Bike randomly changing gears.

[kwackers]

I changed the entire drive a few hundred miles ago, new chainwheel, cassette and chain (didn't touch the derailleur or cables).

Almost immediately the bike started randomly changing gears, up and down. For any given gear the bike will randomly change up (or down) and then a few seconds later it'll change back.
Its completely random and some gears seem worse than others.

At the side of the road I swapped out the split link (it was a U shaped type with detachable plate which I don't like anyway) and the problem stopped.
I examined the split link closely and there was a bit of play in the plate but it wasn't imo that bad, so whilst it obviously contributed I don't think it was the whole answer.

So now the problem is back and so I've had another look.
The cable seems fine - moves freely and doesn't pull as I move the bars. It looks properly seated in all the appropriate bits.
The derailleur seems free to move, the idle wheels rotate ok.
There's no side play in the cassette or chainwheel (and no obvious buckling).
Chain seems fine.
Can't see any evidence of anything snagging the cable when I'm riding.

And of course annoyingly on the stand it all works perfectly.

This is a new fault for me so any suggestions gratefully received. At the moment its a bit of an head scratcher (and bl**dy annoying to ride!)

[Brucey]

could be one of a lot of things but I would check the RD for wear. If there is slop in it you may get spontaneous gear changes as it wobbles from side to side. This happens more easily with a new chain because the new chain is not so laterally flexible as an old (worn) one.

cheers

[Mick F]

I'd go for a worn top pivot.

I have a very nice Campag Chorus that's worn at the top pivot. Still works, but indexing is shot with the symptoms you describe. Could possibly be fine with friction shifting though I haven't tried it.

PS:
Thinking about it, the problem arose in one single ride. Fine before.
I kept stopping to tweak the indexing, but it made no difference.

[NUKe]

My guess would be either a worn cable with a fray somewhere or an outer that is starting to break down

[peetee]

It's possible the cassette position isn't quite the same as the last one.

[peetee]

Does the bike have a separate deriallier hanger plate? Perhaps it isn't tight or seated incorrectly after you replaced the wheel when you changed the cassette.

[kwackers]

Does the bike have a separate deriallier hanger plate? Perhaps it isn't tight or seated incorrectly after you replaced the wheel when you changed the cassette.

No, it's a fairly 'standard' Shimano Deore setup.

The chain and cassette replacements were genuine Shimano.

Currently I'm thinking along the lines of Brucey and MickF's idea.

It has to be a case of the chain moving relative to the gear(s) - which means either the gear moves or the chain moves.
I know the gear can't/isn't moving which leaves the chain and the only reason the chain can move is if the derailleur moves it.

I'll pull it off tonight and have a look. I think I'm expecting the bush the derailleur sits on to be worn but we'll see...

[Mick F]

You don't need to pull anything off.
Hold the bottom of the jockey cage and pull outwards and push inwards,
If the top pivot bush/bolt are worn, you'll see the movement easily.

Bottom of mine moved maybe quarter of an inch, perhaps slightly more.
The gears were working fine one day, then the next, they weren't.
Probably went out of tolerance. Once there's some wear, the wearing happens exponentially more and more.

I fitted a new Campag Comp mech and instantly the indexing was spot on, and the bottom end moved hardly at all.
This was back in August 2014, and I've just looked a few minutes ago, and play is returning.
All still fine and dandy, so no issues as yet with indexing.

The Chorus rear mech did 13,000 miles (ish) before its demise.
Present Comp rear mech has done a tad over 6,000 miles ............. so I reckon past its best and beyond halfway to the end.

Moulton has a Deore XT "Shadow" rear mech now after ditching the Tiagra mech, and it doesn't have a top pivot. All the swinging is done via the parallelogram system. I would like to think that it'll last more than a standard rear mech ........... but we'll see. It's done only a couple of thousand miles so far.

[Mick F]

PS:
Just realised that the Tiagra rear mech I had also wore out the top pivot.
It had done ONLY 4,300miles.

This was my post on the subject.
viewtopic.php?f=5&t=118316&hilit=deore

[kwackers]

Moulton has a Deore XT "Shadow" rear mech now after ditching the Tiagra mech, and it doesn't have a top pivot. All the swinging is done via the parallelogram system. I would like to think that it'll last more than a standard rear mech ........... but we'll see. It's done only a couple of thousand miles so far.

Bikes done close to 25,000 miles so perhaps I should just buy a new derailleur...
I wonder if the "shadow" is a direct replacement.

[meic]

A quick temporary test and repair is to exchange the top and bottom pulleys.
The top has built in float, the bottom doesnt. The excessive wear acts pretty much the same as the float.

At 25,000 miles I would expect Tiagra pulleys to be totally worn out, even if you had serviced them fairly conscientiously.

[Brucey]

there 'shadow' 9s mechs that are a straight swap for older 9s mechs. Provided you are using a wide range cassette, they work really well. Only other snag is that most models lack a barrel adjuster on the mech.


RD-M772

cheers

[kwackers]

there 'shadow' 9s mechs that are a straight swap for older 9s mechs. Provided you are using a wide range cassette, they work really well. Only other snag is that most models lack a barrel adjuster on the mech.

Thanks for that.
I've ordered one, can't hurt and I have a sneaking suspicion the problem will go away once it's fitted.
There's no barrel adjuster on my mech anyway. Adjustment is either by the 'stepped' frame adjuster or the screw adjuster on the brakes.

[Mick F]

A quick temporary test and repair is to exchange the top and bottom pulleys.
The top has built in float, the bottom doesnt. The excessive wear acts pretty much the same as the float.

At 25,000 miles I would expect Tiagra pulleys to be totally worn out, even if you had serviced them fairly conscientiously.

The top pivot I'm referring to, it the pivot at the top where the mech as a whole bolts to the frame.
The whole mech swings backwards and forwards as you go up and down the cassette and it wears it out.

The Deore XT is bolted solidly and it doesn't swing.
The "shadow" system is the parallelogram design that follows the sweep of the cassette cog diameters.

[meic]

A derailleur which is worn at the pivots tends to have difficulty getting into gears but is pretty constant in its "wrong" position. The worn pulleys however make the chain wiggle from side to side, changing gear all the time.
I have come across this on a friend's bike (coincidentally with a Tiagra) and swapping the pulleys bought some time and confirmed what the fault was.