description here: enter image description here

I assumed that the derailleur should be more verticle. Also the small cog nearest the wheel seems to be nearly touching the rear wheel cassette.

The derailleur is a Shimano Acera, which was a like for like replacement. I also replaced the derailleur hanger too.

It does seem to work, but we haven't been for a proper ride. I'm concerned that once we are out it something will go wrong.

Can anyone give any advice?

Thanks to everyone how has replies so far. Re. the chain is the original so it shouldnt need reducing. Re. the B-screw here are a couple of photos. Not sure how useful they are. TBH adjusting it doesn't seem to do much. enter image description here enter image description here

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    Hard to tell without a close up from the back of the bike. There’s normally a screw (B-screw) in the derailleur that should be against a lug in the hanger. The photo indicates that possibly the chain is too long and you might need to remove a few links. Also look at this parktool.com/blog/repair-help/rear-derailleur-adjustment – Warren Burton Jun 26 at 13:06
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    Looks about right to me. You're in the small sprocket, which means that the derailleur cage is wrapping up a lot of chain, which is why the cage is horizontal. Shift to a bigger sprocket and you'll see it's more vertical. I agree the chain may be too long, and you may need to adjust the B screw. – Adam Rice Jun 26 at 13:45
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    Possibly chain is too long, and the bit for the B screw not on properly. – abdnChap Jun 26 at 14:09
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    The chain looks very clean. Is it new as well? – Carel Jun 26 at 14:47
  • The chain does look long, but its also in smallchainring-smallcog or maybe middle chainring. It would look better if the chain was on the big ring at the front. Try shifting into big-big and see how far forward the derailleur cage comes. – Criggie Jun 29 at 11:56
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From your picture, there are a couple of questions.

  1. Is the B screw in the correct place.
  2. Is the chain the correct length

Please watch this video on installing a deraillieur and 1 minute in, they mention the B screw and it's corrct position. Note the two different types of fittings, the direct bolt to the hanger, and the bolt to the tab and then to the hanger. The video shows both methods.

Or update the question with close ups of the area where you attached the derailleur to the hanger.

You didn't mention whether you replaced the chain too. If you did, please make sure it is the correct length. These links discuss more on chain length.

Chain length link 1

Chain length link 2

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  • Hi I have updated original question with close ups on the B Screw. The chain is the original one. – James Jun 29 at 11:32
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That might be perfectly normal, depending on a few factors. However, the position of the arm isn't really indicative of whether the derailer itself is installed correctly, but rather an indication of how long the chain is.

To determine if the chain is the correct length, what you'll need to do is shift onto the biggest cog and check the arm at that position. It should hang vertically or lean slightly forward. I prefer it slightly forward but it depends on the bike. If the derailer struggles to bend far enough forward as you're shifting onto bigger cogs, you will need an even longer chain. If, however, as is more likely the case, the derailer arm still points back even on the largest cog, then the chain is in fact too long and ought to be shortened.

Likely what you'll see is that on the biggest cog the derailer arm hangs vertically or slightly forward. That is normal and good! That arm is responsible for pulling out the slack on the chain in order to maintain chain tension, so when less chain is being used by the gears (on the smaller cogs), the arm has to pull further back to account for the extra chain not being engaged.

The installation of the derailer is really more about setting up the limit screws to block out the range of motion that the derailer can travel in, and you can't determine that from this picture. The b-tension screw is used to pull the upper jockey wheel (small gear) on the derailer away from the cassette so they don't smoosh the chain. The b-screw shouldn't be used to account for a chain that is too long or too short, ideally.

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    That's not the correct way to size a chain. The chain needs to be long enough to get on the largest chainring and rear sprocket. See these instructions. Shimano has a slightly different procedure for single chainring setups. – Argenti Apparatus Jun 26 at 21:35
  • What part of my explanation struck you as incorrect? Having sized thousands of chains in my 10+ years as head mechanic at a professional shop I feel confident in my methods. – Nathaniel Hoyt Jun 27 at 23:10
  • check out the park tool video. Either the chain on the largest sprocket and chainring the derailleur should actually be near the limit of its forward motion and the lower jockey wheel will be quite far forward - the cage will definitely not be vertical. – Argenti Apparatus Jun 27 at 23:22
  • that is indeed how i prefer it, and how i would normally go about setting it up. when i say here that the derailer arm hanging vertically is fine, that is indeed i mean: it is fine! on a bike such as this for the purposes it is likely being used, it will work without issue. a too-long chain is always far preferable to a too-short one. is it ideal? perhaps not, but i would rather this user understand that it's more than "good enough" before they run for the chain breaker and potentially make anything worse. – Nathaniel Hoyt Jun 28 at 6:49

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