Bicycle Mechanics - How do you gauge cassette cog wear?

I was wondering what criteria people use to decide when their cassette is worn out. I just put on a new old stock 7 speed uniglide cassette on my roadbike about 900 miles ago and I clean my chain, chainrings and cassette cogs about every 300 miles. I've already noticed the silver colored plating is coming off the teeth on a few of the more used cogs. Is this purely cosmetic? I do ride in the rain sometimes but I think I do a decent job of keeping a clean drivetrain. I just got a Park Tools chain checker and it says that the wear on my SRAM PC-69 chain is about 0.5% which means it isn't worn out yet. I just would like to know if there is some kind of objective test to determine cassette cog (also chainring) life. Partly I'm concerned because I'm running Uniglide but the supply of these is very limited, and if I need to convert to Hyperglide sooner rather than later, I'd like to prepare myself.

The cassette is DEFINITELY worn out when the gears start slipping.

Chain wear and cassette wear contribute to each other in a vicious cycle (read Sheldon Brown's chain article for the details), so if you check your chain for wear, and replace the chain when it reaches 0.5% elongation, your cassettes won't wear too much. On the other hand, if you let your chain wear out to 1% elongation or so, then replace it, the cassette will probably be too worn and the chain will slip on the small cogs. :(

I was wondering what criteria people use to decide when their cassette is worn out.

I had good experiences using the Rohloff cog wear indicator:

http://www.rohloff.de/en/products/hg_ig_check/index.html

In my experience, it has been hard to wear out the largest cogs, but the middle to small cogs indeed deteriorate. Still, you need to ride for a much longer distance than 900 miles and losing some coating is not enough for condemning a casssette. When eventually some cogs go bad, you might replace just those. On the other hand, separate cogs could add up to a higher cost than a whole cassette.

I've already noticed the silver colored plating is coming off the teeth on a few of the more used cogs. Is this purely cosmetic?

Yes, it's just cosmetic, but that is where you'll begin to watch for wear. These cogs are so nice and thick, they last much longer than the Hyperglide. That's one reason I love them so much. That cassette should last you well over 10,000 miles easily, and at least twice that if you keep the drivetrain immaculately clean and change your chain very often. My strategy is to use a less expensive SRAM PC-48 chain, and change it as soon as there's ANY detectable wear at all. Like around every 1500 miles. These are good chains, same rollers as the more expensive ones, but only about $7 each from probikekit.com.

Thanks for the replies. I'll look into the Rohloff cog thingy...I might order one if its not too expensive. I'm using a PC-68 chain not a PC-69, it does have over 2,000 miles on it. Since it has about 0.5% elongation on it, am I hearing you correctly that its time to retire it? These go for like $28 each, although I saw where biketiresdirect has them for 19.99.

Personally, I would retire that chain, yes, but some people run them longer. Personally, I'd rather buy the less expensive PC-48 for around $7, and not be reluctant to retire it. That just works better for me. I used to use the PC-68, it's a beautiful chain, but it doesn't last any longer than the 48 because the pins and rollers are the same. And I don't care about the shiny plating. I'd rather have a newer chain all the time and save the wear on the drivetrain. But that's just me. :)

I'm going to need to start following that paradigm, and get off the shiny chain tangent :) . Now where can I find some quantity discounts on SRAM chains?

I had good experiences using the Rohloff cog wear indicator:

http://www.rohloff.de/en/products/hg_ig_check/index.html

In my experience, it has been hard to wear out the largest cogs, but the middle to small cogs indeed deteriorate. Still, you need to ride for a much longer distance than 900 miles and losing some coating is not enough for condemning a casssette. When eventually some cogs go bad, you might replace just those. On the other hand, separate cogs could add up to a higher cost than a whole cassette.
thanks 2 i. I have to have one of those. Can they often be found at bike shops? How about the front cog set? Guess , why would it not work equally well.

I'm going to need to start following that paradigm, and get off the shiny chain tangent :) . Now where can I find some quantity discounts on SRAM chains?

I highly recommend www.probikekit.com. I've ordered from them many times, with no problem ever. They are in the UK, but they ship to the US for FREE! Their current price for a quantity of five or more SRAM PC-48 chains, is about $6.50 each (about $9.50 if you just order one, and less as you go up in quantity). Here's a link to that item: http://www.probikekit.com/display.php?code=K1004

The stock is low right now, down to 5, but if they run out before you order, just ask to be notified when they're back in stock. They get more in fairly quickly. They've got some great deals on lots of cool stuff. And it's hard to beat free shipping. Great customer service too.

thanks 2 i. I have to have one of those. Can they often be found at bike shops? How about the front cog set? Guess , why would it not work equally well.

If you google around, you can find it e.g. at Cambria for $25:

http://www.cambriabike.com/shopexd.asp?id=13754

Whether it is a worthy investment depends on how much you ride. If you're going to replace the cassette once in your life, presumably not. If you go for the tool, it is worthwhile to test its operation on a new cassette, to have a benchmark, before operating on a suspect cassette. The method they recommend for larger cogs should work for front cogs too, but these take forever to wear out.