Chain goes slack, any ideas?

[Spiderweb]

I was out cycling today with my friend Gaynor. Her set up is Shimano 105, 50/34 front and I think 12-34 rear cassette. When she is in the 34t front chainring and moves down to the smaller gears on the rear cassette the chain sags, the rear derailleur doesn’t take up the slack chain. What would cause this? Friction between gear cable inner outer? Knackered spring in rear derailleur?
I can’t see if the rear mech cable is slack as the bike has all internal cables.
Not sure what else it could be?
Everything is ok when she is riding in the 50t front chainring.

Thank you.

 

[roubaixtuesday]

Without a pic and/ or more details of the bike, some possibilities in the order I'd investigate:

1. Pivot partially seized where the derailleur attaches so it doesn't swing all the way forward.

2. Chain too long

3. Rear derailleur doesn't have the capacity for the gears.

If you can post a pic with it in big/big and small/ small and the model number stamped on the rear derailleur, that would help diagnose.

 

[Ming the Merciless]

Chain too long in the absence of cable issues.

Picture, thousand words as above.

Try pushing (by hand) derailleur back as far as possible in problem gear combinations to see if sticky cable or something else.

 

[Spiderweb]

I wish I had a pic.

The bike is a carbon Cube agree which she has had from new.
The gearing front and rear is original spec.
I’ve ridden with her many times and I think this is the first time she has had this issue so I doubt the chain is too long however I could be wrong as she rarely rides in the smaller front chainring.

The bike had a service last summer, new chain included. Surely a reputable bike shop would correctly break the chain to the correct length?

 

[fossyant]

Dragging freehub or an issue with the mech.

 

[Spiderweb]

Dragging freehub or an issue with the mech.

Could be the mech, doubt it’s the freehub as all ok in 50t front chainring.

 

[Paulus]

I would guess that the RD is either gunked up and not swinging back to take up the tension, or the spring is broken, giving the same problem.

 

[roubaixtuesday]

The bike had a service last summer, new chain included

Chain too long becomes prime suspect!

But checking the pivot rotates freely maybe too.

Get her to send you a pic in big/big and little/ little, and post them.

 

[Cycleops]

I would suspect an issue with the mech or cable. First off clean the mech and renew the cable and go from there. Grease the cable.

 

[fossyant]

Check the jockey wheels.

 

[I like Skol]

Nothing to do with the cable as she is getting all the gears ok so cable friction can be ruled out. If there were sticky cable problems then the indexing would be bad and she would struggle to move to the smallest sprockets on the cassette, but still wouldn't have a saggy chain.

If the freehub was seizing she will get chain droop at the top when freewheeling but everything will be ok when pedaling.

If the chain is saggy at the bottom then it is either a sticky mech or weak/broken spring, or the #1 suspect is chain too long.

I assume the 11-34 cassette is the original size?

 

[Spiderweb]

Nothing to do with the cable as she is getting all the gears ok so cable friction can be ruled out. If there were sticky cable problems then the indexing would be bad and she would struggle to move to the smallest sprockets on the cassette, but still wouldn't have a saggy chain.

If the freehub was seizing she will get chain droop at the top when freewheeling but everything will be ok when pedaling.

If the chain is saggy at the bottom then it is either a sticky mech or weak/broken spring, or the #1 suspect is chain too long.

I assume the 11-34 cassette is the original size?

Yes it is the original cassette.

And it can’t be the cable as all the gears are selecting correctly when she is in the 50t front ring.

As I mentioned up thread, the bike was serviced last summer which included a new chain, I’m beginning to think that they’ve not cut the chain. I originally ruled that out as the bike shop that serviced the bike in York has a good reputation.
I won’t see them for sometime now as they are heading back to their other home in Austria (nice for some), I’ll check her chain length when they return.

 

[Juan Kog]

I would guess that the RD is either gunked up and not swinging back to take up the tension, or the spring is broken, giving the same problem.

I agree with Paulus , it could be as simple as a build up of winter crud . It’s sounds like the problem I had with my geared winter bike , solution was a full clean and a dose of GT 85 .

 

[Spiderweb]

I agree with Paulus , it could be as simple as a build up of winter crud . It’s sounds like the problem I had with my geared winter bike , solution was a full clean and a dose of GT 85 .

Definitely not gunked up, the whole drivetrain is clean.

 

[HobbesOnTour]

Not a mechanic but.....

The bike had a service last summer, new chain included

Not very useful information unless we know how far / how often the bike is used

Surely a reputable bike shop would correctly break the chain to the correct length?

I admire your optimism

Given that the small front chainring is rarely used, that everything works fine in the big ring and that everything is clean, the first thing I'd check is the chain length, then spring(s). The derailleur can be checked by moving it by hand.

If it's the chain length there's a bike shop that deserves a few stern words.