AJW2W11E

How do I correct a chain that falls off the big chain ring outwards? When I hit a bump or stomp hard to climb a hill my chain falls the big chain ring outwards (towards the pedals). The last few times this has happened, I tightened up the Upper Limit screw on my front derailleur. That helped, but the last time I tightened it, I noticed I couldn't downshift to the intermediate chain ring. I should have fixed then and there, but I didn't Today I went for a ride and the chain stiil fell off when I stomped hard to clear a small gulley. That was bad news, because now I couldn't use my internediate chain ring, which would have been safer. So my chain came off four times today.

JoeTBM 

Besides proper derailleur adjustments (limits and mounting height) if your chain is too long for the slack to be taken up properly by the rear derailleur it could contribute to the issue.

smd4

Adjusting the limit screws won’t affect your ability to shift to the middle (“intermediate?”) chain ring. If you screw them in too tight, that might be your only usable chain ring.

GhostRider62

I had a professional mechanic get out big honking crescent wrench and bend the front derailleur cage. That worked.

I'd recommend messing with the cage height and angle, some triples ramp profile are sensitive to that. Sorry not too specific, I haven't adjusted a triple in 5 years and it was a tricky on because the range was huge.

Moe Zhoost

Adjusting the upper limit screw should have no effect on downshifting to your middle ring. Are you sure you didn't adjust the lower limit screw? It's easy to confuse the screws. This would have inhibited inward movement of the cage, though I think it would be tough to screw it in enough to restrict shifting to the middle ring.

Thinking out loud, the first thing I recommend checking is your chain. A worn chain can certainly unship unexpectedly, though I'd think that a properly adjusted derailleur would prevent dropping to the outside. The second thing I would check is for wear or damage on your chain rings. This is not likely to be your issue but it's easy to check. Eliminating these factors will narrow the issue to derailleur mounting height and angle. I've worked on dozens of mis-shifting triples in my volunteer work at the local co-op and have found that small adjustments in the cage angle can make a big difference.

I'd assume that your chain length is proper and assures sufficient take-up tension. It's worth taking a look at your rear derailleur to see if any debris is preventing the cage spring from sufficiently tensioning your chain.

2_i 

A chain guard put on top of the external ring would help.

55murray

Your limit screw is for shifting, if it is just jumping off while riding your chain does not have enough tension. So I would think either too may links or the RD spring is not working (gummed up, broke).

Iride01 

Your high limit on the front DR shouldn't affect your shifting into the middle ring from the big ring. If it did, then there is more wrong with your front DR than we can tell from what's been described. Possibly the FDR is sitting too low or is skewed at a angle from where it should be.

I agree that limit screws are for limiting how far the DR can over shift to get the chain into the sprocket quickly. Their affect on other reasons of chain drop is minimal but there is some. So look to your chain being too long or the rear DR is gummed up and can't keep the slack out of your chain, or you are running cogs and/or rings that put you outside the spec's of your rear DR.

AJW2W11E

Oops. Thanks!

Iride01 

Can we infer that you confused the two limit screws and adjusted the low limit?

I'm sure none of us have ever done something like that. But I'd be lying if I was to say I never have.

mpetry912 

the front derailleur adjustment should not have anything to do with this problem.

I'd suspect that either the chain is worn, too long, or possibly there's a bent tooth on the outer ring.

/markp

oldbobcat

Dropping a chain while not in the act of shifting indicates a worn out chain, a damaged chain, a chain that is too long, or a bent or otherwise damaged chain ring. Once the shift is complete, as long as the front derailleur is not making contact with the chain, the problem is not the front derailleur.

urbanknight

Could the derailleur be too high?

Kontact 

Not a derailleur problem.

AJW2W11E

I had the limit screws confused. I made the derailleur lower and checked the angle.Off I went and the chain dropped again. ​​​​​​​Never experienced this before. Also the bike won't tune well.This is a Giant Defy, a straight forward bike and trouble free.​​​​​​​

Kontact 

It has been suggested you look for bent chainring teeth, chain wear, chain tension and other things not involving the front derailleur.

Have you?

oldbobcat

All the derailleur is for is knocking the chain off the ring and guiding it to the other ring when you shift. The derailleur does not keep the chain on the ring. The derailleur should only touch the chain when you're shifting.

And the derailleur height is OK.

AJW2W11E

What causes this?

AJW2W11E

She shifts perfectly now. The moment I stomp on her, the chain pops off.

smd4

Time to spend some money! You might want to measure that chain and find out just how worn out it is!

Moe Zhoost

Excessive chain wear. It's worn and needs to be replaced. It's likely that the worn chain has caused damage to your cogs and perhaps your chainring.

urbanknight

I always figured having the limit screw too wide or having the derailleur too high would be like leaving an open door for the chain to fall through.

Kontact 

If you were jumping or something. Otherwise only during shifting.

veganbikes

It looks like a bike that needs new parts and a tune up. The chain and cassette is likely worn (and possibly to front chainrings) your front derailleur cable is fraying and things look dirty. Never a good sign. If you can't or don't want to do a full tune up and replacing everything that is worn which is possibly more than listed but can only see so much in a single photo, your local bike shop can work you up a quote and would be a good thing to do yearly so you can prevent issues.

Stomping on a bike is not a good thing either.

AJW2W11E

I take good care of my bikes actually. When you're riding on a level surface, you use a regular rhythmic pedaling motion. Now let's say you suddenly hit a tough hill which is about every 3 miles out here Northwest. Then you push hard and yes at the point where you are just at the crest and doing 2 mph,and your eyeballs are coming out of your sockets, you are ..not being polite to your bike.that's the way it is out here, hills and hills...you just have to put some ....I like your attitude towards a clean bike. I maintain mine pretty well, but with the daily rain and mud out here, you have to use messy wet lube and it's safer to keep the hose off them rather then soak your bearings with hose spray and just let them be. It's heart breaking to see a muddy bike,....but I have to ride everyday outside.I'm glad I don't own a Crux or Roubaix. I would never ride them out here.