Is this chainring toast?
 

Just changed chains and the new one keeps falling off the big ring up front. At first it seemed to have been cut too short, so I lengthened it. Situation has improved, but still there, chain falls off the outside edge off the front chainring when under load. Doesn't happen while shifting, just normal pedalling.

Can't think what's going on other than maybe it's time for this ring to retire.
https://cimg0.ibsrv.net/gimg/bikefor...92f6957a28.jpg

Thanks

Looks to me as if your front derailleur is too high

As stated, move the FD down until it just clears the teeth when moving the chain to the big ring. It makes a huge difference in both shifting response as well as keeping the chain on that ring.

^This^ x 2.
But...if it's not while shifting and just happens when you're riding along you've got something else going on. The derailleur being too high won't make the chain shift itself off the ring.

Thanks guys, I'll get that adjusted

Yeah I agree. The cassette is being a bit of a nightmare too, hard to index. I'm wondering if it's worn and the root cause of all this.

Might need to re-visit chain length if after correcting the DR height you still have the chain fall off occasionally. I usually would expect too long a chain having slack in some gear combo's to be more prone to falling off the front.

Chain length will be low on my list of other things to check normally for dropped chains. I only bring it up because you said you lengthened it.

Shimano rear DR? They tell you how to size the chain in their DM's for that model DR.

https://si.shimano.com/

Thanks. It's actually 105 5800 up front and Ultegra 6800 at the back. I'm hoping to have time to sort the FD tomorrow.

It's an 11 speed crank.
How many speed chain & cassette?

This is how I make a quick in situ chain length measurement.
Put the chain on BIG:BIG.
You should have between 1 to 2" slack.
https://cimg7.ibsrv.net/gimg/bikefor...0ac04727e2.jpg

Agreed on the FD height, but regarding the chainring teeth, if they aren't worn yet, they might be on their way. A closeup photo would help, but new chains and new teeth always play better together.

Since you are moving the front derailleur, you should start from scratch and readjust it.
This Park Tool repair guide is always the first place I go to. They have easy step-by-step instructions.
Front derailleur adjustment

Not just the height, but the limit screws and cage angle, and cable adjustment too.

It's hard to see if the teeth are worn, since they have multiple odd shapes. I'm guessing that the small chainring gets more wear, since there's high chain pressures when climbing hills.

But one thing that can wear out cassettes and chainrings is keeping a very "stretched", worn out chain for too long. It puts high force on just a few teeth at a time.

My Ultegra chainring has almost 30,000 miles on it, from 2014. It's still good: no shifting problems or unusual chain wear. (And the Di2 system still works perfectly.) It's so hard to tell just by looking.


https://cimg8.ibsrv.net/gimg/bikefor...e52c5315dd.jpg

Big chainrings take a very long time to wear out, tens of thousands of kilometers. They take much longer to wear out than small chainrings. The one in the OP's photo is pretty dirty, but doesn't look all that worn

Yes, I'm aware that different rings wear at different rates; that's why I only said the OP's might be getting worn.

Whenever I replace a chain, I take the opportunity to evaluate the cog and chainring teeth, especially if I experience any skipping, dropping, or missed shifts with the new chain.

Is there any more to the story than just a changing of the chain?

Changed crank or chainrings? Swapping out components with another bike? Just bought the bike from someone?

I wouldn't be surprised if a new chain skipped on that big chainring, but I'd at least expect it to stay in place. Perhaps if you're in a high enough gear (small cog), the chainline is crooked enough that the chain will also derail when it skips.

Full story is I bought this bike from someone else in September. Soon after buying I replaced the chain and cassette like for like, same tooth count and same chain length. The chain snapped on its second outing, I didn't think much of it at the time, just popped in a master link. Since then I don't know how many miles I've done on it, but somewhere around 300-400 in pretty gross conditions (think having to stop to pull mud out of your fork and chainstays 4 times a ride gross).

The other day I noticed the shifting wasn't great and checked the chain - it was worn, so I swapped it out. I cut the chain to the same length again, but this time got a kmc x11 rather than a Shimano. Immediately noticed terrible chain issues when in the big ring, tried to shift to big and big but couldn't. It's a mystery to me why the Shimano didn't behave this way and the kmc did, but it seems I've been running a short chain for a while without realising.

Added a bit of length to the chain, can shift into big big now, but other problems persist.

I should mention the previous owner had to replace the inner ring due to wear. No idea how many miles the big ring has done before I got it.​​​​​

Since the front and rear DR aren't from the same group, then a previous owner has been changing stuff out. Was the original group on the bike 105 or Ultegra? Possibly the limits on the front DR are not set correctly if the front DR was changed along with the crank. That will let your chain fall off too, but get the DR height over the ring correct first.

The exact model and version numbers will be on all of your Shimano components somewhere. Usually the backside and sometimes you won't see it without removing the component. But with those numbers, you can find all the tech documents for them at the link I gave in an earlier reply. The DM's will give you the best information to check if they are installed and adjusted properly.

How would this happen? If you're pedaling the bike the 'tension' on the chain is what it is depending on how hard you're pedaling. The chain has no more or less 'slack' because the top run of chain will have tension on it. The bottom run has tension from the pulley cage of the derailleur. The only place you could have slack is on the bottom run in the smallest cog or 2 when in the small ring if it was actually too long. As long as there is tension on the bottom run of chain in small/small it'll be the same everywhere else.

Seems we agree. I just wasn't as verbose. See our comments in red.

When I'm getting a chain dropping off a ring, I put the bike up on the stand to see exactly what's causing it. Sometimes it will always happen on the same section of ring. In that case I'm looking for a damaged tooth or pin on the ring, or the ring has a wave to it Sometimes it happens on the same section of chain. Then I look for a stiff link or a bent plate. To check for a worn out ring, put the new chain on the big ring, pedal gently, and look for daylight between the front of the tooth and back of the chain link.

Thanks. This is chain with the extra length added:

https://cimg9.ibsrv.net/gimg/bikefor...7ecd4669bb.jpg
seems like way too much. What's between the two master links in the photo below is what I added:

https://cimg4.ibsrv.net/gimg/bikefor...783cb7ebfb.jpg
I just took the additional length out, this is where we're at now:

https://cimg1.ibsrv.net/gimg/bikefor...8848ca3742.jpg

however the derailleurs can't get the chain in big big. I put it there manually, but the minute I start pedalling it shifts down.

This is the derailleur in big big with the smaller chain:

https://cimg0.ibsrv.net/gimg/bikefor...8022daa733.jpg
https://cimg0.ibsrv.net/gimg/bikefor...bff8c64c53.jpg

I haven't yet lowered the FD, will do that after work today.

Put it on the BIG ring and 5th smallest cog. (assuming it's an 11 speed)
Stand behind the bike and sight down the chain.
Note how much of an angle the chain is at vs "straight ahead".
Repeat with chain on the SMALL ring and 5th largest cog.
The angle should be extremely similar, but to the other side.

This should give you a rough idea if the chain line is correct.

Modern Shimano 4-arm chainrings wear significantly quicker than you would expect. Likely due to the heavy shaping they get to aid shifting, and 11/12spd needing narrower chainrings so there is already less material to work with.