Why does my chain come off the Chainring?

[sleuthey]

On average every 3 miles, when shifting in the middle rear gears (3-5) when the chain is fairly parallel with the bike, it will fall off the outer side of the Chainring.

I have googled and checked all suggestions eg. Stiff links, BB wobble etc. I have swapped the chain and the crank/chainset.

It's a late 90s GT frame with original Spindle/cup BB, new Sunrace 7 speed shifter and new 14-34T Freewheel. Both Cranksets are unbranded but have worked well on other bikes.

Any suggestions on the cause of solution please?

 

[T4tomo]

er... because you have neither a chain guide /catcher , narrow wide chain ring or clutch derailleur to keep it on. a 1x set up neds at least one of these...

something like this is what you need

shop around to find best value etc, I just snipped first photo i found for you

 

[Ajax Bay]

Plus maybe the single chain ring is well further in than the designed chainline. I'm guessing this had a triple on before.
Besides tomo's main likely issue and solution, you might improve this by replacing the (original) cartridge BB with one with a longer spindle. Of course this may result in the chain falling off inwards.
Edit: I'm also going to hypothesise that the chain comes off when you're changing down (ie to a larger sprocket) and easing off the torque (as is normal) while doing so. Such reduction in chain tension will make any lateral perturbation as the chain meets the ring more likely to throw the chain off.
https://sheldonbrown.com/chainline.html

 

[ColinJ]

Maybe the derailleur is gunged up or worn out so it isn't keeping enough tension on the chain, especially if the road is a bit bumpy when changing gear?

I was watching a slow motion video of a gravel bike descending and the lower run of chain was flapping up and down. That bike may well have had a clutched rear mech. Yours won't have so the derailleur would be able to bounce about and make chain flap worse.

 

[GuyBoden]

Dog Fang are cheap and work well. About £3-£4.

https://www.ebay.co.uk/itm/373861584413

 

[T4tomo]

Dog Fang are cheap and work well. About £3-£4.

View attachment 705380

https://www.ebay.co.uk/itm/373861584413

that may stop it falling off the inside of the chain ring, but not the outside

 

[Sharky]

Is the chain coming off when you are pedalling or when freewheeling?

 

[GuyBoden]

er... because you have neither a chain guide /catcher , narrow wide chain ring or clutch derailleur to keep it on. a 1x set up neds at least one of these...

something like this is what you need
View attachment 705369

shop around to find best value etc, I just snipped first photo i found for you

You might as well use a front derailleur.

 

[sleuthey]

er... because you have neither a chain guide /catcher , narrow wide chain ring or clutch derailleur to keep it on. a 1x set up neds at least one of these...

I have a 36T Narrow Wide Crankset on order. £15.80 on eBay. Also thought that might be the case.

If you have a 21 speed (3x7) and we're using the middle chainring and shifted from 3rd to 4th on the rear then I would not expect the chain to come off as the chain angle would not be excessive. Thus I don't understand why what I am doing is any different. Happy to be corrected.

Plus maybe the single chain ring is well further in than the designed chainline. I'm guessing this had a triple on before.

It did indeed have a triple. I'm just confused why it's doing it when the chain in well aligned in the middle gears. If it did it with 1st or 7th I would get it! 🤔

Edit: I'm also going to hypothesise that the chain comes off when you're changing down (ie to a larger sprocket)

Nope. 90% of the time it's when I'm upshifting.

Maybe the derailleur is gunged up or worn out so it isn't keeping enough tension on the chain, especially if the road is a bit bumpy when changing gear?

It's an Altus mech that's done well over 10,000 miles. It did work fine on another 1x bike but that had N/W Chainring. I'll give the pivots a clean.

Is the chain coming off when you are pedalling or when freewheeling?

Only when peddling. More so over bumps.

You might as well use a front derailleur

Appreciate the advice but I can't stand the things! 🤣

 

[Mike_P]

My ebike had an elongated chain guard which managed to get tangled up with chain on my trip for the first covid vaccination. Chain has only come off once since and that was due to changing gear as a tree root bump was reached

 

[Ajax Bay]

36T Narrow Wide Crankset on order. £15.80 on eBay

Best of luck. From their fromagerie range I assume. What length of BB spindle do they specify? Same as you have in already?
Anyway, the merits of a good chainline are over-rated.

 

[Gunk]

Looking at how the chain is falling off, I think the chain ring needs to go slightly inboard, all these chain guides do is mask the real problem, you shouldn’t need one if it’s set up properly.

 

[sleuthey]

Best of luck. From their fromagerie range I assume. What length of BB spindle do they specify? Same as you have in already?
Anyway, the merits of a good chainline are over-rated.

I didn't check as I have a collection of spindles.

Here it is:

https://www.ebay.co.uk/itm/31455375...ar=613105145552&widget_ver=artemis&media=COPY

 

[sleuthey]

Looking at how the chain is falling off, I think the chain ring needs to go slightly inboard, all these chain guides do is mask the real problem, you shouldn’t need one if it’s set up properly.

That's exactly why I have come to cc 😀

 

[Deleted member 121159]

That's exactly why I have come to cc 😀

You should be fine once you install the narrow wide chainring. Don't need to worry about spindle length too much as long as there's a reasonable chainline.