Chain falling from middle to small ring on 105 triple?
 

Hi all. Especially when on the larger cogs in the back, my chain just loves to drop from the middle ring of my triple to the granny gear. If you pedal backwards, it happens immediately, pedaling forwards, randomly after a couple of minutes. It also sometimes skips from the big ring down to the small one when shifting, missing the middle ring.

I don't think the front derailleur is causing the problem, and it may have started happening when I replaced by rear derailleur and chain. The whole drive train is 105, but the rear derailleur is a couple years newer now. I can't tell what's causing the issue, "it just happens". Any likely candidates for this misalignment? The limit screws seem correctly adjusted. Thanks.

I'm not an expert, but I don't see how it can be anything but the front derailleur. A misadjusted rear derailleur could cause this when pedaling backwards, but not when pedaling forward.I would suggest you try turning the barrel adjuster for the front derailleur cable (assuming you have one) counterclockwise (tighten the cable) 1/2 turn at a time until it stops happening. You can also tighten it until the chain rubs on the inside of the derailleur on the middle-big combo, then back off a bit.

Actually, it can't be the front derailleur at all. Its job is to move the chain to a certain chainwheel, not to keep it there, and when pedaling backward the chain comes off at the bottom, not the top of the chainring.

Pedaling backward unships the chain when on the larger chainwheel and larger rear cogs because the chain is traveling from the bottom pulley, at an angle ,to the bottom of the chainwheel. The combination of the angle and low tension makes it easy for the chain to come unshipped. It would not be surprising if the chain occasionally came off the cassette in some combos when pedaling backward.

When pedaling forward the chain coming unshipped would tend to be due to poor chainline, a bent tooth or chainring, chain problem, etc. Will take more observation, details and perhaps some measurement to determine the exact cause.

Backpedaling in random gears is not a good practice, serves no purpose.

In general what makes the STI stuff shift easily also allows the chain to come off more readily
than it did , before the engineers started adding shift ramps, lift pins and shortening teeth tips..

I had the same problem on one of my triple bikes. It turned out that my rear derailleur hanger was bent inwards which caused the chain at the bottom to be too close to the spokes and threw the bottom of the chainline off. So when I back pedaled, the chain coming off the bottom jockey wheel was pushed so close to the spokes, it caused the chain at the front to drop to the smaller chainrings.

Thanks all. Sounds like more investigation is warranted, with the rear derailleur/hanger setup being a prime suspect. Will keep working on it.

No, the fact that one poster had that experience does not make it any more likely than other possible causes, and would only apply to the back-pedaling "problem." When pedaling forward the rear derailleur has no effect on the chain unshipping from the chainwheel. When diagnosing problems one needs to use observation and logic to avoid going down the wrong path. It's also helpful to know when any problem you have began, and of any incidents or maintenance that immediately preceded the problem.

My $.02: check your chainline
It's crucial, IMO, that you have about a 43mm chainline if you expect the drivetrain to behave, esp. in adverse conditions (back-pedalling would qualify in my world).

Why was the RD replaced?

The chain line for your triple should be approximately 45 mm: http://www.sheldonbrown.com/chainline.html

If the chain line is ok I would try adjusting the front derailleur cable with a 1/2 turn at a time, counter clockwise. This will add tension to the cable. But first confirm that the cable fixing bolt is tight and that the cable is not slipping at the derailleur.

Conversely if you have too much tension in the cable it can cause the chain to jump from the big ring to the smallest ring on a triple due to elastic rebound when the cable is released when shifting down from the big ring.

If the chain line is too far to the right check the bottom bracket and crankset are both tight.

Again, the front derailleur is not there to hold the chain in place. The front derailleur cage is not designed to function as a chain keeper - pushing the chain back on when it starts to unship, and will typically fail at the task.

Normal drivetrain operation allows the chain to travel from most of the large/inboard cogs (on my 8 speed all of them) to the large chainwheel on a triple and stay in place without any involvement of the front derailleur. If that is not possible then there is something out of spec, period - backpedaling excepted.

Thanks for the suggestions all. Old RD was trashed due to incorrect low limit screw setting my owner with limited technical experience = long walk home. As cny-bikeman says the FD isn't the issue. Inspected further, loosening the b-limit screw seemed to help a bit. Then I inspected the middle ring teeth and found what looks like quite a bit of wear, see pic. The cassette looks fine. http://bikeforums.net/attachment.php...hmentid=352596

Funny my LBS replaced this middle ring about a year ago, explaining my bike had come with a cheap one which was worn. When I pedal there's quite a bit of horizontal play in the chain, maybe due to this wear?

Those teeth look like the shortened shifting assist profile teeth present on some chainrings. Look to see if there are other shortened profile cut teeth on the chainring that are roughly 180 degrees from each other. If so then that is by design and not worn chainrings.

You know the teeth aren't worn on the opposite side, and I now notice my chain drops exactly when that most worn tooth toward the right connects with the chain. The middle is by far my most used ring. Maybe not adjusting the RD correctly when I replaced it hastened its wear.

Hmmm?
 

Horizontal, as in lateral (side-to-side) or horizontal, as in along the chainring forward to backward?

Not related to horizontal play, but I was thinking chainline, too. Have you made any crank changes recently?

possible causes

bad chainline
too narrow chain
bent chainring teeth
loose bottom bracket

i had the same problem on a bike with fsa cranks
i always chalked it up to bad chainring design
or perhaps a chainring designed for nine speed chains used with a 10sp one

edit
also
the wear you show is definitely not wear
chainrings dont wear like that
the teeth get to looking taller and skinnier and like shark fins
but not worn down to nubs

Phil - horizontal as in when I spin the crank I can see the chain moving laterally (toward, then away from the frame), though never touching the FD. I never touched the crank before this started happening, but at a recent bike mainenance class we removed our crank arms and I was told I'd need a new bottom bracket soon as it had a bit of play and was spinning freely, as if without bearings.

Wilfred - thanks for the thoughts, funny my outer chainring said "FSA" on the outside, but the bike shop explained the manufacturer had used cheaper small/medium rings, and replaced the middle ring (I believe) with an FSA.

Maybe the next step is to replace the bottom bracket and/or ring, and "see what happens".

that makes no sense

i have never seen a factory bike with chainrings and cranks from different manufacturers
shimano cranksets generally come with chainrings installed
and the time necessary to remove one set and reinstall another
would probably offset the savings pretty quickly

i suppose they could then sell the shimano rings
as the retail price on a set of those costs more than a set of cranks
but most bike manufacturers are not in the business of selling loose chainrings

it would be nice if you could post a picture of the whole crankset

So I took it to the pros...they think the bottom bracket spindle is too wide (I assume it's 118 mm), and are going to stick a 113 mm on there. The BB needs to be replaced in any event. We'll see if that fixes things.