I've had the chain come off on my singlespeed MTB three times in the last few weeks. The first two times I thought maybe some random trail detritus had derailed it as I was pedalling down some narrow singletrack but then today it came off after a landing a couple of (not enormous) drops.
The chainring, chain and sprocket only have about 500 miles on them, the chain tension is good (not a lot of slack at all) and the chainline is correct. The chainring is a narrow wide Works Components one but the sprocket is a Gusset 6 bolt disc mount one because I'm running a Gusset XD adaptor and an X01 chain because that's meant to wear slower than most. It's always the rear sprocket that loses the chain.
So my thinking is I need a better rear sprocket (this one has quite thin shallow teeth) and to get that I need to swap to an HG freehub. The wheels are DT Swiss XM1700 from 2023's CRC demise sale. Have I missed anything? Any suggestions for an 18t that'll keep the chain on and allow me to ignore my drivetrain for a long time (which is surely the whole point of singlespeed?!)
It is the slack side of the chain that allows it to come off the rear sprocket - so under pedalling loads it is the lower run of chain that gets flappy. Under drops I'm not quite so sure - maybe just starting the pedal stroke whilst the chain is flapping about.
Similarly, it can be wear in the chain, so the top run gets pulled tight and all the play ends up in the bottom run.
Also I'm having more success with KMC singlespeed specific chains that have flat edged links and no chamfering.
So it can be a combination of things. As well as deeper sprocket teeth, guide discs either side of the sprocket help, but harder to do with that bolt on sprocket.
"Under drops I'm not quite so sure - maybe just starting the pedal stroke whilst the chain is flapping about."
I'm not sure I actually pedalled at all this time, but maybe I switched my foot position for the tight berm after the second drop - I pulled up because it felt like the chain had come off but I didn't have any moments of actually putting power into the derailing chain, if that makes any sense?
I'll check the wear on the chain but it's really not done many miles!
Most of my derailling issues stemmed from the chain, SRAM PC-1 or whatever their singlespeed chain was at the time just constantly shipped, I think frame flex was enough to derail it!
Swapping to KMC with deeper sided plates solved it.
By that logic fitting a sprocket with deeper teeth might also work 👍
Is the tensioner keeping the chain taught when you backpedal?
"Is the tensioner keeping the chain taught when you backpedal?"
No tensioner, it's a Pipedream Moxie with sliding dropouts. If I push as hard as possible on the middle of the chain there's only about 12mm of vertical displacement.
I'm just using these Gusset SS cogs on my Moxie - they're fine!
BIKETART.COM Had 1 chain drop in the last 990km / 5 months (and that was only because there wasn't enough tension in the chain).
I'm running cheap SRAM 9sp chains, and a VeloSolo Aluminium SS chainring.
allow me to ignore my drivetrain for a long time (which is surely the whole point of singlespeed?!)
Well. You say that!
I've probably been tinkering with my SS drivetrain more than I do on my geared bikes (although that's partly because my Moxie is my most-ridden bike currently ...)
Started off with a 19t cog (32 on the front), swapped to a 20t for most of the winter. Swapped back to a 19t for a weekend away to Suffolk (couple of flat-ish rides). Back to a 20t when I got back to Yorkshire. Tried a 22t for a ride at my local steep/off-piste/Enduro spot. Back to 20t. And now back on 19t now the trails are dry and I've faster rolling tyres on.
I also usually have to adjust the chain tension every 2-4 rides slightly!
But that finding the magic gear (yeah, I know, I kid myself!) for the type of ride/conditions is all part of the fun of Singlespeed?!
surly cogs are great but probably still only HG I guess?
otherwise i think the works components kit is pretty decent. think a mate is running that on his moxie. might be cheaper to get the freehub and surly sprocket though, added to which it's thick/thin, so even numbers of teeth only.
WHEELSMFG.COM “I'm just using these Gusset SS cogs on my Moxie - they're fine!”
They look quite a bit quicker where the chain sits than the Gusset 6 bolt ones I have. Fancy doing some measuring with calipers?
“They look quite a bit quicker”
Thicker!!!
Fancy doing some measuring with calipers?
Hmm. Would love to. But I don't have an calipers 🫣
I'll try a tape measure ...
“Hmm. Would love to. But I don't have an calipers 
”
Are you sure you’re allowed on here?! 😉
Is your particularly flexy along the stays I wonder?
I had a frame a few years ago with a flexy plate dropout and it would occasionally throw the chain.
I've been using the Gusset kit on my current SS for a while without issue, usually with an 8/9 speed chain. The cog teeth on mine are pretty tall looking to me, poking right through the chain as pictured.
I think some others do these bolt on cogs you could try though, Velo solo being one name I recall from my fixie bothering days.
I had a rear cog that slipped and dropped chains - i think the manufacturing tolerances were crap.
A new cog and all good. I do run a solid tensioner though. I found the sprung ones let the chain skip under load (due to my massive power output! haha)
I`d tighten the dropouts - and get a more robust chain.
If you do decide to go hyperglide freehub then these AbsoluteBLACK Single Speed NarrowWide Sprockets are good (although unfortunately 18t is out of stock).
"Is your particularly flexy along the stays I wonder?"
When I had chainline related droppage last year I compared this to a singlespeeding friend's Sonder by doing the lean the bike over and push the crank into the ground test and I was surprised that my skinny-ish steel Moxie was miles stiffer around the BB than his boxy carbon frame - IIRC the Moxie moved less than half as far.
