Time for a new rear mech.

[PpPete]

Finally, got round to fitting my new STI's (9 sp. Sora) ..after years of being a downtube shifter luddite.

Got everything adjusted on the work stand after a little playing about. All shifting perfectly.

Quick circuit to check.... no probs on middle and big ring, drop to the granny ring for the first time.

Loud noise, end of forward motion, only just managed to unclip in time.

Check rear wheel

Fortunately hadn't gone far from home.

Took some doing to disentangle everything. Thank goodness for steel frame, the hanger could be bent back. Wheel is 1/4" out of true with a few spokes that need replacing.

But I "think" the mech is FUBAR

The limit screws were fine when same rear mech was on downtube shifters and OK so I should have double checked them after I'd adjusted everything else, but what I'm really baffled by is why it should have happened at all? I wasn't changing the rear at all when it happened, only the front.

Is there some peculiarity about STI shifters that can cause this?

 

[I am Spartacus]

From my understanding, you must have released the tension on the rear mech when swapping the shift mechs?
If so, then you would have had to set up the hi and lo limits and the indexing from scratch.

 

[jimboalee]

Some people get miles from home before something goes wrong.

PS This is what's refered to as a 'Pringled rim'.

 

[AlanW]

Is there some peculiarity about STI shifters that can cause this?

No not at all, so I'm not sure how this could have happened.

I can only assume that it was the chain that was the root cause, but I'm really guessing?

What was the alignment of the chain like prior to the "incident"?

 

[GrasB]

Could the mech have got caught in the spokes & been pulled round somehow?

 

[PpPete]

From my understanding, you must have released the tension on the rear mech when swapping the shift mechs?
If so, then you would have had to set up the hi and lo limits and the indexing from scratch.

Agree with your first sentence... but not your second. Surely the limit screws define the maximum travel of the mech, whether there is any tension on the mech or not?

 

[PpPete]

Could the mech have got caught in the spokes & been pulled round somehow?

That is undoubtedly what happened .... but why?
The (previously set) limit screw should have prevented....unless I've totally misunderstood.

 

[PpPete]

No not at all, so I'm not sure how this could have happened.

I can only assume that it was the chain that was the root cause, but I'm really guessing?

What was the alignment of the chain like prior to the "incident"?

Middle ring to middle of cassette was no more than a couple of mm out. Like I say, it ran fine on downtube shifters, noisy on big-big & small-small as one would expect, but I was always careful to avoid that anyway.

Chain had no more than 500 miles on it.

 

[GrasB]

Front mech pushes the chain towards the wheel, it then drops the chain a chainring causing slack that needs to be taken up quickly. This isn't taken up fast enough by the rear mech & a ripple of chain in the horizontal plane travels along the chain which is enough to be grabbed by a spoke. End game.

 

[gwhite]

Front mech pushes the chain towards the wheel, it then drops the chain a chainring causing slack that needs to be taken up quickly. This isn't taken up fast enough by the rear mech & a ripple of chain in the horizontal plane travels along the chain which is enough to be grabbed by a spoke. End game.

^^^^ Sounds right to me. Lack of tension on the return spring of the rear Der. causing the above sounds about right. The same thing can cause the chain to kink and foul up the front Der.

 

[PpPete]

Return spring on the old rear derailleur seemed OK to me. Even now it doesn't seem any weaker than the new mech I've fitted.

However what I did notice was that even with the L screw on the new mech as finely adjusted as I could....only just getting enough to get onto the big sprocket, the cage next to the upper jockey wheel was only millimetres from the spokes.

I can see that with a bit of whiplash in the chain, and flexing in the cage of the mech it could happen again all too easily. I'm dreading that, because I don't know whether the mech hanger will take much more being bent to and fro.

Just wondering about fitting one of those really naff looking spoke protectors behind the cassette. Any other advice gratefully received.

 

[Amanda P]

'Course, you'll have to change your avatar now...

 

[PpPete]

'Course, you'll have to change your avatar now...

Damn! Should have thought of that. Just have to buy N+1 with DT shifters now.

 

[bonj2]

did the mech snap right at the limit screws? if so check http://yacf.co.uk/forum/index.php?topic=19519.0
would be interested to know if it did

 

[GrasB]

Return spring on the old rear derailleur seemed OK to me. Even now it doesn't seem any weaker than the new mech I've fitted.

However what I did notice was that even with the L screw on the new mech as finely adjusted as I could....only just getting enough to get onto the big sprocket, the cage next to the upper jockey wheel was only millimetres from the spokes.

I can see that with a bit of whiplash in the chain, and flexing in the cage of the mech it could happen again all too easily. I'm dreading that, because I don't know whether the mech hanger will take much more being bent to and fro.

Just wondering about fitting one of those really naff looking spoke protectors behind the cassette. Any other advice gratefully received.

It may have been as strong but it may not return the arm as quickly as it should.

Could your hanger be bent inwards making the arm closer than it should to the wheel? Spoke protectors look naff but from riding quickly on rough back roads they do a rather good job of stopping your spokes getting chewed by the chain. I duno if it would have helped with this one.