Rear derailleur not shifting into lowest gear

Discussion in 'Bicycle Mechanics and Repairs' started by MrPorridge, 10 Dec 2018.

  1. MrPorridge

    MrPorridge Active Member

    Location:
    North West England
    Hello. I've just bought a Triban RC520 from Decathlon.

    However, the rear derailleur (Shimano 105) wasn't shifting into either of the lowest two gears (small chainring and biggest two cassette cogs). So having googled the problem, I did the following:

    Pushed the derailleur with my fingers to establish that the derailleur was physically capable of moving to the right position and able to select the lowest gears. It did this OK so I *think* this ruled out the limit screw as the cause of my problem.

    I then twisted the rear derailleur cable barrel adjuster anti-clockwise by several turns which I *think* increased the cable tension. This was partially successful as I can now select the second lowest gear via the shifter.

    However the barrel adjuster will now not move any further out and I still can't select the lowest gear. The chain keeps hopping on and off the biggest cog with each rotation of the cassette.

    So... my (eventual) questions are:
    1. Do I now need to undo the cable bolt and pull the cable tighter to get even more tension?
    2. If so, is this something that a mechanically incompetent person can do without special tools or will the cable somehow ping back and the whole transmission implode into a thousand pieces in a comedy fashion?
    3. Will adding tension to enable the lowest gear selection have implications elsewhere, i.e. impact higher gear selections in any way?
    Sorry for the length of the post and my obvious lack of knowledge. I've tried to google it but don't want to risk knackering up my lovely new push iron. Many thanks.
     
  2. Specialeyes

    Specialeyes Über Member

    Location:
    Essex
    If the barrel adjuster is at its maximum extent and you still can't engage the largest sprocket then you do indeed need to tighten the cable... so you were absolutely on the right lines!

    Slacken the barrel adjuster back almost all the way off and put the derailleur into the smallest sprocket, to put little or no tension in the cable, then it won't want to ping back quite so much. Undo the clamp screw, tighten the cable a bit (long nose pliers can be handy here and you can kinda roll the cable round a bit, then tighten the clamp back up.

    So to answer your questions:

    1. Yes.
    2. Yes - no it won't implode.
    3. No.

    Once you've tensioned the cable, you need to index the gears with the barrel adjuster so they run smoothly on each sprocket.

     
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  3. OP
    OP
    MrPorridge

    MrPorridge Active Member

    Location:
    North West England
    Thanks @Specialeyes

    Unfortunately, I think I've reached the limit of my mechanical competence (it's a very low limit).
    I tried loosening the cable bolt and it just won't budge - I'm really giving it some with my hex key but it's not budging.
    Also, the barrel adjuster is now impossible to move! It seemed to go anti-clockwise fine but now won't go either way without me having to use pliers - and even then it's a struggle.

    I think I probably need to book it into Decathlon for a competent person to see what the problem is. I'm bothered that I'll void any warranty if I break anything!

    Thanks again.
     
  4. Brandane

    Brandane Fair weather cyclist.

    Location:
    Ayrshire.
    To tighten rear derailleur cable, first use the shifter to select the smallest cog on the cassette, this will slacken the cable. Loosen the clamp nut holding the cable, and rotate the adjuster barrel clockwise all the way in. Then pull the cable firmly with pliers, but not stupidly tight (or you'll end up with it TOO tight and unable to drop the chain onto the smallest cog). Fine tuning should then be possible with the barrel adjuster. No doubt all that info is in the above video, but I can't see it just now!
     
    MrPorridge likes this.
  5. OP
    OP
    MrPorridge

    MrPorridge Active Member

    Location:
    North West England
    Thank you too @Brandane

    I've tried to tighten the cable several times but I just can't get any more tension in there. The result is always the same - I still can't select the lowest gear (although I seem to be consistently able to select the second lowest, which is progress - of a sort).

    The barrel adjuster seems to be doing nothing now and I in the lower gears (bigger cogs) the derailleur is sometimes doing a weird sort of "judder" thing where it looks like it's moving, as if to change gear, even though I'm not doing anything with the shifter. It was doing this earlier on but the barrel adjuster tweak seemed to stop it.

