Front mech keeps throwing the chain.

[Andy in Germany]

The Elder Son's bike needed some serious repairs including changing the crankset. This is now done, swapped for an idential newer unit and a new chain, cassette and rear mech but now the (original) front mech consistently throws the chain when changing into the big ring. Attempts so far to resolve this have included:

• Adjusting the adjuster screws. Ineffective; either the gears wouldn't change or it threw the chain again.
• Tensioning the cable. Ineffective; same as above.
• Lowering the mech to within 1mm of the big ring: No noticable difference.
• Turning the mech so the tail was towards the frame: Ineffective.
• Turning the mech so the tail was further from the frame: Some improvement; the chain now seems to stay on, but it still tends to go too far before slipping back into place.

Any other suggestions for what may be causing this or how to deal with it?

 

[Cycleops]

Has the new chainset got the same offset as the old one?
Try moving the front derailleur by hand without the cable attached.

 

[Mike_P]

Double or triple?

 

[Andy in Germany]

Has the new chainset got the same offset as the old one?
Try moving the front derailleur by hand without the cable attached.

If there was a difference it was no more than 1 mm. The chainset was identical apart from colour; I had a box full to choose from. I would have thought the discrepancy would surely be within the mechs tolerance.

 

[fritz katzenjammer]

Is the derailleur the correct width for the chain? A derailleur for an eight speed chain has a wider cage than one for an eleven speed chain and so can chuck the chain right over the top ring as the outer wall of the cage is well outboard of the ring by the time the inner wall of the cage is able to push the chain up onto the ring.

 

[Andy in Germany]

Is the derailleur the correct width for the chain? A derailleur for an eight speed chain has a wider cage than one for an eleven speed chain and so can chuck the chain right over the top ring as the outer wall of the cage is well outboard of the ring by the time the inner wall of the cage is able to push the chain up onto the ring.

It's an 8 speed system; I replaced the old 8 speed chain, which had been in use for a good five or six years with no problem, and put in the standard XLC 6-8spd chain we always use for replacements.

 

[IanSmithCSE]

Good afternoon,

I am often surprised by what does and does not work and when confused just give up and replace everything with new bits.

I noticed that you said newer, not new which opens many cans of worms, such as a bent ring or a few teeth dodgy teeth.

This chainring worked except under full load


but the inner ring which appears unworn slipped with both old and new chains.

Kind Regards

Ian

 

[Paulus]

Is the chain worn beyond usable life?
It could flexing just enough to tip it over if very worn.

 

[Ajax Bay]

bike needed some serious repairs . . . now done, [plus] a new chain, cassette

Is the chain worn beyond usable life?
It could flexing just enough to tip it over if very worn.

See OP @Paulus
Andy - your list is comprehensive and I'd just keep tweaking the last bullet.
And suggest to 'elder son' that he pushes rather than 'shoves' the lever to shift up, easing off power a bit (obv).

 

[Ming the Merciless]

Does it only throw it when your son rides it, or also when you have on stand adjusting?

 

[Andy in Germany]

Does it only throw it when your son rides it, or also when you have on stand adjusting?

It kept throwing when ridden, more than on the stand. Tomorrow is the next test ride: I'd got it behaving better on the stand so we'll see if it rides better.

 

[Andy in Germany]

See OP @Paulus
Andy - your list is comprehensive and I'd just keep tweaking the last bullet.
And suggest to 'elder son' that he pushes rather than 'shoves' the lever to shift up, easing off power a bit (obv).

Thanks, that'll be the first thing I try tomorrow.

 

[Andy in Germany]

I am often surprised by what does and does not work and when confused just give up and replace everything with new bits.

I've had mystery clicks and jumps on other bikes and ended up replacing everything, so that's encouraging. I still have one bike where replacing all parts didn't make a difference. I solved the problem by hiding the bike in the storeroom.

I noticed that you said newer, not new which opens many cans of worms, such as a bent ring or a few teeth dodgy teeth.

We have boxes and boxes of clean and sorted parts and this one had no obvious defects or damage, but I've known a 0.5mm discrepancy cause problems before...