Chain goes slack, any ideas?

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wafter

I like steel bikes and I cannot lie..
Location
Oxford
Lots of good suggestions already so I think you have enough to work with until more information comes to the fore.

To add, I have a a lazy freewheel on the router; however for some reason the chain seems to sag less on the larger chainwheel; so don't discount this ;)
 

I like Skol

A Minging Manc...
seems to sag less on the larger chainwheel

That's because the additional tension of the rear mech spring when on the larger chainring is enough to overcome the freehub drag. Likewise, mechanical advantage of running on a larger cassette sprocket will similarly mask a dragging freehub, it's easier for the hub to drive the chain around when on the smallest sprocket.
 

Sharky

Guru
Location
Kent
A 12-34 cassette is a fairly wide range and your RD would have to be up for that.

But you'll find that you have quite an overlap on gear ratios. A 34x12 for instance is very similar to a 50x18. Always best to switch to the big ring well before moving down to the smaller sprockets with the small chain ring, then chain slack wont be a problem.

I've heard the expression "Shimano Shuffle" before. Basically when speed is increasing, change up on the front and two down on the rear when you are getting past the middle sprockets. When slowing, do the opposite, down on the front and two up at the rear. You need to study gear tables to make sure that you are getting progressively higher or lower rations when you do the "shuffle".
 

wafter

I like steel bikes and I cannot lie..
Location
Oxford
That's because the additional tension of the rear mech spring when on the larger chainring is enough to overcome the freehub drag. Likewise, mechanical advantage of running on a larger cassette sprocket will similarly mask a dragging freehub, it's easier for the hub to drive the chain around when on the smallest sprocket.

Never thought of that - makes sense; ta!
 

Broadside

Guru
Location
Fleet, Hants
My money is on a dragging free hub too. I’ve had it on many bikes in the family fleet. Get the rear wheel off and remove the cassette, it should be fairly obvious. If you don’t want to open the free hub up a good squirt with wd40 works as a temporary fix.
 
OP
OP
Spiderweb

Spiderweb

Not So Special One
Location
North Yorkshire
A quick update; My friend is back from Austria now so I had a quick look at her bike last weekend.
Chain length was fine but the lower pivot, where the jockey pulley tension spring is, was seized so not preserving the tension in the chain.
WD40 to help loosen then a few drops of lube has fixed the issue.

Thank you again for all the advise.
 
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