Chain slipping off top cog... and I've made it worse

MTBDanno
MTBDanno Posts: 60
edited April 2017 in MTB workshop & tech
So my chain started slipping off the biggest cog on my cassette while out riding yesterday. Not sure how my derailleur has managed to get so out of whack, doesn't look bent to my eye.

Anyway, I set about trying to adjust it (cant be that hard right?) having never done it before I watched some youtube videos and first off tried to adjust the cable tension with the barrel adjuster.. only my SRAM NX rear mech doesn't appear to have one?!

So I then went on to adjust what I thought was the equivalent to a barrel adjuster, which I now realise is probably the B tension adjuster?!

Anyway, the long and short of it is that I'v probably just made the whole situation worse. Can anyone tell me if i can use the barrel adjuster on the handlebars to adjust the rear mech?

Can someone also run me through the basic principles of adjustment and tell me how I can effectively 'reset' the adjustments and start from scratch?

Thanks in advance

Comments

  • JGTR
    JGTR Posts: 1,404
    Parktools website has all the info you need, step by step instructions.
  • MTBDanno
    MTBDanno Posts: 60
    Funny you should say that! I've just found it.

    http://www.parktool.com/blog/repair-hel ... -section-6
  • FishFish
    FishFish Posts: 2,152
    Take it easy and take it step by step and work out what you are trying to do. Once you have grasped what the tension is for and what the left, right and B adjustments do then you will see it is not hard. But if this has just started then look again more closely at the rear mech and the chain line as something might have changed which is nbot due to indexing. Make sure the BB is tight and the cables are clean and free to move in the sheath.
    ...take your pickelf on your holibobs.... :D

    jeez :roll:
  • The Rookie
    The Rookie Posts: 27,812
    As the barrel adjuster changes cable tension and the b-screw is nowhere near the cable I would suggest some proper research before touching anything else!
    Currently riding a Whyte T130C, X0 drivetrain, Magura Trail brakes converted to mixed wheel size (homebuilt wheels) with 140mm Fox 34 Rhythm and RP23 suspension. 12.2Kg.
  • larkim
    larkim Posts: 2,474
    My experience has been that once you start messing inexpertly (as I have done in the past, though decreasingly so as my experience has built up) you end up just throwing everything out and have a complete hotchpotch of a mess.

    The parktools step by step is spot on - go through it systematically. Put it all back into default starting positions and you'll learn quickly what each bit does. And I'd also recommend starting with a new cable. This may not be essential, but I take some comfort in using a new cable more or less every time I disconnect the mech from the cable as I end up with a mashed cable.

    So unbolt the mech from the cable, reset the barrel adjusters at the handlebars (hold the cable between your fingers so you can feel which way the barrel adjuster does in fact adjust the cable if you don't instinctively know which way to turn it - you want it to be as long as possible before you add the couple of twists to add some fine tuning ability) and then follow what parktools says about reconnecting it, getting it adjusted to work fine in the smallest cog, then getting it adjusted to move up and down just one gear, then getting it adusted for the biggest cog on the cassette etc etc.

    Once you can see what the high and low screws actually do and you can watch them moving the rear mech inwards / outwards it all starts to make sense. Ditto for the barrel adjuster at the handle bars, play around with it once you know what the baseline is and you can visualise the changes it makes.
    2015 Canyon Nerve AL 6.0 (son #1's)
    2011 Specialized Hardrock Sport Disc (son #4s)
    2013 Decathlon Triban 3 (red) (mine)
    2019 Hoy Bonaly 26" Disc (son #2s)
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  • mattyfez
    mattyfez Posts: 638
    Yeh H and L limit screws are the first thing, they physically stop the mech from throwing the chain off either end.

    B screw, the long one, is just to keep the mech from fouling the largest cog, when the chain is on the largest cog.

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