Chain jamming *in* rear derailleur

[Andrew Cooke]

Hi,

Has anyone else experienced the following (and knows how to fix it)?

Three times now (the last two while riding on a fairly rough surface at speed, so with a fair amount
of vibration) the chain has jumped off the outer wheel of the derailleur and jammed between that
wheel and the cage frame. By outer wheel, I mean the toothed wheel nearer the ground (and not the
other one nearer the rear wheel hub; together these two wheels form the chain into an S shape) -
what Shimano calls the "Tension Pulley Unit" at
http://bike.shimano.com/product_images/RD/ev_images/RD_M330-8_EV.pdf

It sounds impossible, but if you look at an Acera derailleur, the frame is actually quite wide where
the chain leaves (apparently on purpose, the sides of the cage are pressed with a step shape to
increase the width).

First time I stripped and cleaned the derailleur. This evening I've done the same, but also bent the
cage sides a little to reduce the gap.

I can't believe this is normal - I know this is a fairly cheap component by some people's standards,
but in my experience Shimano's moderately priced stuff works well when correctly adjusted (as far as
I can tell, this is correctly adjusted - which is why I'm asking for help!).

Probably unrelated, but the whole derailleur feels a bit stiff when shifting to larger sprockets.

Thanks, Andrew

--
http://www.acooke.org/andrew

 

[Michael Dart]

"andrew cooke" <[email protected]> wrote in message
news:[email protected]...
> Hi,
>
> Has anyone else experienced the following (and knows how to fix it)?
>
> Three times now (the last two while riding on a fairly rough surface at speed, so with a fair
> amount of vibration) the chain has jumped off the outer wheel of the derailleur and jammed between
> that wheel and the cage frame. By outer wheel, I mean the toothed wheel nearer the ground (and not
> the other one nearer the rear wheel hub; together these two wheels form the chain into an S shape)
> - what Shimano calls the "Tension Pulley Unit" at
> http://bike.shimano.com/product_images/RD/ev_images/RD_M330-8_EV.pdf
>
> It sounds impossible, but if you look at an Acera derailleur, the frame is actually quite wide
> where the chain leaves (apparently on purpose, the sides of the cage are pressed with a step shape
> to increase the width).
>
> First time I stripped and cleaned the derailleur. This evening I've done the same, but also bent
> the cage sides a little to reduce the gap.
>
> I can't believe this is normal - I know this is a fairly cheap component by some people's
> standards, but in my experience Shimano's moderately priced stuff works well when correctly
> adjusted (as far as I can tell, this is correctly adjusted - which is why I'm asking for help!).
>
> Probably unrelated, but the whole derailleur feels a bit stiff when shifting to larger sprockets.
>

Is the cage smooth in its travel? If it is hanging up the chain can go slack and be allowed to jam
in strange places. Also is that pulley excessively worn? You might consider springing for at least
an LX derailleur.

Mike

 

[James Connell]

andrew cooke wrote:
> Hi,
>
> Has anyone else experienced the following (and knows how to fix it)?
>
> Three times now (the last two while riding on a fairly rough surface at speed, so with a fair
> amount of vibration) the chain has jumped off the outer wheel of the derailleur and jammed between
> that wheel and the cage frame. By outer wheel, I mean the toothed wheel nearer the ground (and not
> the other one nearer the rear wheel hub; together these two wheels form the chain into an S shape)
> - what Shimano calls the "Tension Pulley Unit" at
> http://bike.shimano.com/product_images/RD/ev_images/RD_M330-8_EV.pdf
>
> It sounds impossible, but if you look at an Acera derailleur, the frame is actually quite wide
> where the chain leaves (apparently on purpose, the sides of the cage are pressed with a step shape
> to increase the width).
>
> First time I stripped and cleaned the derailleur. This evening I've done the same, but also bent
> the cage sides a little to reduce the gap.
>
> I can't believe this is normal - I know this is a fairly cheap component by some people's
> standards, but in my experience Shimano's moderately priced stuff works well when correctly
> adjusted (as far as I can tell, this is correctly adjusted - which is why I'm asking for help!).
>
> Probably unrelated, but the whole derailleur feels a bit stiff when shifting to larger sprockets.
>
> Thanks, Andrew
>
> --
> http://www.acooke.org/andrew

