efficiency

When I am riding, the two pulleys on the derailleur "arm" are getting pulled forward then released in a snap (the arm is swinging back and forth as I pedal). This is happening intermittently while riding, not while off bike, and it happens on flat ground (I haven't tried it on hills).

The bike is new, and in an attempt to solve the problem, the LBS has changed the chain and rear derailleur, but the problem still persists. They also checked for things that would cause chain suck, but the cranks are new so there is none of the normal wear that would cause chain suck.

What else could the problem be?

Pete Hamer

Try a different, new crankset.

DMF 

Yeah, sounds like the front rings are very worn.

The chain could be climbing the cassette, too.

Al1943

[QUOTE=DMF]Yeah, sounds like the front rings are very worn.
QUOTE]

He said it's a new bike.

Check for a tight link in the chain. Even though the chain has been replaced there could be a tight link where the chain was joined. I think this is the most common cause of the problem described.

Al

idcruiserman

+1

DMF 

He also said that they'd already changed the chain.

They could be using the wrong size quick-link, but that doesn't sound like a mistake a LBS would make.

operator

Sounds like a stiff link.

rmfnla

...and apparently didn't do a proper job of it.

Have them check where they re-joined the ends during the installation; they most likely didn't do the final "flex" that ensures the links will not bind.

Yours are binding.

Pete Hamer

[QUOTE=Al1943]Based on the limited information given in the original post I'm guessing that we aren't going to get the necessary details like type of chain, connecting pin vs. connecting link, crankset type, gear combination, frame alignment, full suspension, etc. to fully hash this one out thus my simple initial response. What I was thinking might be happening is unusualy large shift ramps/pins on the chainrings combined with a bad chainline was creating contact between the two across the bottom of the chainring. When pedaling, the shift ramps/pins try to pick up the chain and wrap it around again but the tension on the chain from pedaling was causing the chain to fall back down. I've seen this on multiple occasions on brand new bikes. The fact that it's a new bike doesn't eliminate the cranks/chainrings as the culprit.

If you read the original post carefully you'll notice that his question was, "What else could the problem be?"

DieselDan

Agreed. Not hard to fix with a chain tool.

efficiency

Thanks for the feedback. The bike is at the shop right now, but I will check into the stiff link. How do I fix a stiff link? I have a chain tool if that is necessary.

operator

https://www.parktool.com/repair/readhowto.asp?id=53

urodacus

if it is/was a stiff link, change the LBS if you still have problems. they should not send you out with a bike like that. you can fix a stiff link by flexing the chain sideways at the stiff pin. you will get greasy hands doing this.

could also be shift pins on the big chain ring (at front) picking up the chain and then dropping it back on the same ring. in which case you may have had a cheaper, wider-than-recommended chain fitted by said LBS. or a bent chain ring. or a poory adjusted chainline and/or rear derailleur. or something else. again, change the LBS if you still have problems.

Prevent mechanical malpractice, use a torque wrench.
And if that doesn't fix it, use a bigger torque wrench.
(sorry, pete)

adun111

Is the cassette working probably? The freewheel on a friend's bike was seized and so that caused the derailleur to pull forward.

Pete Hamer

If that doesn't work use a hammer.

No need to be sorry. Nothing wrong with with a little good natured razzing.

efficiency

The bike is still in the shop getting the freehub replaced under warranty. We'll see if that fixes it in a few days.

charlisity

I would ride it again in an empty lot and watch the chain. Where does the climb? Front? Back? I'd wait for it to malfunction again and stop pedaling when the pulleys are forward. Stop and see what's going on.

miamijim

In general a stiff link will not cause the problem mentioned. Chainsuck occurs when a link doesnt 'release' from the teeth on the chainwheel. A stiff link will still release. A stiff link will 'click' as it moves through the rear derailleur and will pull the cage forward when pedaling backwards.

If a link is the wrong width it will cause the problem but I dont see that as being an issue in this case.

Pete Hamer

Efficiency, did you get this problem worked out yet? What was the solution?

rmfnla

Miami sucks for most things (except bugs) but you are wrong; a stiff link will cause exactly this problem.

Fortunately, it's easily fixed (unlike the bugs).