Hello, i'm new here. But i've been riding my fixed gear bike for some months now on my commute; I know how to skid and skip stop, but when skip stopping after a while, the chain falls off (while I'm stopping) and gets stuck between the cog and the hub. I don't know why, cause I keep my chain properly tensioned.
I wanted to know your opinions, can you help me please? :'(

Get a front brake and stop worrying about your chain falling off?

even with a front brake throwing a chain is definitely something you don't want happening and should seek to resolve. your chain ring may be out of round causing slack in some areas where the chain has enough play to come off. another issue could be that maybe you're not tightening your axle nuts enough and the wheel is sliding forward in the track ends

So then should I buy another chainring?

I've actually got both...but not using them (all the time) is the most fun of having a fixed gear, I think.

not necessarily, is isn't always only the chain ring, and being a little out of round is common on most bikes. just rotate the pedals and keep an eye on the chain for an area with significantly more slack. at that point if you can pull the chain off the rear cog then you know you should slide the wheel back a bit. just make sure the chain won't also bind at the tightest point.

Are you using a 3/32 chain? Most of them are designed for geared bikes and will flex side to side (in order to facilitate shifting), making it more likely to pop off your cog when skip stopping.

Not until you're certain that's the source of the problem. If it's because the crank spider is out-of-round, sometimes you can compensate for this by rotating the ring one position at a time on the mounting bolts until you find a place where effect is minimized.

What kind of ring do you currently have? If it's intended for a road crank, the ring itself may be out of round, and most modern road rings have modified teeth to make derailing easier for gear changes. If that's the case with your current ring, either find a vintage ring without the modern shifting improvements, or get a ring intended for track use, which may cost more because more attention is paid to getting them properly round.

1/8" chain with cogs and chainrings also 1/8" will give you the most secure drivetrain. But whatever you have, there are things you can do to minimizes chain deraillements.

Spin you cranks (with the rear wheel off the floor). You will see the chain go tightish then slacken. This is either the cog or the chainring not being a perfect circle centered on your BB axis. There isn't much you can do with the hubs/cog and they, even less expensive ones, are usually not too bad. Cranksets can vary a lot. Cranksets made for geared bikes may well be made cheaply with little thought or care given to making everything perfectly round and centered because on a derailleur bike, that matter not at all and may even improve shifting.

So ... you see the chain go tight and loose. Adjust your hub location until the chain has just a touch of clack at its tightest, then check to see how loose it goes. If the chain is within about 3/4" of play total up and down when tugged, you should be OK - if you do not have chainrings and chain designed to shift easily. If you do, change them! (And seriously consider spending the bucks to go 1/8") If the chain ever goes completely tight, it will shorten the life of the bottom bracket bearings and hub bearings a lot.

I've been riding fix gear forever. I love the ride. Never skidded. Always had two very good brakes. Always rode with good road tires and skidding would have put me in the poor house years ago. I have derailled a few times, sometimes (in fact usually) at high speed going downhill. It can get expensive. Paint, spokes, tire and chain usually suffer a little to a lot. (Better be carrying a bunch of dollar bills. You may well need them to stuff in your tire to get home.)

Ben

chainline

+1 Chainline is the line the should be taking from your front chairing to the rear cog. It should be with a couple of millimeters of exactly parallel to the frame. To check this, site down the chain from behind the bike, pre3ferably up on a stand, with your eye just above the top of the chain. The chain should be exactly straight. Even better, do the same sighting with the chain derailled but the wheel in place. No you can see if the cog exactly lines up with the chainring. If it doesn't, that's a real problem and needs to be fixed. To be completely kosher, the chain should be 42 mm to the right of the centerline of the bike. If so, you can swap to other standard cranksets and hubs and not have issues. (This is the universal track standard.) But as far as running well, the bike does not care if your chainline is kosher. It only cares that the chainline is straight.

Ben

+1 on chainline. I would make sure the sprocket is aimed at the chainring. There may be an issue with your frame where the tire is centered between the chain stays and the sprocket is crooked with respect to the chain ring. If your chain has a master link you could take the chain off with the wheel in place. Then sight down the chainline to see if they are aligned. You could also use a string to see if the two gears are parallel.

I doubt this is the case, but is your sprocket tight. Perhaps it is unscrewing and coming out of line.

Ride Safe,

Joe