Bigger gear not changing

CyclingObsession
CyclingObsession Posts: 314
edited May 2012 in Workshop
I have an issue with my bigger gear it wont shift it is as if it is thinking it's on the last gear but I had a look and I can see there is one more gear, Is there something I need to adjust? I have shimano gear set

Comments

  • smidsy
    smidsy Posts: 5,273
    Did it work before? Have you changed anything? Front mech or Rear?

    Could be the limit screw on the mech.

    Need more info to help you.
    Yellow is the new Black.
  • Yes it worked before the bike is just over 2 months old, I adjusted the black screw at the back to c if that did anything and did not work, THe screws are all tight, The chain is a new one could it be just slack in the chain?
  • smidsy
    smidsy Posts: 5,273
    edited May 2012
    The black screw at the back? If you are talking about the barrel adjuster that simply alters the amount of tension in the cable. Correct adjustment with that will make the gears shift smoothly between cogs but it does not change how far the mech moves in and out.

    If the mech is not shifting far enough over to the inner or outer limit you need to alter that with the high or low screw accordingly. Turning these alters how far the body of the mech moves and hence it needs to allow the chain to move far enough to reach the cog you can not currently access.

    If it worked before it is unusual that it has stopped now (assuming the mech has not been bent). The only way it changes is with the limit screws.

    Youtube is your friend - search for mech adjustment or tuning gears.

    The workshop section on here also has vids of how to do this. Look under the 'Bikes & Gear' Tab in blue at the top of the home page.
    Yellow is the new Black.
  • Cheers for that will check it out
  • thecrofter
    thecrofter Posts: 734
    If it used to be OK but isn't now. Then before you start adjusting screws, try cleaning all the pivot points on the rear mech as best you can, then lightly oil them and remove any excess oil. Remove the cable from it's routing and slide the outers away from where they normally sit on the cable then clean and oil the inner cable, run the outer up and down a few times and them put it all back on. A sticky cable is the most likely cause of your problem (if it was OK but now isn't).

    If it's never been right then you need to adjust the stop screws.
    You've no won the Big Cup since 1902!
  • Sounds like to me that initially the problem was cable stretch, you said it wouldn't go into the biggest sprocket which requires the most tension to get there. If the bike was new you would expect some cable stretch and you could fix it by simply tightening up the barrel adjuster. If however you adjusted the limit screws rather than the adjuster you would have changed the maximum amount of movement in derailleur so no matter how much tension you have in the cable you'll never get there. It is fairly low-tech maintenance job to fix; get the bike up on a work stand or turbo so you can peddle the bike with your hands whilst making adjustments. Youtube is the best place to get info, but the way I do it, to set the limit screw for the smallest sprocket I completely disengage the cable so there is zero tension, then tweak the screw 1/4 turns at a time until it is seated correctly on the sprocket (i.e. not dropping off the bottom or trying to change to 2nd smallest gear). For the largest sprocket, I reengage the cable, shift as far as it goes (i.e. try put the chain onto biggest sprocket/easiest gear), then add extra tension by pulling on the cable with my hand. Again I tweak 1/4 turns at a time until it is right. Important to make sure than putting max tension on the cable doesn't pull the chain off at the top or the derailleur into the spokes. Once that is all done, return chain to smallest sprocket, open up the barrel adjuster as far as it goes (so any future turns add more tension, not less), with your hand take up any cable slack , fix the cable in place then start peddling slowly, making small adjustments on barrel adjuster until the gears shift smoothly between smallest 2 gears. It isn't really necessary to repeat this for other gears as one click on the STI moves the derailleur a set amount so if it shifts smoothly between the bottom 2 it should shift smoothly for all the gears.
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