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B Tension screw affecting chain skipping

bruce225bruce225 Posts: 129
edited May 2021 in Workshop
Hi bought my daughter a second hand MTB and put a new chain on it using the old one to measure against.Set up the rear derailleur for smooth changes but the largest cog the chain will not sit on and rides along the top of the teeth.But noticed if I back off the B tension screw i.e further away from the cogs it stops the chain skipping but soon as I wind it back so the jockey wheels are nearer the cog it starts again.Is this a chain length issue.

Posts

  • reaperactualreaperactual Posts: 1,120
    edited May 2021
    Could be a few causes that may or may not be related to the b tension adjustment.

    Could the chain be skipping over the top of the big cog's teeth rather than riding on them? If so more likely worn cassette that isn't meshing with the new chain.

    If the top jockey wheel is bouncing up and down underneath when on the big cog maybe the b tension gap isn't big enough?

    Could be the low limit screw needs unscrewing a little to allow derailleur to move inwards a bit more?

    Unlikely the chain is too short if measured off the old one unless that was too short in the first place.
  • bruce225bruce225 Posts: 129
    Thanks done all adjustments going noticed if I wind the b screw so the jockey wheel goes nearer the cogs the jockey wheel at the bottom starts skipping that's what then causes the chain not to sit correctly.
  • reaperactualreaperactual Posts: 1,120
    edited May 2021
    Can you post a pic at the point of when the chain rides up, it could help?

    So all shifting and gears performing well except for the big cog?

    What set up for you have and any idea of the normal setting for the b tension gap?

    It's unusual but are the jockey wheels okay and not sloppy, loose or worn? Any excess slop in the derailleur itself?
  • nibnob21nibnob21 Posts: 207
    If it doesn't skip when the screw is eased out a bit, like you say, and everything shifts fine like that then I'd leave it. If not I'd maybe look to the L screw limit. With the b screw wound in such that skipping occurs, I'd tweak the L screw limit to see if a clean shift can be achieved (without going too extreme obviously).

    While a worn cassette is possible, I'd be surprised if it was only manifesting as skipping on the largest cog.
  • nibnob21nibnob21 Posts: 207
    Another thing to check on a 2nd hand bike is derailleur alignment. It only takes a small knock to put one out of line.
  • slowmartslowmart Posts: 4,208
    edited May 2021
    It’s SFA to to with the b tension screw.

    1 step is to push the rear mech towards to largest ring in the rear cassette. Take a look from behind , do the jockey wheels on the rear mech align with the larger cog?

    If they do align ideally you’ll need a bike stand, put the bike in, turn the cranks and change up until the chain can’t move any further , at which point turn the tension screw on the rear mech until the chain sits squarely in the big ring and it’s sounds smooth.

    If the jockey wheels don’t align you’ll need to move the limit screw on the rear mech until the cogs align.

    Once you’ve set the limit screws you can fine tune the smooth shifting by increasing or decreasing the tension on the gear cable

    There are plenty of online tutorials on how to set up gears .

    “Give a man a fish and feed him for a day. Teach a man to fish and feed him for a lifetime. Teach a man to cycle and he will realize fishing is stupid and boring”

    Desmond Tutu
  • pilot_petepilot_pete Posts: 2,120
    edited May 2021
    bruce225 said:

    Hi bought my daughter a second hand MTB and put a new chain on it using the old one to measure against.Set up the rear derailleur for smooth changes but the largest cog the chain will not sit on and rides along the top of the teeth.But noticed if I back off the B tension screw i.e further away from the cogs it stops the chain skipping but soon as I wind it back so the jockey wheels are nearer the cog it starts again.Is this a chain length issue.

    You need to start from first principles -

    1. Ensure the derailleur hanger is straight with an appropriate tool (by eye is not usually good enough for accurate shifting).

    2. Ensure the jockey wheels aren’t excessively worn. Also ensure the cable inner and outer aren’t worn, kinked etc.

    3. You’ve put a new chain on, but what are the cassette and chainring teeth like? Excessive wear can make a new chain not run/ shift smoothly.

    4. Set the limit screws to limit derailleur movement to a fraction beyond the big cassette cog (towards the spokes) and a fraction beyond the the smallest cog (towards the dropout).

    5. Set the B tension screw with the chain on the smallest chainring and the largest cassette sprocket. The top pulley wheel should trap the chain against the biggest sprocket, and then be adjusted away until the chain just runs freely without being trapped. This means it is as close to the sprocket as possible to make for snappier shifts.

    6. You may need to reset the cable clamp and tension to achieve point 5 above. To do this shift the shifter all the way to the smallest cog and if necessary pull gently on the exposed inner cable to ensure there are no more ‘clicks’ in the shifter - so it is in the correct position for the smallest sprocket. Unclamp the inner cable and set the tension adjuster at around the mid point, screwing it in or out as required. Usually you want it slighter further ‘in’ if anything, to allow you to adjust it further ‘out’ to take up any slack once the cable is clamped. This will give you the best range of adjustment to index the shifts. Ensure the cable is routed correctly into the clamp (it can be easy to route it incorrectly and get some of the gears correctly, but not all of them). Gently pull the cable inner end to take up any slack and nip up the clamp. Check the inner for any excess slack and if necessary repeat until you have it slack-free, but not tight pulling tension!

    7. Now shift up the rear cassette a few sprockets (check the dealer manual for your particular derailleur), but three or four up from the smallest (depending on how many sprockets there are) should be about right. The derailleur should shift up one sprocket with each shift of the lever. Once you have it on the correct sprocket (corresponding with the number of shifts), now you can fine tune the indexing using the barrel adjuster - wind the adjuster out (anti-clock) to move the top pulley wheel inwards, and in (clockwise) to move it outwards. The usual method is to move the pulley wheel in towards the centre of the bike until you hear the chain ‘tinkering’ on the next biggest sprocket as you turn the pedals, then back it off (clockwise, moving the top pulley wheel outwards) until the tinkering stops. The top pulley wheel should then be almost exactly aligned vertically below the sprocket the chain is running on.

    And that should be it. Park Tool explain it well here; https://www.parktool.com/blog/repair-help/rear-derailleur-adjustment
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