tandem drive chain problem

Discussion in 'Cycling Equipment' started by Jim, Jun 26, 2003.

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  1. Jim

    Jim Guest

    I recently had a bike shop replace the drive chain, rear derailleur, and chainring on my tandem. A
    recurrent problem has developed when shifting from the 16t cog to 14t cog: the drive chain derails
    from the new chainring. Is this a symptom of the chain length being too long? I've adjusted the
    derailleur tensioner screw (sram 7.0) and it has no effect. This is a bike friday tandem with
    internal hub, so there is only a single chainring.
     
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  2. David Ornee

    David Ornee Guest

    "Jim" <[email protected]> wrote in message news:[email protected]...
    > I recently had a bike shop replace the drive chain, rear derailleur, and chainring on my tandem. A
    > recurrent problem has developed when shifting
    from
    > the 16t cog to 14t cog: the drive chain derails from the new chainring. Is this a symptom of the
    > chain length being too long? I've adjusted the derailleur tensioner screw (sram 7.0) and it has no
    > effect. This is a bike friday tandem with internal hub, so there is only a single chainring.
    >
    Jim, It could be a lot of things. Here are some things to consider: Chain too long. You can shorten
    it to be just safe in the largest cog. Chainline with too much bias towards the inside/larger cogs.
    This may have been the case all along, but could be adjusted if your bottom bracket provides for
    adjustment/ New chain not limber enough in the lateral direction.

    David Ornee, Western Springs, IL
     
  3. Jim,

    It could also be a horizontal oscillation in the chain which occurs sometimes when you shift.

    I have a Moulton with single front chainring, 7-speed cluster and 3-speed SRAM hub. Had problems
    with dropping the chain off the front chainring. Some research indicated that the front derailleur
    blades (which of course I don't have since the 3-speed hub does the job) damp this sort of chain
    oscillation and prevent the chain dropping off.

    I brazed up a small forked device from a piece of tubing and two lengths of welding rod which fixed
    the problem. It is mounted to the seat tube bracket for the (unused) front derailleur, and straddles
    the chain with a leg on each side, about 1.5" behind where the chain goes onto the chainring. The
    legs are adjusted so they just barely clear the side plates of the chain when it is on the largest
    and smallest cogs. I could also have used a conventional front derailleur, with the limit screws
    adjusted to fix the desired position permanently.

    After making this modification, I realized why Moultons (which traditionally don't have front
    derailleurs), traditionally DO have a plastic ring on the outside of the chainring which extends a
    bit above the upper edge of the chain. My wife's bike has this arrangement and she has never
    experienced the dropped chain problem. Evidently the damping only needs to be applied to one side of
    the chain to do the job.

    Martt

    PS: I can send a picture if it will be helpful. E-mail me at [email protected]

    ------------------

    David Ornee wrote:
    >
    > "Jim" <[email protected]> wrote in message news:[email protected]...
    > > I recently had a bike shop replace the drive chain, rear derailleur, and chainring on my tandem.
    > > A recurrent problem has developed when shifting
    > from
    > > the 16t cog to 14t cog: the drive chain derails from the new chainring. Is this a symptom of the
    > > chain length being too long? I've adjusted the derailleur tensioner screw (sram 7.0) and it has
    > > no effect. This is a bike friday tandem with internal hub, so there is only a single chainring.
    > >
    > Jim, It could be a lot of things. Here are some things to consider: Chain too long. You can
    > shorten it to be just safe in the largest cog. Chainline with too much bias towards the
    > inside/larger cogs. This may have been the case all along, but could be adjusted if your bottom
    > bracket provides for adjustment/ New chain not limber enough in the lateral direction.
    >
    > David Ornee, Western Springs, IL
     
  4. Jim

    Jim Guest

    David, Martt- Thanks for your replies. Yesterday I measured the chain length using the sram
    instructions (wrapping the chain around the largest cog & chainring) and found that the chain was
    four links longer than the specs. Being conservative, I removed two links and we rode the tandem
    this morning. The chain no longer jumped off the chainring with a lot of rigorous shifting except
    once on the highest 8 speed gear. Previously it had jumped commonly when shifting the middle cogs.
    So, I think that removing one more link may solve the total problem. This bike has no bottom bracket
    eccentric adjustment; the timing chain adjusts through lengthening the frame where the tubes come
    apart for disassembly. The horizontal oscillation theory is interesting and I will take a closer
    look at Martt's fix if the chain tightening doesn't fix it.
    ***********************************************
    "Martt Harding" <[email protected]> wrote in message news:[email protected]...
    > Jim,
    >
    > It could also be a horizontal oscillation in the chain which occurs sometimes when you shift.
    >
    > I have a Moulton with single front chainring, 7-speed cluster and 3-speed SRAM hub. Had problems
    > with dropping the chain off the front chainring. Some research indicated that the front derailleur
    > blades (which of course I don't have since the 3-speed hub does the job) damp this sort of chain
    > oscillation and prevent the chain dropping off.
    >
    > I brazed up a small forked device from a piece of tubing and two lengths of welding rod which
    > fixed the problem. It is mounted to the seat tube bracket for the (unused) front derailleur, and
    > straddles the chain with a leg on each side, about 1.5" behind where the chain goes onto the
    > chainring. The legs are adjusted so they just barely clear the side plates of the chain when it is
    > on the largest and smallest cogs. I could also have used a conventional front derailleur, with the
    > limit screws adjusted to fix the desired position permanently.
    >
    > After making this modification, I realized why Moultons (which traditionally don't have front
    > derailleurs), traditionally DO have a plastic ring on the outside of the chainring which extends a
    > bit above the upper edge of the chain. My wife's bike has this arrangement and she has never
    > experienced the dropped chain problem. Evidently the damping only needs to be applied to one side
    > of the chain to do the job.
    >
    > Martt
    >
    > PS: I can send a picture if it will be helpful. E-mail me at [email protected]
    >
    > ------------------
    >
    > David Ornee wrote:
    > >
    > > "Jim" <[email protected]> wrote in message news:[email protected]...
    > > > I recently had a bike shop replace the drive chain, rear derailleur,
    and
    > > > chainring on my tandem. A recurrent problem has developed when
    shifting
    > > from
    > > > the 16t cog to 14t cog: the drive chain derails from the new
    chainring. Is
    > > > this a symptom of the chain length being too long? I've adjusted the derailleur tensioner
    > > > screw (sram 7.0) and it has no effect. This is a
    bike
    > > > friday tandem with internal hub, so there is only a single chainring.
    > > >
    > > Jim, It could be a lot of things. Here are some things to consider: Chain too long. You can
    > > shorten it to be just safe in the largest cog. Chainline with too much bias towards the
    > > inside/larger cogs. This may
    have
    > > been the case all along, but could be adjusted if your bottom bracket provides for adjustment/
    > > New chain not limber enough in the lateral direction.
    > >
    > > David Ornee, Western Springs, IL
     
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