had a couple of chain drops when going down not-that-rough stuff today, it’s tight enough and the chainline is correct, i’m pretty sure (measured) – measured because it came off a couple times when i was pushing hard last week resulting in shredded calves.. moved the cog a couple mm.

it’s a cheap narrow cog at the back and a 1/8 chain – might a fatter cog improve things?

if not, can you get like a rear chain guide that sits in the spacers? what would they be called?

it really shouldn’t be necessary.. but if it stops the chain coming off it might be worth while.

if it doesn’t exist i suppose i could just stick a couple bigger cassette cogs on, one each side 😉

am now finding out that 1/8 cogs aren’t too common.. do they exist?

On a SS your chain should be held on because it is the same length as the distance between the teeth on the front and the back.

ie, the chain will only come off if it is too long, or the rear wheel is not far enough rearward in the dropouts (assuming its horizontal dropout).

Hardtail or full sus ?
Sliding dropouts, EBB or tensioner ?

hardtail, sliding dropouts

the chain is definitely not too long, any tighter and it makes that horrible “tight chain” sound.. obviously it’s loose enough to come off i suppose but still.. argh i don’t know

only other thing i could think was that it might be squint but it certainly doesn’t look it.. unless the frame is twisted.

if not, can you get like a rear chain guide that sits in the spacers? what would they be called?

Discos.

http://www.mudandcowbells.com/blog/2009/7/18/rocking-the-discos-singlespeed-chain-guide.html

ah cool – not finding them online but now i think about it they’d be a piece of piss to make.. cheers!

still would like to get to the bottom of why it’s happening though.. it’s not common, right?

chain is new and undamaged btw

could buy them briefly, but they never really took off.

what sort of dropouts?

Is your chain tension even all the way round ?
It’s not uncommon for chainrings to be off centre.
You might find it’s too tight at one point, yet loose enough to drop off at another as you turn the cranks.
Try moving the chainring round on the spider.
If they are both out, you might find the errors compounded at the moment.
If you move it round, they might cancel out.

ah yeah that’s a thought.. it is a little off currently though i didn’t think it was too bad. will investigate.

Bent chainring?

Non-bending link in the chain?

nup

i’ve often found smaller cogs to drop, eg 15, 16t wheres chains usually stay put on 18t if the chain is a little slack. (though yours sounds not to be)
i doubt the width of the cog will make a difference, never has for me, maybe just me but i see no reason to use a bmx style chain, just a cheapo steal plate cog and 8 speed or kmc singlespeed chain works great.
try flipping the cog and see if that helps?

it’s an 18 :/

cog seems pretty symmetrical!

“i’ve often found smaller cogs to drop”
I use a 13t on my Rohloff, which is effectively a single speed, and had no problems.

stiff chain link? or using wrong joining link?

I use a 13t on my Rohloff, which is effectively a single speed, and had no problems.

yea i know it can work just fine but smaller cogs are more susceptible to poor setup than larger cogs

so i tried the large-cog-each-side “disco” technique, worked a treat! no offs, i did hear it rattling off them a bit on the rough stuff so think they did the trick..

i’ll probably grind the teeth off to avoid horrendous calf severing and would like to know why it happened in the first place.. but yeah, sorted!

If you use chains, cogs, or chainrings that are for derailleur bikes, then they’ll do what they’re designed to do. That is allow the chain to slip off sideways easily.

Use proper singlespeed chains, chainrings, and cogs, get your chainline and tension correct, and the chain will stay on without any guides.

Unless…

Your frame is flexing laterally under pressure. What bike is it?

I’ve not got a vendetta against SRAM or anything, but I found simply switching from an SRAM PC-1 chain to a KMC equivalent solved all my dropping problems. Guess perhaps the PC-1 could flex sideways more or something?