what gear you using?? small ring/ small cogs on a wide range tranny don't leave enough tension on
the chain. you don't say under what conditions this problem happens, most common would be down hill
in a small gear ( pedaling every now and then. if it happens on the road under lite riding chech
that the cage isn't buggered/bent/twisted between the upper and lowwer pulleys. as for the shifting
problem, how old is your bike and a you sure the LBS lubed the cables when they assembled it?
--
James Connell

 

[Chris]

> It sounds impossible, but if you look at an Acera derailleur, the frame is actually quite wide
> where the chain leaves (apparently on purpose, the sides of the cage are pressed with a step shape
> to increase the width).

That's it right there, my man..."Acera" is the symptom and the disease. For as little as $28 US, you
can never have this problem again - http://www.jensonusa.com/store/product.asp?number=14050.

Chris

 

[Andrew Cooke]

James Connell <[email protected]> wrote in message news:<[email protected]>...
> what gear you using?? small ring/ small cogs on a wide range tranny don't leave enough tension on
> the chain. you don't say under what conditions this problem happens, most common would be down
> hill in a small gear ( pedaling every now and then. if it happens on the road under lite riding
> chech that the cage isn't buggered/bent/twisted between the upper and lowwer pulleys. as for the
> shifting problem, how old is your bike and a you sure the LBS lubed the cables when they
> assembled it?

I can't remember the details, but I'm normally pretty careful to avoid extreme gear combinations and
I think I would have noticed if it was small ring/small sprocket. And thinking about where I was
riding, the first time it occured would have been the large ring, as it was coming down a road from
the mountains here; the later times it would have been on the middle ring as I was on the flat and
going pretty quick. Both times it was bumpy (first time broken up tarmac, later times packed earth)
- this isn't happening on smooth surfaces.

Again, as far as I can tell, there's no twisting. I did hit a rock, a couple of months back now,
that broke the rear drop-out (had to order a new one from the States; no-one in Chile stocked
them...). Maybe that damaged it. It looked OK. I'll check again. Travel is smooth and the plates
forming the cage seem flat.

I wouldn't be surprised if the bike shop didn't lube the cables (S American service isn't exactly
reliable) - I've kept them lightly oiled, but I'll loosen everything up and get some grease inside
the tubes. It's maybe 6 months old and gets a fair amount of use on and off-road (me on the weekends
for fun, my partner to get to work during the week).

Thanks to everyone who posted (I'm replying to all suggestions here). That's given me a few things
to try. If necessary I'll spend money to get something better, but I'd rather understand what's
wrong and fix it - it's always a pleasure to pass people on fancier bikes ;o)

Cheers, Andrew

--
http://www.acooke.org/andrew

 

[Andrew Cooke]

Ebarassing to relate, but I believe this was down to a lack of oil on the pins in the
"parallelogram" part of the derailleur. I'd stripped, repacked and rebuilt the cage/pulley wheels
and greased the cables, but somehow overlooked lubing the derailleur "body". Once I did oil that,
the stiffness disappeared. And I've just returned from a ride without having the chain jam so
hopefully that is cured too (that may have been helped by narrowing the cage slightly).

It's also cured an annoying intermittent click that I thought was coming from the front chainset...
Ah, the interconnectedness of all things Shimano!

Cheers, Andrew

[email protected] (andrew cooke) wrote in message
news:<[email protected]>... [...]
> Three times now (the last two while riding on a fairly rough surface at speed, so with a fair
> amount of vibration) the chain has jumped off the outer wheel of the derailleur and jammed between
> that wheel and the cage frame.
[...]
> Probably unrelated, but the whole derailleur feels a bit stiff when shifting to larger sprockets.