Spesh Stumpy 2017… 11 Speed XG cassette, SRAM x1 rear mech, RF Aeffect Chainset. run with a stock chain retention device.

I have never dropped the chain. Ever. in 4 years. Until two weeks ago. and now it has happened 4 times in successive rides, and wrapped around the crank arm. (on the same bumpy fast descent)

But the retention device is still firmly in place. There is no moving it, and no room for the chain to go back on. IT’s like one of those puzzles you get in a Xmas cracker where once it is undone, there’s no getting it back. So I have to remove the device to get the chain back on, and then re-fit it.

It’s puzzling me as you cannot remove the chain by hand once it is back on. I’ve positioned the device so that it (shouldn’t be) physically possible for the chain to come off. it’s fractionally above the chain ring teeth.

Drive train seems good. Super quiet, shifting is fine.

What am I missing!? Why’s it happening?

Chain ring and cassette are about 4 chains in… 2500 miles (this bike is only used in summer….my HT takes the brunt of winter). Rear Mech seems to have plenty of spring. Always measured the chain against the one coming off….so this chain is no longer or shorter than any of the other chains, and indeed rode fine for several miles before this started happening.

Help!

I had something similar and it was the rear derailleur spring was not working as it should. Chain was quite loose in the higher gears (but everything looked fine in lower half of the cassette). Worth a check?

I have been having the exact problem that you describe over the last few rides. Pretty sure it’s the rear derailleur (Sram x01) not taking up the slack properly when the rear suspension compresses fully.

Yep, sounds like the spring/clutch given up in the mech, presuming your chainring is fine.

Chain ring and cassette are about 4 chains in… 2500 miles

I wouldn’t rule out the chainring, with NW rings it’s the retention that can go first, even though the drive from the teeth is fine. Just might not be gripping the chain as well as before.

I had the exact same issue on my Enduro which had the exact same parts, I found it would happen when I dropped my pedals from level to foot down for turning or dodgying stuff. When I move my front foot down, I pedal forwards. When I drop my back foot, I back pedal to do this and the chain would then come off the chainring from the bottom and come out of the guide, think it was linked to the clutch in the mech wearing and the chainring teeth wearing meaning it was already off at the bottom during bumpy terrain and the back pedal would bring it through the guide.

To stop it I replaced everything including the very cheap superstar chainring which wore really quickly compared to what it replaced, I also picked up a csixx gravity guide with top and bottom guides which was a bit overkill but havent had this issue again, although I did get stung a treat on import by HMRC, almost as much as the guard cost!

Narrow wide chainrings start dropping the chain when they are worn, even though the chain does not slip under load.

If the chain comes off at the bottom and you back pedal a 1/2 turn it will force the chain off past the guide due to the leverage/body weight, but you will be unable to put it back on by hand.

If the mech is not holding the chain slack tight you can get the same symptoms.

I wouldn’t rule out the chainring, with NW rings it’s the retention that can go first, even though the drive from the teeth is fine. Just might not be gripping the chain as well as before.

100% this.

Also don’t underestimate how much bike components flex under load. That explains how the chain can pass through a seemingly non-existent gap.

I’d an inkling it may be the chainring. The mech feels fine. Plenty of spring….I’ll replace the ring first. And then the mech if no success!

Thanks!

Check the chain ring is straight, no bent teeth, no loose chainring bolts. Check the chain has no stiff or bent links, check powerlink is the correct size.

I have had pretty much the same and discovered the cassette was slightly loose on the freehub so worth popping that out and tightening it up – no idea how it came undone! Plus the chainring as above.

It’s puzzling me as you cannot remove the chain by hand once it is back on. I’ve positioned the device so that it (shouldn’t be) physically possible for the chain to come off. it’s fractionally above the chain ring teeth.

The amount of force the chain can generate when it’s lifting off the ring is huge, way more than you can generate trying to do it manually, it’ll find a way when you can’t believe it. (think about what happens to people who get their fingers between chain and ring…)

I had my chain keep coming off on a recent ride. No other issues. Turned out to be snapped axle! Quite surprised everything else was fine. The only thing was the freehub occasionally sticking just a tiny bit, and that was enough to throw the chain.