so twice in the past week i thrown my chain off.both times on a fast winding descent,where i was really spinning.ive been able to come to a nice safe stop both times,but the law of averages states that-im going to die next time this happens......the first time it looked like my one of my rear axles nuts wasnt tight enough and it slipped.but today it was still on dead tight.chain was taut,chainline seems straight,no idea why the chain would throw off.....any ideas?

are you running 3/32"?

mine popped off frequently, mostly when i was first starting out or going really slow. i used to run 3/32, and ran it pretty tight and i'm guessing it just popped over one of the teeth on the ring somehow with the crank flex. i've switched to 1/8" and haven't had it happen since.

oh shiz.
thats pretty scarry for me. My new buildup is going to be a 3/32"
I think I should really get some dough to convert my chainring and cog to 1/8"

i have been running a 1/8" on a 3/32" setup for like 8 months... no problems. i am, however switcing over soon when i finally buy new cranks

Chain width has very little to do with throwing your chain. Your chainring might be off center giving you slack, or your chainline might be off. I'd double check it.

Yeah, a 3/32" chain is NOT going to fall off more often than an 1/8".
Check your chainline. Make sure the chain is not loose (but not too tight either). Don't use chainrings made for geared bikes that have ramps, pins, and short teeth. Get a chain tensioner like the Surly Tug Nut (this will guarantee the wheel axle can't slip forward under hard pedaling). But most of all - check your chainline. Get a bike shop to use a chainline tool because it is really hard to tell if the chain is straight or not just by eyeballing it.

is this spencer rangitsch by any chance?

a 3/32" chain is going to have easier side to side flex movement than a 1/8" chain because it is thinner... try bending a each kind the wrong way and you'll see what i mean

Never thrown a chain. Put a wrench through a window once, though.

*rimshot*

*****ob*?

Chain width has nothing to do with being easier to fall off. Chainline and chain tension do.

If anything, I would think a 1/8" chain on 3/32" ring/cog will fall off easier because of the extra 1/32" slop between the chainring and cog.

maybe you're right, but i have never thrown/broken a 1/8" chain on my setup, but it was frequent when i was running 3/32" on a straight chainline

It's really hard for me to comment on your anecdotal evidence.

It does, however seem weird that a 3/32" setup would throw chains, and upon switching to a 1/8" chain, no more throwing occurrs.

I'm still betting chain tension.

like i said the chain is pretty tight.any tighter would seem too tight.im gonna stop by the lbs and have them take a look...it just sucks cause for now i dont trust my bike.any noise(which is usually my creeking seat/seatpost)any i get scared.and im especially scared to bomb hills...which is just fun.....

sorry,no.spencer is actually my last name.

interesting how we all have our own empirical reasonings/anecdotal evidence and they often times conflict...

I have a 3/32 chain (old road HKK for my 12speed (6spd.freewheel) 80's peugeot) on my conversion, I've never 'centered' my chainring (it isn't centered, as my chain is tighter in diff crank positions than others, although not significantly), I'm missing a chainring fastener bolt, my chainline is visibly off by at least around 10mm, and I ride breakless and fast in a hilly area (skipping plenty) ... and I have never thrown my chain

I do, however, make sure that my chain tension is propper..

I'm not trying to push my 'luck', though, as my newly ordered single stack chainring bolts will allow me a chainline (on the other side of spider) more close to within around 2 mm of my cog in the back...and I'm getting a new KMC z610 chain because mine has been stretching a bit. although the kmc is designed for singlespeed, I hope that going from bushinged chain to bushingless will not allow for it to waver laterally more

I'm gonna go with junioroverlord and say that you prob should check your chain tension at all crank positions

'bout the only other reason I could think for a bike throwing a chain other than chain tension is that on a conversion if your chainline is off enough, or your cranks/ring can flex enough, or you have a chainring with shifting platforms, you could theoretically 'shift off ' under very high load.

But I've run 3/32" chains and know a fair few folks who do it and no-one has ever thrown a chain. Maybe your hub is slipping in the dropouts?

It's been mentioned before in this thread. SS/fixed specific chainrings work best. Otherwise, the ring is designed to drop the chain with enough side-loading. Surly makes some affordable steel rings in a lot of bcd's.

A lot of SS'ers have the same problem when using hyperglide cassette cogs on a cassette hub conversion.

Maybe a loose bottom bracket? That would allow some looseness, sloppiness or "wobble" of the chainring. This isn't exactly "flex," but the effect would be the same, which could explain the dropping as well.

i am gonna guess something different, frame flex....

is it by chance an old steel frame? does the botom bracket noticably sway from side to side when really mashin?

i would think maybe, just maybe, you are using your legs to try and slow you down so much the whole frame is flexing throwing your chainline off and therefore making your chain pop off.

quasi-tangential, but I hate it when folks assume all 3/32" chain is derailleur chain. Plenty of good 3/32" single-speed chain out there with lots of lateral stiffness.

i run an SRAM 9 speed chain on a BioPace chainring, it's never fallen off, or broken. it's the quietest ive ever heard my drivetrain. 1/8 chains stay straight, so if you have a bad chainline the chainring is obviously going to jump off. and a 9 speed chain is built for strength too, multi speed chains break more often cause of shifting, so they build them really strong. and if used on a single speed they work great and are quieter than cheap bmx chains. non of us are grinding so there's no need for bmx chains

il try and answer all the questions:1/8 KMC BMX chain,old steel frame,i do use my legs to slow down but not primarily(i have a front brake).....chainline looks dead on but i guess you cant eyeball it....i a havent noticed the BB slipping,but maybe.....gonna head over to the lbs tomorrow and try ang get it figured out...thanks for your help guys.

so i still havent stopped by the lbs,but ive just been taken it easy the past few days.no big hills or real speed.but last night i was riding down a pretty mellow hill,using only my legs to slow down.as soon as i really started to resist---there it goes,chain throws off.so at least now i know when it happens.i guess that narrows it down a little bit.

Is your cog 1/8 as well. The only time I've thrown chains is when I was running a 1/8 chain on a semi-worn 3/32 cog. A new 1/8 cog cleared it right up.

Is your chainring super worn-out? Is it a one-sided chain that's upside down?