Rear Derailleur in horizontal position
 

Hello,

The rear derailleur on my son's CX bike is in horizontal position when it is on the smallest cog of the cassette. Also it does not shift properly anymore and cannot shift to the largest cogs anymore. It seems that the chain is too long but what is strange to me is that there has not been any problems for the past 6 months with the same chain. I checke the chain wear and that is ok. Also cassette is in good order. Could it be that the RD is damaged and that this is resulting in this horizontal position when on the smallest cog. Or any other reason and how to fix this?

Obviously it could be damaged. Would be great to see photos, guess we'll wait til you can do that.

I would like to post a photo of it. But I understood I am not able to do this now as I just became a member. The RD does not appear to be damaged to me.

How does it feel if you push down on the pulley cage? Or push/pull on the main body? Is there a distinct lack of spring tension?

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If you can uploaded them, make a comment in this thread to let us know.

push down on the pulley cage feels normal also on the main body. There does not seem to be a distinct lack of spring tension as well

Bike is all original stuff? No one has replaced the chain or changed the cassette or chain rings?

Cassette and chain ring have not been replaced and my son has been riding fine on it the for past 4 months or os. Several high intensity CX trainings without any issue

If the chain and the rd are ok the only thing i can think of is a compromised hanger or that the wheel slipped forward. For the latter to happen it would need horizontal dropouts or the dropouts and/or the rear triangle are compromised in some way.

Does the derailleur have a "B screw"? Was it accidentally unscrewed?

The "B" screw is not for chain tension.

BTW normally the RD is only mostly horizontal when your are in the large gears both front and rear. I would guess it is a spring problem.

The 'horizontal position' you refer to - I assume you mean the jockey wheels are lined up horizontaly.

As long as the chain is not dragging across itself in this position then there is nothing to worry about.
As for the trouble shifting into larger cogs, this is likely unrelated. I suspect the poor shifting begain and during inspection you saw that the derailleur was pointing straight back.

It is possible the derailleur hanger is bent, but this almost always results in the shifting going too far toward the larger cogs... unless the wheel was off and the derailleur took a whack from behind to bend the hanger in the less common outward direction.

Once you determine if the hanger is straight (bike shops have a tool for this) then you just need to ensure the cable is properly connected and running smooth, the hanger is properly tightened and the wheel is in straight and tight, then look at the derailleur setup from step 1 (check that limit screws are set properly). Then step two (set cable position/tension so the derailleur indexes with the clicks in the shifter).

Indeed, B screw is not for adjusting the chain tension (although it might bring a very small help in extreme situations on "small - small" positions if the chain is almost too long and the spring at the lower limit).
But the OP complains of:
1. "Horizontal position": if this means horizontal cage, then is good to know that unscrewing B screw will pull the derailleur cage toward the horizontal position, especially on smaller cogs.
2. "Can not shift in larger cogs anymore": Unscrewing the B screw will change geometry and bring the upper pulley closer and closer to the cogs (vertically). If unscrewed too much, then, there will be not enough space between the upper pulley and the cogs and the chain can not "find" the space to lift up (to shift up) on larger cog. It happens especially at the largest cog (or maybe the largest 2 cogs, depending on cassette type).
Furthermore, the op. said he can't feel a lack of spring tension and even if he did, I think it should create issues around the small cogs, not around the large ones, where springs tension increases.

Sure, the above is just a scenario, there are also other possible reasons.

Actually the "B" screw is there to adjust the jockey wheel to cassette clearance to the largest cog in the cassette. My Sram adjust is listed at 6mm for instance.

If the OP is say horizontal I am assuming that he mean in big gears the bottom tension wheel of his RD is pulled clear forward almost to the horizontal. That can happen when the chain is too short.

His concern is with the position when in the small cog, see his original post.:foo:

It is really hard to diagnose without pictures or at least some details on the rear derailleur and frame, but it sounds like the b-tension screw or plate is not properly hooked on the derailleur hanger. In the case of derailleurs that use a metal plate or tab that hooks onto the hanger for b-tension, it it easy to mount the derailleur without hooking that plate on the hanger. The derailleur may function ok for a while, but eventually the derailleur will rotate around the mounting bolt and cause the issues that the OP describes.

If you mean that the problem occurs in the small/small combination, that gear combination should be avoided for several reasons, among which a horizontal derailleur cage would probably be one of the less significant.

If it has been shifting fine for 6 months than the chain length is unlikely to be the problem.

Have you adjusted the cable tension? That is the most obvious first thing to look at first.

Also, the hangar could be bent.

As far as chain length, look up the manufacturer’s instructions to determine proper length if you are interested.

Looks right. Could use a clean and oil.

What's wrong with that? If the bike is in the smallest cog and small ring (if it has 2) that is fine. It can't have changed because the orientation of the pulley cage is governed by chain length. In other words it can't be in any other position. The bike is a mess, it should be cleaned.