    I've probably completely knackered it. I'm tired and fed up. I would take it back to Decathlon but it was such a palaver even getting the bike off them that I'm not sure I'd trust them to be able to fix it. They'll probably say (possibly quite rightly) that I've damaged it by messing about. Still, it was a nice bike in the three days I had it until I actually tried riding it!
     
  6. Wind the barrel adjuster clock wise until it won’t go any further, put the rear onto the smallest sprocket of the cassette. Then undo the bolt holding the cable at the rear mech, and give the cable a sharp tug. Re tighten the bolt holding the cable on the mech. Then shift up the sprockets, until you can’t get it any further. Then begin to turn the barrel adjuster anti clockwise until you get the chain up over the largest sprockets. If you can’t get it up onto the biggest sprocket, shift to the next smallest sprocket, then turn the barrel adjuster a bit more anti clockwise, then try to shift to the biggest sprocket again. Repeat until it works. It’s probably just that everything has settled into place / been shaken down from the ‘factory fresh’ settings, as cables and grommets / ferrules have settled in.
     
    MrPorridge likes this.
  7. slowmotion

    slowmotion Quite dreadful

    Location:
    lost somewhere
    MrPorridge, Blue Hills and raleighnut like this.
  8. raleighnut

    raleighnut Guru

    Location:
    On 3 Wheels
    Never use pliers to turn a barrel adjuster, if it has come out of the thread in the mech completely then you need to re-engage it carefully to avoid 'cross threading' it.

    For me it'd be back to the shop with it (and don't admit to fiddling with it)
     
    MrPorridge and Cycleops like this.
  9. Cycleops

    Cycleops Guru

    Location:
    Accra, Ghana
    ∆ As above, don't mess with it. Take it straight back to Decathlon and let them sort it out. If you're not sure of what you're doing you could cause damage.
     
    MrPorridge likes this.
  10. OP
    OP
    MrPorridge

    MrPorridge Active Member

    Location:
    North West England
    Thanks for the advice. Unfortunately, I think it's too late.

    The barrel adjuster does not actually have any thread as such but there's a threaded end of a cable that I think the barrel adjuster tightens/loosens. This has indeed come out of the mech and I can not get it to screw back in so I think I may have cross-threaded it. I've taken a photo of the bit that won't go back (with the barrel adjuster pulled back - it's on some kind of spring).

    Not that it matters. Feeling pretty sick that I've busted my new bike. Was panicking about the shifts as I needed to commute on it yesterday (wife uses car) and as a result of being in too much of a hurry, I think I've ****ed it up. So annoyed with myself.

    45547565774_6786c793f5_b.jpg
     
  11. Blue Hills

    Blue Hills ^

    Location:
    London
    Looks good, very calm and methodical.

    Must admit that I had doubts before hitting play though as he makes an initial cardinal error.

    Never undertake even the most apparently minor job on a bike wearing beige pants.
     
  12. Cycleops

    Cycleops Guru

    Location:
    Accra, Ghana
    Don't panic @MrPorridge! You haven't busted it. Easy mistake to make if you're not used to working on bikes.
    Take it back to the shop, Decathlon have very good customer service and will sort it for without a mention of blame.
    (It's an easy fix)
     
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  13. raleighnut

    raleighnut Guru

    Location:
    On 3 Wheels
    ^^^^^ yep, easy without the cable in to re-set the adjuster then re-fit the cable
     
    MrPorridge likes this.
  14. Smokin Joe

    Smokin Joe Legendary Member

    You need to unclamp the cable to screw that in. It is under tension at the moment and that makes it difficult to line the threads up.
     
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  15. Specialeyes

    Specialeyes Über Member

    Location:
    Essex
    Just chipping back in to say... best photo of mechanical problem ever :notworthy:: clear, well lit, in focus and annotated.
    Chapeau! And you'll have it fixed in no time.
     
    MrPorridge, Blue Hills and Cycleops like this.
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