Chain falling off

Hi,
Having some problems with my bike.. The chain has begun falling off with increasing frequency - it fell off occasionally when I first got it, but has recently started to fall off pretty often. The chainline is bang on, and the chain isn't slack. I took it to a bike shop and they said it may be because my chainring was designed to be used on a geared bike and thus a small nudge will cause the chain to drop down a gear, or completely off in my case. Is this possible or the bike shop trying to get me to buy a new chainring off them?
I bought the cranks & chainring off someone on here so I wouldn't expect it'd be the wrong thing for a single speed?
thx

Pictures from a few angles of chainring and rear sprocket

If they are modern road cranks then they might be 'notched' in order to aid shifting, have a look and see if any of the teeth have anything like that on them. Pics would be better. If you're sure the chain's not slack, it shouldn't able to come off, it would just seize instead...

I've only ever used road chainrings so that's not likely to be the problem. (including modern ones). Have you checked every single tooth on your sprocket and ring? Even a tiny little bend or kink in one tooth can give problems; it's hard to see how a chain that is tight enough can fall off without something 'pushing' it. Are the teeth worn down to a "shark's fin" profile? Is your bottom bracket axle straight; no side-to-side movement in the chain ring? Is the chain ring straight? Has your bike ever been cursed by anyone?

it's hard to see how a chain that is tight enough can fall off without something 'pushing' it.

;)

Its possible that the rear is twisted and out of line

The most obvious is chain ring wear plus a combo of slack chain and a bad chain line will cause this.

loose bottom bracket ?
loose chainring ?
chainline ?

bent chainring? remove it and put on a flat surface to check. maybe a bent link in chain? Should be something obvious as if the chainline is good and chain is tight it should work.

that or a worn out chain, how long has it been on there?

if it's a conversion on a cassette hub, then even the tiniest amount of play (often not very noticeable) in the spacers can cause this.

Your chain is too loose or your bike has teh AIDS.

Probably the bad AIDS

With Good Aids, you'd get 'slippage' not derailment.

have you got a singlespeed/track chain or one designed for shifting with loads of side to side play? will exacerbate any of the problems already mentioned

with good aids, you'd get 'slippage' not derailment.

lol

have any of the teeth got height differences (ie: some look ground down and some sit tall and proud) some chainrings have them to aid shifting................could be that

Nope.. Aids...

YouTube - Brass Eye - Good AIDs vs Bad AIDs

Ha!
YouTube - Brass Eye clip from 2001 Pedo Special

biopace ring?
http://en.wikipedia.org/wiki/Biopace

but agan pics help

We're getting all esoteric now...

do any of the teeth look like people who live in west london: ie: they look ground down? And do some sit tall and proud like people who live east? ................could be that

.

Chainring.

Are you riding brakeless?

This can affect things, particularly if you have risers.

Do you ride a Bianchi Pista with motorcycle style risers- i.e. with the reinforcing bar between the two sections where the grips go?

If so I can tell you why your chain keeps falling off.