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Broke my rear derailleur a week ago along with the hanger.

On my Spark 720 year 2015, the old hanger said 142-RWS12 IDS-SL so I ordered a new one from eBay: https://www.ebay.com/itm/SCOTT-23528...-/291980522554

Bought a new Shimano XT RD-M786 derailleur from a local shop. Both say "Shimano RD-M786 VLA" in the recess.

The parts fit together but it's clear that the reach of the derailleur will never reach the largest sprocket no matter how the adjustment screws are set, so my suspicion is that I've got the wrong hanger for the frame+derailleur combination, despite the markings being the same as the old equipment. The derailleur can't reach as close to the wheel as needed.

Which is the correct hanger?

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    Sounds like your replacement derailleur is a short cage when you needed a medium or long, or a medium when you needed a long. Measure the distance between the bolts of the two jockey wheels to be sure. It may be possible to swap the cage over if its undamaged.
    – Criggie ♦
    Commented Jul 1, 2018 at 9:51
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    thanks for the input. The bolt distance seems to be 95 mm on both the old and the new derailleur though.
    – Adam.at.Epsilon
    Commented Jul 1, 2018 at 10:14
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    When you say 'derailleur will never reach the largest sprocket', do you mean that derailleur is too far outboard and the cage will not move inboard enough, or that the derailleur is too high and strikes the largest sprocket?
    – Argenti Apparatus
    Commented Jul 1, 2018 at 10:49
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    Also, some pictures of the installed derailleur will help a great deal
    – Argenti Apparatus
    Commented Jul 1, 2018 at 10:49
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    @Adam.at.Epsilon do you have the old broken one to compare with (or parts of it)?
    – Argenti Apparatus
    Commented Jul 1, 2018 at 17:03

3 Answers 3

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Derailleur hangers are typically made from a flat plate of alloy, and are not bent to position the derailleur in or outboard.

I'm guessing, but if you have the correct hanger I suspect you have it mounted on the wrong side of the frame. Googling the serial number for images of the hanger I see the hanger mounts on the inside of the frame and there is a matching plate that mounts on the outside.

The google images I found seem to match online images of the Spark. If your replacement hanger looks like those you probably have the correct part.

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  • the outside plate did not fit perfectly. The outside plate has a circular fairing which goes into the hole on the frame. This makes me suspicious that it's the wrong hanger, somehow.
    – Adam.at.Epsilon
    Commented Jul 2, 2018 at 11:23
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    Yes it seems you have an incorrect component then. Sorry to post an answer that suggested you were doing something dumb. FYI a picture of the installed hanger and derailleur will really help you get a good answer. Also, have you tried contacting a bicycle store that sells Scott bikes or Scott Sports directly?
    – Argenti Apparatus
    Commented Jul 2, 2018 at 11:44
  • yes, I concacted a store yesterday. The mechanic suggested that the problem would be much worse if it was somehow the wrong hanger.
    – Adam.at.Epsilon
    Commented Jul 3, 2018 at 9:11
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I suspect that you have not set the cable length correctly yet or the cable has unseated at one of the frame guide points.

The hanger looks good for that frame and being a Shimano derailleur. When you say 'no matter how the adjustment screws are set', it implies that you have adjusted all three.

When you set your limiter screws all the way out and you actuate the cable with your fingers, you will quickly see the actual reach the derailleur can manage.

If the b-limit screw is limiting the range you will not reach the top as the sprockets get in the way.

However, if you can pull the cable and the derailleur can then go all the way to the top, you have too loose a cable. That means the shifter can't pull in enough cable to get it all the way. Therefor you need to shorten the cable by using the barrel adjuster(s) or re-fasten it tighter at the derailleur.

If you pull the cable with your fingers and reach a hard stop before the top, you can look into the derailleur and should see the stop hitting the limiter screw. In this scenario you can say it's out of range and in this scenario you start looking at if the derailleur may be bent or incorrectly mounted. It is also important to check to make sure that the derailleur 'hanger-bold' lug is on the correct side of the hanger stop.

At this point it is also a good idea to check the entire length of the cable to check if it had not slipped off (or out of) one of its frame guides. This can cause a loss in cable tension that will also give these same issues. You want to check that its in place and also that the ferrules are seated correctly.

To actuate the cable with you fingers, you can pull on a exposed section of cable. If there is no open cable, you can use the shifter to gear down, which will take the tension out and allow you to pull the point of entry near the shifter or the derailleur. With the change attached you can expect this to be difficult unless you are turning the cranks over to move the chain along as well.

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Finally bit the bullet and took the bike back to the shop where I bought the derailleur. The mechanic claimed that the derailleur was a bit twisted, which is strange since I took it straight out of the box. Perhaps I accidentally dropped it on the floor?

So the mechanic couldn't fix this with the adjustment screws, he took out an alignment tool which fit into the hanger and then twisted first the hanger a little. Then he fit the derailleur and twisted it a little, too. Now it works fine.

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