tandem drive chain problem

"Jim" <[email hidden]> wrote in message "]news:[email hidden]...

Quoted message said:

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

Quoted message said:

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

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 hidden].

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

David Ornee said:

"Jim" <[email hidden]> wrote in message "]news:[email hidden]...

Quoted message said:

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

Quoted message said:

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

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 hidden]> wrote in message "]news:[email hidden]...

Quoted message said:

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 hidden].

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

David Ornee said:

"Jim" <[email hidden]> wrote in message "]news:[email hidden]...

Quoted message said:

I recently had a bike shop replace the drive chain, rear derailleur,

and

Quoted message said:

Quoted message said:

Quoted message said:

chainring on my tandem. A recurrent problem has developed when

shifting

Quoted message said:

Quoted message said:

from

Quoted message said:

the 16t cog to 14t cog: the drive chain derails from the new

chainring. Is

Quoted message said:

Quoted message said:

Quoted message said:

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

Quoted message said:

Quoted message said:

Quoted message said:

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

Quoted message said:

Quoted message said:

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