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** Edit: Solved **
When riding yesterday, I experienced a lose of tension in my RD.
If I deliberately put the bike in the 9th gear (or above), whilst still in the lower chainring, the chain would become extremely loose and saggy. spilling over the chainstay.

I appreciate you’re not supposed to cross chain. But I’ve not had this issue before. Example, like when the bike is on a stand and you’re cleaning the chain, running it through all gear combinations.

I took the bike to Halfords. They suggested cleaning the drivetrain, shortening the chain and replacing the gear cable.
I’ve cleaned the parts and shortened the chain. But shortening the chain doesn’t seem logical, as I’d want the RD to move towards the rear of the bike to maintain tension.
I haven’t replaced the cable (only tried to add tension), because I thought the cable only moved the RD in/out and not towards the rear or front of the bike.

The RD was bought from Halfords last July, so it’s still under warranty. I unsure if I’ve still got the old one, so maybe I could refit it, to check the tension.
The chain isn’t worn and neither are the other parts of the drivetrain.

The bike is a 105 R7000 11 speed. 52-34 on the front and 11-40 on the rear. Chain is normally 114 links (standard no. of links in packet), now 112 links.
I’ve used this setup for many thousands of miles and never had any issues until yesterday.

Thanks for any advice.

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HoarseMann wrote:

HoarseMann wrote:

That sounds about right. When you pedal backwards, you are tensioning the chain the other way, so it won’t sag. – actually this depends how much friction there is. Thinking back, I was able to turn the pedals backwards initially, but as it got worse, the freehub drag was more than the derailleur spring could handle and pedalling backwards just caused the hanger to move fully forward, with the chain still slopped all over the chain stay.

The reason it only happens in the smaller cassette cogs is due to leverage. If the freehub is only dragging a little bit, then there’s not enough force to cause the chain to sag when it’s on the larger cogs.

Thank you.
So, to clarify should I be able to rule out the freehub body as the problem?

[HoarseMann]

That sounds about right. When

That sounds about right. When you pedal backwards, you are tensioning the chain the other way, so it won’t sag. – actually this depends how much friction there is. Thinking back, I was able to turn the pedals backwards initially, but as it got worse, the freehub drag was more than the derailleur spring could handle and pedalling backwards just caused the hanger to move fully forward, with the chain still slopped all over the chain stay.

The reason it only happens in the smaller cassette cogs is due to leverage. If the freehub is only dragging a little bit, then there’s not enough force to cause the chain to sag when it’s on the larger cogs.

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wycombewheeler wrote:

wycombewheeler wrote:

shortening the chain is logical, as it could be that the rear mech is fully extended and there is still slack in the chain, especially if the chain is capable of passing round 52 at the front and 40 at the back. That;s a lot of teeth, reducing to 34×11 so you are looking for the rear mech to absorb 47

Of course if you never had this problem before it doesn’t sound like the answer.

I’d try a bit of WD40 and grease on the rear mech pivot and spring.

Thank you.
From, memory. I would say there’s less tension in the chain (114 and 112 links cut down version) throughout the entire possible range of gears, than there was before.

[wycombewheeler]

shortening the chain is

shortening the chain is logical, as it could be that the rear mech is fully extended and there is still slack in the chain, especially if the chain is capable of passing round 52 at the front and 40 at the back. That;s a lot of teeth, reducing to 34×11 so you are looking for the rear mech to absorb 47

Of course if you never had this problem before it doesn’t sound like the answer.

I’d try a bit of WD40 and grease on the rear mech pivot and spring.

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HoarseMann wrote:

HoarseMann wrote:

If it only happens when you stop pedalling, it could be a sticky freehub. I had this. Unfortunately, my freehub was shimano, which needs a massive hex key to remove, there are lots of different versions and servicing is not really possible as it’s full of tiny ball bearings. As I was at the point of possibly getting a new rear wheel, I just sprayed a load of GT85 into it and that, so far, has worked. Done another 3k miles.

Thank you.
I’ve only checked the stop pedalling part, when the bike was on a stand. And yes, it did sag when I stopped pedalling or pedalled backwards.
Edit: From memory, pedalling backwards wasn’t possible.

