I desperately need help with this. I have a fixed gear bike I built myself (Ebay wheels, old Schwinn frame, various other parts left over and lying around). I ride it to commute, and have put around 2000 miles on it with very few problems. However, lately my chain has been falling off, and today I couldn't get it to stay on. Had to call my wife to pick me up and then drive into work.

I've tried replacing the crankset/chainring, and cleaning the chain. I suppose the chain could be stretched. Any other suggestions? The chain appears to be straight.

I'm incredibly frustrated right now with this, and am even seriously considering going back to my road bike for commuting purposes. I love riding fixed, but I can't have this much of an interruption of my commute. I'd be grateful for any suggestions people can offer.

R.

check the tension...it shouldn't be hard to get the chain back on the sprockets...you said you replaced some parts did you replace the chain as well? of your chain is worn than it wont work well on the new chainring or cog

IF:
the chain line is fairly straight (within a few mm)
both cog and chain ring are the same width
all components are of approximately equal wear
the chain tension is adequate
the chain is appropriate for your gears (1/8" for 1/8" or 3/32" gears, and 3/32" for 3/32" gears only)

THEN:
your chain won't fall off.

The last link in the chain needs to be connected to the first link.

Seriously, post pictures of your chain-line.

The chain needs to be connected? D'oh.

Thanks to everyone for the comments.

Seriously, here's a pic of the chain line, or at least, the way the chain line was last March. It looks straight to me. If it isn't straight enough, how would I go about fixing the path?

I suspect chain wear, and am planning to upgrade the chain. I think it's the right size. It was a 1/8" chain on 3/32" cogs. I'm planning to go with a 3/32" chain this time around since Sheldon said to on one of his fixed gear pages.

That is probably the main part of your problem.

Get the 3/32 and run a bit tighter tension to make up for the imperfect chainline.

Also, try getting a fixed specific, unramped chainring.

That chain-line looks so-so to me. Looks like the chain-ring could be shimmed over a couple/few mm's.
If it was running good before, I'd blame wear..

What does one use to shim a chainring? Should I just put washers between the chain-ring and the crank arm?

not sure about washers if you did that you would need different chainring bolts...what you can do is move the chainring to the inside of the spider if its not already that can correct things just enough

Are those vertical dropouts? If so, getting proper chain tension is going to be difficult. Sounds like your chain has worn enough so that it's too slack.

"I love riding fixed, but I can't have this much of an interruption of my commute. I'd be grateful for any suggestions people can offer."

Have you considered using a freewheel and chain tensioner for a reliable single-speed instead of a flaky fixed gear?

I think they're those old horizontal slanted drop-outs.

You can move the chainring around mm's by using PROPER chainring stack spacers. You can get them in different thicknesses, down to like 0.5mm They're pretty good! You might need longer chainring bolts as well, but most likely not.

How used was each element in your drivetrain prior to assembling, and were they of different age? What amount of tension were you running? Is your chainring centered (ie no "tight" or "loose" spots in the pedal stroke)?

Once you get a perfect chainline (a shop can use a chainline guide tool to check perfectness) then get a new chainring/cog/chain all at once. Get them all in 3/32 or all in 1/8.

Beyond that, maybe it's the old frame? But I would think it's just the drivetrain.

In my opinion fixed gear bikes should be on a good, solid frame, with good solid parts all around. I would not ride a cheap fixed gear bike.
As mentioned above, a super-reliable cheap bike is an old frame, singlespeed with "set and forget for years" chain tensioner and cantilever brakes.

If you cranked the chain off under tension, your chainring might have bent some teeth?

Oh yeah and make sure it's a non-ramped chainring!

Check it ...https://www.sheldonbrown.com/chainline.html

"Chainline Adjustment-Front
If you need to adjust the front chainline, there are several different options to accomplish it:
Change the Bottom Bracket
Most bottom brackets made since the mid-'90s are symmetrical, they stick out the same amount on each side. If you replace your present bottom bracket with one that is, say, 4 mm shorter, it will move the chainline 2 mm to the left, because it will be 2 mm shorter on each side.
If you go this route to a narrower chainline, make sure that there won't be a clearance problem between the frame and the crank or chainring.

Bottom Bracket Spacer
If you need to increase the chainline (move the chainring to the right) you can usually add a spacer washer between the right-side bottom bracket mounting ring (or cup) and the bottom bracket shell of the frame. The usual spacer for this is a Sturmey-Archer sprocket spacer. These fit all threaded bottom brackets except Italian size. Sorry, I don't know of any source for a spacer that will fit an Italian (36 mm) bottom bracket.

Chainring Spacers
For a single chainring, you can add spacers between the chainring and the crank spider.If your chainring is mounted on the outside of the spider, you can move it to the rigtht this way. If it's mounteed on the inside of the spider, you can move it to the left as shown.
This uses the same 10 mm I.D. spacers normally used on rear hub axles.

Suitable spacers are available in 1, 2, and 3 mm thickness. You may need to use longer "double" stack bolts, especially for the 3 mm size.

Phil Wood Bottom Brackets
Phil Wood BBs are super quality, available in many different lengths, and they also feature the ability to adjust the chainline over a range of several millimeters.
Phil Wood stuff is expensive, but it's the Very Best.

This will push the chainring in. It looks as if it need to go out.
Adding washers to push the chainring out should work.

Heres a pic of a chainring on the inside of the spider.