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Chain tension/slack in drive line
#1
Let me open by saying how much I appreciated all the help and insight members shared with me last season. Now I'm back with a new(to me) bike and a new problem.

Upon each revolution slack develops in the chain between the freewheel and chainring. It's as if the freewheel was pushing the chain towards the chainring. No much slack develops. Advancing the crankset less than 5 degrees takes it up, but next revolution it's back. There's an audible 'click' each time the slack is taken up. Speed, load, gearing combination doesn't change this condition.

I might add that this bike was in storage for several years. I've cleaned and oiled the chain. It shifts correctly. And I have no experience with this Shimano RX100 derailleur.

What is causing this? and how can I correct/make adjustment(s)?
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#2
To clarify:
* happens once per revolution of the cranks. If this is not correct, the following are not candidates.
- pedals?
- chain ring(s)? are they flat? all teeth look good?
- BB?
- crank arms tight on the BB axle?

* stiff chain link would happen approximately every third revolution, but would not sync with the crank revolutions in most gears.

* once per crank revolution is highly unlikely to be associated with the RD or freewheel/cassette-freehub.
Nigel
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#3
Nigel, thank you for taking interest.
Yes, slack appears between the freewheel & chain ring once per revolution of the crank.
No, not in the pedals...
Chain rings are in good condition.
No 'play' in the BB-spindle vertically or horizontally...
Crank arms are tight on the taper.

The chain tension seems light compared to another on my bikes. In the photo, can you see the upper chain run sagging ever so slightly? If I push the RD body down toward a more vertical position the chain tension increases. I haven't change this adjustment yet, and I don't know what other affects might come from changing the RD angle. or, if that would even correct the condition.





In the photo, can you see the upper chain run sagging ever so slightly?
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#4
If you coast, does the slack get really bad? If so, then it sounds like your freehub (or freewheel) is not spinning freely. You may be able to get some lube into in and improve it.
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#5
Is it possible that removing some links from the chain could correct the problem? Perhaps do a chain length calculation and see if you fall within tolerances.

Just guessing here.....
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#6
Given the age of the derailleur, a possibility is that the spring has aged, and is no longer providing sufficient force.

Do you have another rear wheel that would fit that you could try out ? For a quickie test, the rear brake would not be required, so anything with the correct O.L.D. would be fine.
Nigel
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#7
Thanks y'all for the suggestions.
DaveM, I don't think so... at least not when the bike was on the stand. But I will lube the freewheel and the jocky wheels too. My bad; I neglected to do so when I lubed the chain.

RBurrelli, I too suspect that chain stretch could be involved, if not the culprit. I'll take a measure and verify it within tolerance.

nfmisso, the derailleur tension does seem light compared to a slightly older Simplex on another bike. May be I can wind another turn on the spring?
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#8
(05-31-2013, 06:14 PM)mikes105 Wrote:  ....... May be I can wind another turn on the spring?

Not likely. You may be able to make a new spring if you have access to the materials and tools to do so. Of course, when you take the derailleur apart far enough to change the spring, you probably send a few of the microscopic parts into orbit, and have to make replacements of those too.

Were you able to find any tech docs on Shimano's site for it?
Nigel
  Reply
#9
Got this one fixed! Thanks y'all for all the great advice. Turns out there is an adjuster screw for this condition. It's just below the mounting bolt on the rear of the arm. Shimano's tech document on this derailleur does give the parameters to set the adjustment, but does not mention that it controls chain tension. Also, the adjustment is counter-intuitive (to me anyway). Tightening the adjuster screw lowers chain tension. Backing the screw out increases tension.
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