Middle ring Chainsuck ... WTF?

[matt24777]

Been out today for the 2nd ride on my latest project and suffered from repeated, random chainsuck when in the middle ring.

Now I've never suffered with chainsuck except very occasionally before, but it happened today at least 8 time in 15 miles! Always with the chain in the middle ring but with various rear sprockets (2,3,4 or 5).

I now have some nice scratches to the chainstay half way along ! the chain seemed to just stay stuck on the ring til it got nearly back to the top!

First question reasons this happens? is it the chainring? the chain? the rear dérailleur not having enough tension?

It seems to be completely random, would do it in fits, climbing or decending.

has pee'd me right off cos I can't replicate it to see what's going on.

Suggestions?

 

[LikeClockwork]

Re:

What chainset is it Matt and how new/worn is the middle ring??

 

[shedobits]

Re:

Before you start replacing the usual suspects try comparing the "return" spring tension of the rear mech with a known good one. This was the last thing I tried after wrecking my Kleins paintwork, [it was a new XT mech too].

 

[matt24777]

Re:

the chain is new 9 speed, the rings are new Aerozine, the rear mech has just been rebuilt, but I must admit I did think the spring tension was a little on the light side.... will have to strip and see if I can go to the heavier setting or if its already there.

The only thing is its a lightly (and I mean lightly) used 8 speed cassette.

Another thing I noticed, if I pedal forwards all is quiet and slick, if I spin the pedals backwards a turn or so the chain jumps down a spocket???? weird!

Oh when you say return spring I take it you mean the one at the top of the cage? not the one around the mounting bolt?

 

[Raging_Bulls]

Re:

Jumping when pedaling backwards could be due to the position of the rear mech. No worries there, it's intended to move in one specific direction only.
It MAY be caused by bad alignment of the mech hanger, so get behind the bike and see if the cage is pointing straight down.

As for the middle chainring : check all the tooth and the areas in between for burrs, pointy edges etc. If the chain gets caught in one of those, it can also get sucked all the way up.
I had a burr on the side of one tooth and that caused the same problem a few times, but luckily I was riding an E-stay so there was no paint to damage. Still, that means that chainsuck can go further than on a normal frame and can rip my rear mech apart.

 

[shedobits]

Re: Re:

Oh when you say return spring I take it you mean the one at the top of the cage? not the one around the mounting bolt?[/quote]

Whatever should be pulling the chain off the chainring really. Compare the tension with a known good bike' tension and if it seems weak swap the rear mech. Do you know how the mech performed in its earlier life?
As I say I lost a lot of paint [and some metal] from my Klein before I tried this last. Some time later a mate had the same problem and yup it was the last thing he tried too.
We was both daft really as it's simpler to swap a rear mech than a middle ring. :facepalm:

 

[racer x]

Re:

Could be your bottom bracket axle length is to short so when your chain is bouncing up and down it just catches the stay and chainring forcing it into the stay

 

[matt24777]

Re: Re:

Could be your bottom bracket axle length is to short so when your chain is bouncing up and down it just catches the stay and chainring forcing it into the stay

seems unlikely as I have almost no marking behind the chainset, just halfway along the chainstay :facepalm:

 

[matt24777]

Re:

well I've stripped and cleaned the jockey wheels, the chain is soaking over night, I've cleaned the chain rings and closely inspected the middle ring... nothing that I would consider likely. I did file down a couple of sharp edges just in case.

Looking at the rear mech the tension in the springs is good, the only thing is the jockeys look a little odd in the cage but can't put my finger on it, has anyone got a good edge on veiw of what they should look like in a M950 rear mech?

This is an 8 speed mech I think, I'm running a 9 speed chain with an 8 speed block.... do I need to change the jockeys?

 

[worksok]

Re:

Is there any resistance to the freehub turning backwards/coasting?