chiproth99

.Sometimes, when I change gears on the rear, the chain falls off the front chainring.
The chain falls off and gets jammed between the crank arm and the chainring. Difficult to dislodge
This happens mostly when I am coasting along and want to shift up.
This is scram-rival double-tap shifting.
I don't think this is a question of derailer adjustments.
Something worn in the shifter (after only 1000 - 1500 miles)?
Worn chain? Stretched cable? Operator error?

What's your best guess? It's a real aggro.

Thanks,

Chip

Moe Zhoost

More info needed: Single, double, triple up front? Chain drop happens when shifting up or down or on a particular cog? Chain condition? My first thought is that the RD cage spring is weak, but it could be any number of things.

alcjphil

How can you be shifting while you are coasting?

SpedFast

Start by making sure the stops/limits are set correctly on the FD. Do the RD too while you're at it just for S's and G's. Then fine tune the indexing. Even if you do a lot of cross chaining you should not have a chain pop off a chain ring and jam between the frame and crank. That should be your small front to large rear position. Good luck

curbtender

How is the spring tension in the rear derailleur? I've seen the cage flop around on bumps popping the chain off on the small smaller combos. Sometimes related to chains that are too long also.

Camilo

chiproth99 - please give the type of bike you're talking about, the number of rear speeds and the type of shifters you have, and if it's a 1-2-or 3 speed in the front. Nobody can help you if they don't know what you're talking about.

[edit] I see this is the third post and the third thread started by Chip, one in 2017, one in 2020. He didn't make a single follow up on either of these to get his problems solved. I doubt he'll give any more info here, so maybe not waste your time until he follows up providing more info so we could possibly give meaningful advice.

Yelbom15

This definitely needs a lot more info but I can try just taking some hints. Yes, limit screws can play a part but they’re not really there to keep the chain in place. Just to move it from the upper and lower chain rings. Now if the derailleur has too much swing due to limit screws not being adjusted properly, yes, it’ll throw your chain off but you’re saying it falls when you’re shiftingtt in the rear.

It could be a case of a super stretched chain, the chain ring teeth are super worn down, you’re free hub body isn’t coasting well enough which causes your chain to bunch up and fall off the front chain ring or a possible bad clutch in the derailleur not tensioning the drive train we’ll enough.

Also, I think Rival uses something called X-Sync and you need to use a specific chain to run it. Yes, mix matching chains can still get you to point A to B but how well it does doing it is what it comes down to.

FBinNY 

I think this is the key.

Chains drives want some tension in the chain as it feeds onto the sprocket. The tension ensures that each roller is pulled all the way down before the next one engages.

Without some tension friction can cause the chain to begin riding up to where the next link is out of position. Once the chain starts riding high the pitch doesn't match, and it climbs to the top of the tooth. It will stay there until the pitches line up so it can drop in (chain skip), or fall to the side.

If you understand what's happening you can seek specific causes. These include, poor chain lube, dirt or mud, a sticky freehub spooling the chain forward and slackening the upper loop, inadequate RD cage spring tension, and similar ideas I might have missed. Note also, that a worn chain will exacerbate this problem.

I'd start by cleaning the sprocket and lubing the chain. Failing that, I'd check for a sticky freehub as follows: on a repair stand, shift to a high gear, then hang a small weight from the middle of the upper loop to cause it to sag an inch or so. Watching the weight, grab the rear wheel and give it a decent spin. if the weight sags more than slightly, the freehub is sticking, and wants some attention.

79pmooney

Good stuff here. And those bolded sentences - never heard that before but I like!

Miele Man

Is it possible that the derailleur hanger is twisted in the horizontal plane rather than in the vertical plane?

Cheers

FBinNY 

This is a common issue when chains fall off while backpedaling. However, the chain runs directly from the cassette to chainring on top, so the RD cannot cause the problem the OP describes.

The causes of chain derailing when pedaling (forward) are limited, and related to the sprocket, chain, and chain line.

ThermionicScott 

OP has a pattern of starting threads about his bike falling apart, and then never checking back in...

curbtender

Maybe this is why the chain is falling off?
"left right play in bottom bracketI discovered about a quarter inch play in the bottom bracket axle.


In tuning up for the season, I removed the chain and started poking around.


As stated, I found about a quarter inch left/right play through the axle in the bottom bracket. The chainrings then move that quarter inch. Is this typical? Normal?


The BB bearings are sealed, so I assumed everything would stay in place. Maybe not.


I suspect this is the reason for recent shifting difficulties.


So, I guess I need to wade in there and take a look, unless of course, such play is normal."

FBinNY 

OP is a newb with only 3 posts. (and I wouldn't have thought about it until your post)

In any case, I don't think about posting here as a one to one consultation (for which I might charge). I view it as more of communal passing along of knowledge for all that may be interested.

SpedFast

In case you're not being sarcastic, NO that is NOT normal, haha

grumpus

There should be at most barely perceptible movement side to side, such that you can perhaps feel it but not really see it.

alcjphil

Technically a newbie, but a member since 2017

curbtender

I was just quoting the OP from an older post. I'm not sure how you could get a cartridge bb to slide around unless both sides were adjustable. Maybe running an EDCO?


(picture because we needed one)

Camilo

Yeah, like pointed out in this thread and at least one othe of his three posts since 2017 - all of which were thread starters - he has not followed up once, no additional info when asked, no reply of any sort. Three OPs, three posts.

I have to wonder what the heck he's looking for, obviously not looking for actual productive interaction.

SpedFast

He's shy.

oldbobcat

If everything else works fine, it's likely nothing to do with your derailleurs or shifters. You say it only happens while coasting. That's a clue.

First, by chance, are you turning the crank forward as you shift or are you coasting, shifting, and waiting for something to happen? Because derailleurs aren't meant to work this way. The chain has to be moving forward through the cogs for the shift to happen. Second, are you, by chance, turning the cranks backward as you coast? If you are, stop doing it. The chainring isn't milled to seat the chain in reverse.

Next, we're going to see if the freehub freewheels. That is, does the wheel coast without resistance? A rear wheel that doesn't freewheel when it's supposed to pushes the cranks forward via the chain. If there's a force preventing the cranks from turning forward, such as your feet trying to hold them stationary as when coasting, then the chain goes slack from the rear cog to the chain ring and can be pushed off the ring. So with the bike in a stand and the chain in top gear, spin up the crank and then let go. Does the chain continue to push the crank forward or does the crank stop spinning after a while?

If the chain is pushing the crank, then the sudden slack in the chain caused when you stop pedaling can cause derailment. If that's the case, get your freehub overhauled. I'm not going to tell you how to do it because they're all different and there are YouTube videos that can explain it better than I can, and you most likely aren't equipped to perform this kind of maintenance anyway.

If the chain is not pushing the crank, the problem rests with the chain or the chainring or the crank. The chain might be worn or it might have a kink or a stiff link. The chainring might be bent. Or the crank spider might be bent. I can't diagnose what I can't see.

Camilo

You know the OP is ignoring this - like he did with his other three single post threads. He does't really want advice. BUt I guess it's OK to reply since someone else might find the advice with a search.