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Simon E wrote:

Simon E wrote:

Shortening the chain would increase tension, which of course it at its lowest in the small-small combination. However, this won’t help if the chain is an appropriate length in the first place. It may also cause problems when you try to run it in big-big. Although you have a very wide gearing range if it goes from 34×11 to 52×40! Shimano states max casette cog for the GS medium cage 105 is 34T.

Does the RD pivot turn freely? It can start to get stiff and the bolt seize, particularly if used a lot in the wet.

Alternatively it could be the freehub sticking. Do you have another wheel you could try it with? Put a cassette on it and try it across the range of gears as before.

While you’re at it, check that the jockey wheels are in good condition and spin freely.

Thank you.
The pivot doesn’t feel perfect, and cleaning it, didn’t make any improvements.
I did check the freehub. That was replaced in May 2021, with 3490 miles on the clock. It looks in good condition, with no rusty water present. It looked/turned like new really.
No I don’t have any spare wheels.

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IanMSpencer wrote:

IanMSpencer wrote:

This is going to be a problem with either the jockey wheels being worn or the sprung movement being impeded by dirt and lack of lubrication.

I say jockey wheels because I had a bearing failure and the chain was hopping on and off the lower wheel which had massive play, semi-jamming and causing oddness including a sagging chain.

Otherwise it is the sprung movement failing to tension and winter riding will do that for you, make sure it is well lubricated and swinging freely.

You are right, shortening the chain would be silly. It probably is not the gear cable, as you rightly understand that it affects the gear change not the tension. However, of you’ve not cheated the gear cable and done a lot of miles, do check it, they have a habit of breaking in the changer itself. If you find you start fiddling with the tension to try and get the change back to its old sweet ways, that’s the main clue.

Thank you.
The jockey wheels should be in fairly good condition (2150 miles). I don’t have anything to compare them again. Apart from the old RD, if I still have it.
The spring motion of the RD (rear to forward, not side to side), doesn’t feel like it should do. But again I’ve nothing to compare it to.

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Tom_77 wrote:

Tom_77 wrote:

Replacing the gear cable would probably be a good place to start – they’re cheap and they do wear out.

Thank you.
I’ll change the cable tomorrow. It just eliminates a possibility, and generally helps with shifting. So there’s nothing to lose.

[IanMSpencer]

Good call.
Good call.

[Simon E]

Shortening the chain would

Shortening the chain would increase tension, which of course it at its lowest in the small-small combination. However, this won’t help if the chain is an appropriate length in the first place. It may also cause problems when you try to run it in big-big. Although you have a very wide gearing range if it goes from 34×11 to 52×40! Shimano states max casette cog for the GS medium cage 105 is 34T.

Does the RD pivot turn freely? It can start to get stiff and the bolt seize, particularly if used a lot in the wet.

Alternatively it could be the freehub sticking. Do you have another wheel you could try it with? Put a cassette on it and try it across the range of gears as before.

While you’re at it, check that the jockey wheels are in good condition and spin freely.

[HoarseMann]

If it only happens when you

If it only happens when you stop pedalling, it could be a sticky freehub. I had this. Unfortunately, my freehub was shimano, which needs a massive hex key to remove, there are lots of different versions and servicing is not really possible as it’s full of tiny ball bearings. As I was at the point of possibly getting a new rear wheel, I just sprayed a load of GT85 into it and that, so far, has worked. Done another 3k miles.

[IanMSpencer]

This is going to be a problem
This is going to be a problem with either the jockey wheels being worn or the sprung movement being impeded by dirt and lack of lubrication.

I say jockey wheels because I had a bearing failure and the chain was hopping on and off the lower wheel which had massive play, semi-jamming and causing oddness including a sagging chain.

Otherwise it is the sprung movement failing to tension and winter riding will do that for you, make sure it is well lubricated and swinging freely.

You are right, shortening the chain would be silly. It probably is not the gear cable, as you rightly understand that it affects the gear change not the tension. However, of you’ve not cheated the gear cable and done a lot of miles, do check it, they have a habit of breaking in the changer itself. If you find you start fiddling with the tension to try and get the change back to its old sweet ways, that’s the main clue.

[Tom_77]

Replacing the gear cable

Replacing the gear cable would probably be a good place to start – they’re cheap and they do wear out.

[Author]

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