[pimpbot]

Hi all,

I got a new issue.

I've recently and suddenly began having issues with my chain dropping off my middle ring when in the large cog in back. The chain just has a tendency to start riding up on top of the teeth, and then drops into granny when pedaling under no load. I suspect that my chainline has somehow changed, but I don't have any slop in my BB, BB cups or crank arms, and the front deraileur is not touching the chain. I guess I'll check the BB pivot next (FS frame). When in middle/big cog, I do hear a ticking sound from the chainring every crank rotation, but I have not found any bent teeth.

Apart from that, what else can contribute to this?

 

[Dave B]

not sure what is causing it, but a cure.........

is: Don't do that! It's not drastic "cross-chaining" like small front cog/large rear cog, but it is cross chaining. It's going to prematurely wear your drivetrain. Instead of going to the large rear cog, shift to your smallest front chainring and use the middle range of your rear cassette- you won't have the chain falling off and your drivetrain will last longer.

 

[bulC]

bull

is: Don't do that! It's not drastic "cross-chaining" like small front cog/large rear cog, but it is cross chaining. It's going to prematurely wear your drivetrain. Instead of going to the large rear cog, shift to your smallest front chainring and use the middle range of your rear cassette- you won't have the chain falling off and your drivetrain will last longer.

Unlike the big ring/largest cog combination, which you should avoid, using the middle ring with the largest rear cog is perfectly fine, perfectly acceptable, and very useful, as it's a cleaner shift down to the large cog than having to downshift the front to the granny ring and then upshift the rear to get to a suitable gear that's not too low. In theory, indeed, in fact, any ring/cog combination that is not perfectly straight like on a properly set up fixed gear of singlespeed involves some lateral chain deflection. And using any gear combo that does not result in aperfectly straight chain line will wear the ring and cog faster than would a straight chain line. In practice, hey, that's how derailleur drivetrains work, and that's why derailleur chains are flexible laterally.
If the chain falls inboard off the middle ring when on the large cog, yes, a too-long bottom bracket spindle would be the first thing I would look for. But your chainline doesn't change over time, if it was ok before, it should still be ok. spindles don't grow. So I would look for damaged teeth on the middle ring, damaged chain links, bent derailleur hanger.
Ignore the silly advise not to use your middle ring with your largest rear cog.

 

[pimpbot]

Yep

I cross chain all the time. That's not what is going on here. Middle ring/big cog is not cross chaining.

I often use big ring/3rd biggest cog all the time for tight technical downhill, and I understand that this wears out the chian faster, so I only do it for a short amount of time.

I think I'm gonna need to strip my drivetrain back, clean and inspect everything until I find the bent/loose/broken part.

 

[Speedub.Nate]

Have you reset your front derailleur?

It might be even simpler -- in this particular gear combination, how close is the inner plate to the chain? If you've got enough of a gap to allow the chain to drop, the solution may be as easy as adding a turn or two of tension to the barrel adjuster at the shifter.

But if that doesn't do it, I'd disconnect the cable and basically reinstall the FD, checking cage height and alignment off the big ring, verifying my limit stops, and resetting the cable.

It might be that you whacked the front mech on something and just slightly knocked it off kilter.

 

[pimpbot]

well, the thing is....

Have you reset your front derailleur?

It might be even simpler -- in this particular gear combination, how close is the inner plate to the chain? If you've got enough of a gap to allow the chain to drop, the solution may be as easy as adding a turn or two of tension to the barrel adjuster at the shifter.

But if that doesn't do it, I'd disconnect the cable and basically reinstall the FD, checking cage height and alignment off the big ring, verifying my limit stops, and resetting the cable.

It might be that you whacked the front mech on something and just slightly knocked it off kilter.

... if I lift the rear wheel and slowly turn the cranks, the chain drops eventhough it is not touching the FD and not being bounced around by bumpy trails. That, to me, sounds like chainline issues.

I did double check my bolts, and the BB pivot was a tad loose. I wonder if that could have allowed the chainstays to migrate out of line a tad. Anywho, I fixed that issue, so we'll see if that fixes the whole works once I get the cranks and rings cleaned, inspected and reinstalled.

 

[el Turtlehead]

what kind of crank ?

feel the gin

 

[pimpbot]

Found it!

Hi all,

I got a new issue.

I've recently and suddenly began having issues with my chain dropping off my middle ring when in the large cog in back. The chain just has a tendency to start riding up on top of the teeth, and then drops into granny when pedaling under no load. I suspect that my chainline has somehow changed, but I don't have any slop in my BB, BB cups or crank arms, and the front deraileur is not touching the chain. I guess I'll check the BB pivot next (FS frame). When in middle/big cog, I do hear a ticking sound from the chainring every crank rotation, but I have not found any bent teeth.

Apart from that, what else can contribute to this?

It was a burred tooth on my middle ring. A few strokes of the file, a tweak with the pliers, and problem solved. I must have sucked a rock into it at some point and tweaked it.

BTW, 2003 XT cranks, PC89r chain.