How do you tell if a cog is worn out?
 

15t, 3/32. Machined generic brand.

Has about 3500 miles on it, mostly with either a 40t or a 42t ring. I just put on a new chain, and when I went for a ride, it would make this rattle sound from the rear - as if I had a derailleur - when accellerated. The chain was tight and as far as I know, the chainline on this bike is pretty good.

Noise like that probably means it's worn, especially at that mileage. Really, chain rings and cogs are consumables just as chains are. They should all be replaced together if worn, as they form to each other and will only accelerate wear if a new part is introduced without replacing the others.

the best bet is to change everything (cog/chainring/chain at the same time) because when they do wear out they wear out evenly. switching things separately does lead to noisiness

Hmmm... If replacing the cog then definitely replace the chain, as mentioned, the two will wear out together meaning the chain will stretch out and grind the teeth in the cog to account for the stretching. As always, Sheldon has a good article on this:

http://www.sheldonbrown.com/chains.html#wear

As for replacing the chainring as well, I'm not too sure about that one. The article doesn't mention chainrings specifically and it might get worn less since less force is applied per tooth. Not sure though.

I would not replace my cog and chainring every time I replaced my chain. do you guys also light cigars with $100 bills or what? Just replace the chain frequently (I do it once it stretches to 1/16" per foot or so) and use cheap chains. It's almost always noisy when I put a new chain on my drivetrain but it quietens down within 300 km or so.

Jolson you might have borked your drivetrain by waiting too long to replace the chain. How much is the old chain stretched?

Also, a tight chain is very hard on everytihng in your drivetrain: chain, cog, ring, hub bearings and bb. Slack is your friend.

Usually with chainrings the wear has to be visible before you have a problem, so just look at it and see if the teeth are symmetrical or if they look like shark fins. Chainrings take less stress per tooth, but they're aluminum so they wear faster.

Can also run a lower GI since mashing will wear out the chain and cog faster.

I would think it would have to be a really cheap cog to be worn out after only 3500 miles.

Next time drop the extra $10 for a Dura Ace cog and clean your drive train more regularly.

Chainrings don't usually wear out as often as cogs and chains. Even when you have to change the two (chains + cogs), the chainrings are usually o.k.

Post a picture of your cog. Extreme wear is immediately apparent.

This is undoubtedly what happened. 3500 miles is a lot for one chain. Once the chain is stretched past 1/16" per 12 links, the rate of cog wear accelerates rapidly. Putting a new chain on a worn cog produces the effect described by the OP. If the cogs is only worn a little, things will smooth out as the new chain wears slightly. If the wear is more severe, its time to replace the cog, and possibly the chainring.

Actually, the old chain had about 2k on it, maybe less. My park CC-3 reading was between .75% and 1.00%.

I also just flipped the chainring around, which also had less than 2k on it. The only thing that had 3500 miles of use was the cog. That cog came with a rear wheel which came stock on a Raleigh Rush Hour. The wheel was a take-off from a shop that I got new, along with the cog, lockring and freewheel.

And I keep my drivetrain very clean.

I just put on a 46t chainring, flipped the wheel to the 17t cog and put an 18t cog on the other side. The 46t ring is steel, taken off of an old Bridgestone mtb.

I've been trending toward larger chainrings and cogs for a while.

A few of the teeth on the 15t cog are starting to look like shark fins.