Help: Dahon Mariner dropping chain
 

All,

I hope someone can help me diagnose and remedy a problem that arose this morning.

My Dahon Mariner, beginning this morning, is throwing its chain to the outside of the chainring/chainwheel when shifting down, towards the bottom of the cassette (i.e. onto the small gears). It seems to be when moving from the 17t to the-- what is it, 15t?-- next smallest cog.

It happened twice, once cracking the plastic chainring guard, and both times hopping out from under the chainkeeper.

I can't clearly think what would cause this; dirty gears and chain, or maybe the freezing cold causing the chain to stick to the gears and not release as it pulls across the top? I dunno!

Any ideas on cause and course of action?

New Chain and cog will be replaced together , Right ? this is a derailleur bike?

hard to assess without seeing the Bike.

Hey, B Footies...Bobby Feet! ;)

I'll try cleaning and re-lubing before replacement of running gear; I do rather neglect this bike!

here are a couple of pics; I tried to show chainline, but I haven't had the issue in the past, and it's stock, so probably that's not the issue.

https://farm8.staticflickr.com/7525/...09d740c6_b.jpg

https://farm8.staticflickr.com/7473/...c6166452_b.jpg

https://farm9.staticflickr.com/8653/...1756227b_b.jpg

If you rode through a freezing rain, it could be as simple as ice resisting the entry of the ring's teeth into the chain. So it climbs up onto the tips of the teeth, then dumps. Modern chains are very prone to dumping because the plates barely extend beyond the rollers. On older chains the plates extended quite a bit making for a V-pulley shape and helping keep the chain home.

The reduced plate size is intentional because it makes shifting crisper, but what makes shifting easier also helps shifting off sprockets when you don't want it.

You might try to keep a bit more tension on the chain during upshifts so it can't climb but the right answer for a 1X? drivetrain might be a guide roller where the chain meets the ring.

Short of that, try refreshing your chain lube and cross your fingers.

Possibly the rear derailleur cage is moving more sluggishly due to the cold or lack of lubrication. So when you shift to smaller cogs the cage/jockey wheel aren't immediately moving back to pick up the resulting slack in the chain. If you aren't keeping much tension on the chain when shifting that might explain the chain drop.

Ok, thanks, guys. It sounds like a little TLC in the form of cleaning and lubing should be the first approach, so I'll give that a go tomorrow.

I was going to suggest checking the cable run,since this is a folder,when I noticed this:
http://bikeforums.net/attachment.php...hmentid=424354

Might not be the cause,but try wrapping the strap under the derailleur cable. If it's tight enough,it could be causing the cable to drag.

My experience:

When chains wear, they become more able to flex sideways.
When shifting to a smaller cog, the top run of the chain gets a sideways impulse as it drops abruptly onthe smaller cog.
When the chain is worn enough, the sideways impulse travels forwards along the chain and will tend to derail the chain when this sideways snake arrives at the chain wheel.
This is the most important reason a chain guard is needed when there is no front derailer - to prevent that snake from dropping the chain.
To work best, the guard must be as close as possible to the chain to arrest the chain snake. In most cases imho it is not close enough. You want a few mm at most. My Birdy also dropped its chain, until I fitted custom guards that I spaced as close as possible without chain rub when the extreme gears were engaged.

If you installed a new chain, the sideways freedom will be minimum and likely it will be fine again.

Ooh, @dynaryder, that was a tempting one, especially since I'd hastily thrown the lock on the other day, but after removing, I still had the issue.

I also cleaned and adjusted the drivetrain; no dice. I started getting a grinding noise when in the low gears, too, so I pulled the cassette. There was no discernible wobble in the freehub, but it was making an ugly noise when torqued on. I pulled it, and tried to find a replacement locally, but no luck.

I also noticed the cassette gears shift around a bit; the pins seem to be intact, but there's a bit of rotational play, like a couple millimeters. I suppose when the lockring is torqued down, that's a non-issue though, right?

Anyway, after I get the freehub I'll get back to diagnosing the problem, but for now I'm stuck in dry dock. :(