Rear derailleur puzzle

msw
msw Posts: 313
edited August 2010 in Workshop
I’m rebuilding an old bike and I’ve replaced the freewheel and chain. I’m now fitting the rear derailleur (same one as was on the bike before) and I can’t seem to get the position right, but I don’t know if it’s the chain length, cable tension or something else.

The problem is that the top jockey wheel is too high – the spring in the derailleur body pulls it right up until it’s touching the chainstay, and the jockey wheels aren’t vertically above one another but almost horizontal. This is in big chainwheel-small cog position. When the chain’s off it hangs down vertically as you would expect.

I’m using downtube-mounted friction shifters so problems at that end can pretty much be ruled out.

Possibilities:

Chain too long
My first thought was that the chain was too long but I’ve fitted it using the SRAM method (with no derailleur, big chainring, big sprocket + 2 links). I took another 2 links out but it didn’t seem to make any difference. I’ll continue experimenting…

Chain too short
It’s a 52-42 front chainset and 14-28 rear freewheel so I would expect it to need a fairly long chain –could it be too short? My problem with that theory is that it doesn’t seem to be the chain that’s pulling the derailleur into the wrong position, it’s the pivot spring in the body itself. If anything the problem is that there’s nothing pulling against that spring.

Not enough cable tension
It seems that what’s needed is something to pull the derailleur body down and back. Is this the job of the cable? I’ve pulled it as tight as I can in the slackest position i.e. shift lever flat against the downtube.

My favourite theory is still the too-long chain but is there something I’m missing?
"We're not holding up traffic. We are traffic."

Comments

  • amaferanga
    amaferanga Posts: 6,789
    Pictures would help.

    Are you sure you've mounted the dérailleur correctly?
    More problems but still living....
  • soveda
    soveda Posts: 306
    jockey wheels almost horizontal in which direction? if the cage is forward in big chainring-small cog that sounds like a too small chain (too much tension in the dérailleur).
  • keef66
    keef66 Posts: 13,123
    +1 A photo would help enormously.

    When you say the jockey wheels are almost horizontal, is it the lower wheel that's nearest the back of the bike?
  • John.T
    John.T Posts: 3,698
    There should be 2 springs operating in the mech. One in the top pivot (the 'B' spring) and one in the lower pivot. The 'B' spring pulls the mech downwards and the lower one turns the cage so the lower pulley is pushed back away from the front. Correct chain tension is done by the balance between these 2 springs.
    Have you got the 'B' spring adjuster set correctly. It is quite easy to damage this when fitting a mech.
    As above will help.
  • msw
    msw Posts: 313
    Thanks, pictures will be forthcoming
    "We're not holding up traffic. We are traffic."
  • msw
    msw Posts: 313
    Pictures, we got pictures. The jockey wheels weren't horizontal at all. Ho hum.

    How it was before I cleaned it up (old chain & freewheel):

    IMG_0657.JPG

    How it is now, close up:

    IMG_0832.JPG

    How it is now, wider view:

    IMG_0831.JPG
    "We're not holding up traffic. We are traffic."
  • Ben6899
    Ben6899 Posts: 9,686
    Looks like a problem with the B spring possibly - doesn't seem to be hanging properly at the Allen bolt.

    By the way, I'd describe the jockey wheels here as 'vertical'.
    Ben

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  • John.T
    John.T Posts: 3,698
    That is a rather old mech. Chain length looks OK. The problem is with the top pivot. If it has a spring it does not have enough tension. If it clamps solid then it is in the wrong place.
    Some Campag mechs have an internal B spring adjustment. This may be one. Some of the Campag users on here should be able to help here.
    Some mechs do not have a B spring but use the cage tension to push the rest of the mech onto a stop. If it is like this is the stop damaged.
  • Is it the mounting ? In the before photo the angle between the chain stay and the main derailleur body is at a distinct angle but in the after photo the angle is much less and they are much more parallel.
  • msw
    msw Posts: 313
    Ben6899 wrote:

    By the way, I'd describe the jockey wheels here as 'vertical'.

    Yeah, me too. Good thing you asked for pictures.
    "We're not holding up traffic. We are traffic."
  • chriskempton
    chriskempton Posts: 1,245
    Is it the mounting ? In the before photo the angle between the chain stay and the main derailleur body is at a distinct angle but in the after photo the angle is much less and they are much more parallel.

    +1 to this - I think from memory the plate on the back of the top pivot has some lugs that go either side of the derailleur hanger and prevent the whole thing rotating. Try loosening the top bolt and then retightening after rotating the whole rear mech into a more vertical orientation.
  • keef66
    keef66 Posts: 13,123
    That makes sense. The rear mech on my son's mtb was the same. There's a notch on the gear hanger / frame mount that engages a stick-out bit on the mech (or possibly the other way round) Either way, when correctly mounted (read at the third attempt) this engagement stops the chain tension pulling the whole thing forwards
  • Weejie54
    Weejie54 Posts: 750
    John. T wrote:
    There should be 2 springs operating in the mech. One in the top pivot (the 'B' spring) and one in the lower pivot.

    That looks like a Triomphe mech from around 1987 and I'm not sure if they had a spring in the top pivot - I rather think they didn't.

    The chain looks too short.
  • John.T
    John.T Posts: 3,698
    Weejie54 wrote:
    John. T wrote:
    There should be 2 springs operating in the mech. One in the top pivot (the 'B' spring) and one in the lower pivot.
    That looks like a Triomphe mech from around 1987 and I'm not sure if they had a spring in the top pivot - I rather think they didn't.
    The chain looks too short.
    I posted that before I saw the photos. I suspect that this mech should have some sort of stop to prevent the top section moving so far towards the chainstay which in this case is not doing so. The chain could well be too short now but this is not the reason for the angle at the top pivot. The wheel is also further back in the dropout now but this also should not affect things too much.
  • Weejie54
    Weejie54 Posts: 750
    . I suspect that this mech should have some sort of stop to prevent the top section moving so far towards the chainstay which in this case is not doing so.

    It should have a stop:

    4035823138_d9192a29dd.jpg

    It should hang more like this:

    campagnolo_TRI_VIC_6.jpg

    The adjusting screws have been taken out of the dropouts, assisting the wheel being too far back and the chain is too short.
    If the stop is positioned on the hanger properly and the other faults sorted out, it should be back in service!
  • msw
    msw Posts: 313
    Y'all are geniuses, thanks so much (esp John & Weejie). That will teach me to make sure I've definitely put back on ALL the bits I took off.

    IMG_0834.JPG

    IMG_0835.JPG

    IMG_0836.JPG

    533650384_e500ce3a3c_m.jpg
    "We're not holding up traffic. We are traffic."
  • Weejie54
    Weejie54 Posts: 750
    Glad you got it sorted. It might benefit if you replace the fork-end adjusters as it will shift better on the lower gears with the wheel forward a bit. The adjusters make it easy to replace the wheel in the right position each time.
  • msw
    msw Posts: 313
    Weejie54 wrote:
    Glad you got it sorted. It might benefit if you replace the fork-end adjusters as it will shift better on the lower gears with the wheel forward a bit. The adjusters make it easy to replace the wheel in the right position each time.

    Thanks, I will do. It only had one of these when I got it, will scout around bike shops to see if they have any.
    "We're not holding up traffic. We are traffic."
  • Weejie54
    Weejie54 Posts: 750
    SJS have them:

    http://www.sjscycles.co.uk/product-Unbr ... -11607.htm

    And Campagnolo ones available on on eBay.