Chain drooping

[mw52563]

Hi
Hoping someone can help. My chain has started going loose. I've attached a video below. I've also attached a picture showing what I think is the bottom cog that's not keeping tension. When I hold it tight with my finger it doesn't do it. You can see in the video that when I go backwards the bottom cog moves forward.
Does anyone know if it needs a new spring or if there's a tensioning screw or something
Thanks
Video
Mike

 

[Ajax Bay]

when I go backwards the bottom cog moves forward.

What's the problem? Don't go (move the cranks) backwards.
When you do that you're putting more than normal tension on the RD cage which obliges by moving forward. But the tension in the chain reduces as it negotiates the two jockey wheels and by the time the tension is applied to the cassette it may not be enough to overcome the freehub ratchet resistance.
It may be this effect is exaggerated if the guide (top) jockey wheel is not perfectly aligned with the sprocket.
It may be you have a slightly sticky freehub.
ETA: Low bladed spoke count and deep section rims may also being having an adverse effect.

 

[slowmotion]

What's the problem? Don't go (move the cranks) backwards.
When you do that you're putting more than normal tension on the RD cage which obliges by moving forward. But the tension in the chain reduces as it negotiates the two jockey wheels and by the time the tension is applied to the cassette it may not be enough to overcome the freehub ratchet resistance.
It may be this effect is exaggerated if the guide (top) jockey wheel is not perfectly aligned with the sprocket.
It may be you have a slightly sticky freehub.

I more them backwards by maybe half a turn so I can "tee up" the crank to about one o'clock when I'm waiting to push off at lights or whatever. I think most people do.

 

[Tigerbiten]

I've seen that type of sag a few times.
My most common cause is that I've left it to long between oiling the chain.
If the lube dries out a little then the chain gets a little stiff though the derailleur and you get that sag.
After that it's things like the derailleur hanger is a little bent, derailleur not 100% in line with the sprockets, stiff jockey wheel, stiff freewheel, etc, etc.
Basically if it's not something simple like chain lube then you need to check out anything which can cause extra friction in the system.

Luck ..............

 

[Ajax Bay]

The end of the RD cable seems, in the image and video, to 'disappear' - at least there is no cable end visible, and goes into the cage. Does that interfere with the chain, only catching it (like a ratchet) when the chain is in 'reverse'?

 

[Paulus]

I've seen that type of sag a few times.
My most common cause is that I've left it to long between oiling the chain.
If the lube dries out a little then the chain gets a little stiff though the derailleur and you get that sag.
After that it's things like the derailleur hanger is a little bent, derailleur not 100% in line with the sprockets, stiff jockey wheel, stiff freewheel, etc, etc.
Basically if it's not something simple like chain lube then you need to check out anything which can cause extra friction in the system.

Luck ..............

I would say similar about the chain possibly needing some cleaning and a lube, but also if the shifting adjustment is slightly out that might also cause the problem.

 

[iluvmybike]

Freehub sticking?

 

[mw52563]

Hi
Thanks for all the replies
In regards to the chain its brand new.. ive lubed it multiple times.
I've just checked deraliuer alignment again for like the 20th time. It's all shifting fine.
Nothing seems to be catching.
I did change the freehub not so long ago. But the wheel turns freely and when I stop pedaling the wheel keeps going. What are the symptoms of a catching freehub?
Thanks for all the help

 

[ColinJ]

I found that there was so much friction inside the freehub on one of my bikes that it would actually try to drive the chain round when I stopped pedalling - i.e. it was trying to act more like a fixed gear bike! Obviously, a chain that is pushed just folds up and things nearly got messy...

I stripped it all down and it was fine after a good internal clean and lube.

 

[mw52563]

It's all brand new with brand news bearings and lube.. I've taken the deraliuer apart and it seems fine. So maybe it is the freehub, I'll take it apart again tomorrow

 

[mw52563]

Could it be the jockey wheels are too tight? I've lubed and changed the bushings on those as well

 

[Ming the Merciless]

You shouldn’t need to lube a brand new chain

 

[iluvmybike]

A sticking freehub can be intermittent to start with - one of symptoms is if you pedal then stop doing so the hub keeps pushing the chain forward so you get a bit of a clunk when you start pedalling again as you take up the slack. If not attended to then it just gets worse and becomes as described by ColinJ above. At that point if you put it in a stand pedal then stop the pedals, let go of them, you will see them being pushed forward by the hub. You didn't say which freehub you have but i know on some you can slightly overtighten them after servicing and this can cause drag.

 

[Ajax Bay]

I don't think we have the whole picture here. The OP has changed the freehub. Why? The OP is planning to "take apart" the freehub. That's a pretty radical step yet they come here to ask why their chain is drooping when they operate the drivetrain backwards (which most people do as a matter of course to oil the chain on the 'return' section). This seems to me to be a mismatch of expertise.
The OP has changed the bushings on their jockey wheels. Who does that and why? Maybe the replacement bushings are the wrong size.
Maybe a combination of the original lubrication and the oil applied has made the chain a bit stiff.

 

[Mr Celine]

Did no one else watch the video?

When you turn the pedals backwards like that the rear wheel shouldn't move or at worst may gradually start to rotate. The problem is in your freehub. It could be either sticking or the bearings are shot. Not all freehubs are serviceable.