"I had a rear cog that slipped and dropped chains - i think the manufacturing tolerances were crap."
This one does look pretty cheaply made so maybe that's it? I think I have some spares somewhere...
" I do run a solid tensioner though. I found the sprung ones let the chain skip under load (due to my massive power output! haha) "
Sliding dropouts! Not sure of my power output but I got all the way up EP yesterday (first time every on a singlespeed!) Actually, every time it has come off it's been under minimal or no load (thankfully!)
Chain is too slack. A 12mm vertical deflection would be impressive on the track. I would go with 5 mm. Also since chainrings and sprockets are not perfectly round that 5 mm would be at the deepest drop in a revolution. I have never dropped a chain off-road on my genesis io as. I have lost a chain on road when the wheel hit a pothole and moved, but 5 m off road, 10 mm on the track and road.
Odd that it keeps happening! I've yet to drop a chain on either of my ss bikes. Shimano slx narrow wide chainring, thin pressed rear cogs and slx 12 speed chains.
Is the cog slighlty off centre? Could possibly give you a slack spot enabling the chain to drop? I had that on an xd hub before swapping to microspline.
Chain is too slack. A 12mm vertical deflection would be impressive on the track. I would go with 5 mm.
I'd disagree. Just checked mine (same frame as OP) and vertical deflection on the top part of the chain is at least 10-15mm (pressing down as hard as I can), and nearer 20mm at the bottom. I don't have any issues dropping the chain, so suspect OPs problem is worn teeth/short teeth/poor chain line.
Clearly I need to run my chain tighter - I've got at least 25mm of slack at the top! I've run it like that on my main bike for the last 8 years or so and have dropped two chains. Can't be arsed constantly moving the dropouts in and out. Maybe I've just got lucky, but I've been running predominantly Surly sprockets with nice long teeth and either Wipperman 8 speed or KMC narrow SS chains. (On a Moxie for a few years too, so I don't imagine it's a frame issue.) I'd just change sprocket in your case I think.
As above, I don’t think a chain needs to be that tight, the dropouts on my last singlespeed used to slip a bit and the only symptom of a sagging chain was aesthetics.
My current singlespeed has much less chain clearance under the stay and it’s a pain trying to find the sweet spot between enough tension to not rattle against the frame, and too much, resulting in a tight spot.
Belt drive fixed all that, set, forget and enjoy the silence 🙂
I like beefy BMX chairs like Odyssey Bluebird or Shadow Conspiracy . Also run a Chris King King Cog which I think are hard to source thesedays but Wolftooth do an equivalent. Also run as large a cog as you can for more chain wrap.If you need a stiffer gear put more teeth at the front. I've never dropped a chain since running this set up.
sounds like you need a rear cog with taller teeth and I'd give it a bit more tension, I'd take 10-12m slack at the loosest part of the pedal stroke with an elliptical front ring, but on a round rind I would pull that bugger off under stompy load in no time.
Avoid absolute black cogs, as they shed teeth faster than Shane McGowan, likewise avoid SRAM PC-1 chains as they are awful snappy things and will also cause premature wear on alloy chainrings and cogs. Good old 8 or 9 speed chains for win, cheap as chips and take huge amounts of abuse. (run them on 8 of my single speeds without issue for years)
I'd swap the chain to a good old fashioned KMC 3/32 SS chain, it's cheap and would at least rule an obvious problem. I've not had any issues with my gusset sprocket (other than the threaded part will seize onto the XD freehub, I hope this isn't a wheelset you ever want to swap back!).
I don't think chain tension is the problem, I've run bikes with visibly sagging chains and not had them skip as long as the alignment is good. I just set it so it's not tight at any point in the cycle. Bear in mind it can't be too tight or frame flex will be as much of an issue again.
And I don't know about anyone else, but I always struggle too estimate chain deflection visually. The difference between 0 and 12mm deflection is only about 1/3 of a turn on the adjusters (assuming a 0.75 pitch thread).
There’s a lot of variables aren’t there?! I went with a 30 tooth chainring for more log seesaw ground clearance, accepting the small decrease in efficiency without considering the increased chance of the chain falling off. And an X01 chain because they’re meant to last ages - again not considering it would fall off more easily than a singlespeed chain. And a modern width chain so it would hold better onto a narrow wide ring - but instead it falls off at the back!
So I’ve checked for variations in tension as the cranks and wheel turns and they seem very subtle. I’ve tightened up the chain and will be more vigilant in future.
If that doesn’t work I’ll try a singlespeed chain!
And if that doesn’t work it’s just dawned on me that I have a spare 29” wheel off my mulleted ebike which has an HG freehub! There is the minor cosmetic detail that it won’t match - although annoyingly it would if DT Swiss made their factory wheel rims look the same as their aftermarket rims because the current wheels are XM1700 (XM481 rim on a 350 hub) and this spare wheel is an EX511 on a Pro 4 hub…
I’ve tightened up the chain tension until the chain feels like it can’t come off but that does increase the drag when I spin the cranks by hand.
I’m thinking that one of these chains could be the answer:
KMCCHAIN.COM Tall and thick plates, no chamfering, narrow-ish and I can’t see any reason why it won’t work on my narrow wide ring or the rear sprocket.
Yes that is the chain I mentioned in the very first reply. Works fine with narrow -wide.
“Yes that is the chain I mentioned in the very first reply. Works fine with narrow -wide.”
Well it’s one of them. 😉 If KMC’s website is correct they make 35 (THIRTY FIVE!!!) different singlespeed chains!
KMCCHAIN.COM 
