I changed my chain the other day. I measured the new chain alongside the old chain. Now, when I "mash" on the pedals the chain jumps off or skips forward. Is it possible the chain is too long or is there something else I should consider?

The chain length probably isn't the issue but a worn cassette or freewheel is. Chains and cogs wear together and if the chain is replace on old cogs, the chain and cog tooth spacing no longer match and the skipping you are experienceing is the result.

So, you now need to replace your cassette or freewheel. The preventative in the future is to change chains frequently or expect to change the chain and cassette together.

After my first post I did a little research on sheldon brown's website and did a search here. I measured my old chain and the length is +1/16" approximately. The front rings have some wear, but otherwise look fine. Nothing stands out on the rear cogs, but I'm going to go over it more later.

Theres really nothing to go over, Hillrider is right, you need to replace the cassette/fw. You can put the old chain back on and probably go for another 2-3 years with the same combo, but you can never use another wheel with a different cassette/fw with the chain you have, and you will probably need to replace the rings also when you replace the chain/cass/fw.

If you ask for advice and receive a firm answer (esp. from someone who has posted 14,000 times) it's not wise to second guess it. One does not need to measure anything with the scenario you describe. New chain, old cassette/freewheel, skipping (and no other changes) = worn cogs. Period, end of story.

Terse, but correct.

Even if the chain is changed at .5% elongation, the cogs will still wear out. I use a 3-chain rotation so I never get new-chain skip and get about the same mileage from 3 chains that most people do from 6.

OR, cny, it might be a front chainring. With the 42T middle ring on my triple, I tend to run a lot in the middle ring and all over the cassette. So I replace the middle ring about twice as often as the cassette. So it's one or the other. When you stand on the pedals to make it skip, look down at the chainring. You'll see it jump if it's that. If you don't see it jump, then you need a new cassette.

If it's the cassette, the other thing you can do is put the old chain back on and run it until it breaks (carry a master link). Cheaper that way. I've a friend who puts on 40K miles/year. He has sets of two-cog "cassettes", chainrings, and chains, all stretched and worn to match. When one component gets too worn to work with the others, he just moves it to a different component set. That many miles, and pristine equipment starts to run into money. Because he replaces all but two cogs with spacers, he doesn't have the problem of the chain skipping on some cogs and not others.

Could you explain "jump" more? I tried to narrow the problem more, no I'm not trying to second guess anyone here, I'm just trying to see what parts I need. I have a triple crank and could only get the chain to "jump" in the middle ring, in several of the rear cogs. In some cases the chain jumps to the small ring other times it goes all the way off the front ring. Regardless of what the problem cassette or front ring is it common to have this problem with only 3,000 miles on the bike?

Are you sure you got the right chain for your drivetrain? A 9spd chain on a 10spd drivetrain can easily catch the ramps on the front chainrings and mis-shift, causing it to jump when not expected.

Maybe it's my old eyes, but I sometimes cannot tell just by looking from the saddle what gives. My old bike had the same problem after its rear derailleur and hanger twisted all out of shape. After replacing RD+hanger, cassette and chain it still jumped when a lot of force was applied to cranks, and it was violent enough that I couldn't tell by looking (I was busy trying not to hurt myself). I had to put a dab of toothpaste on chain and chainring, then make it jump, then rotate cranks backwards to see if the marks still matched.

--J

My use of "jump" means that the chain rides up the teeth in the chainring or cog until it drops down into the next valley behind that tooth. On my bike, it usually stays on the same chainring or cog while it's doing that. Looking down at it when it's under heavy load, it looks like the chainring suddenly gets larger as the chain bushings all slide over the top of the tooth that was driving them. If you look at the chainring very closely, and it's the chainring that's the problem, you'll see that the valleys between the teeth are asymmetrical - one side is nice and rounded and the other looks kind of hooked. Look at your cassette, too, and see what the teeth look like there. Are the teeth on every cog about the same shape, or are there one or two cogs that have differently shaped teeth, perhaps because they are your favorite gears?

Yes, it's entirely possible to wear out a cog or chainring in 3000 miles. Moreso if you ride in the rain. Moreso if you haven't been religious about keeping your chain clean and oiled. Clean and lube your chain every time it looks dirty to you. Grab the chain between your fingers and twist it. Does it have a gritty sound? Needs maintenance. Well maintained chains last longer, and make the rest of the drivetrain last longer, too. I use one of those boxes that you put solvent into, hook onto the rear der and the chain passes through it. Works well and it's quick. Then dry it very carefully. I use a heat gun with a shrinktube attachment. Then try Finish Line Ceramic lube. Use the heat gun to warm that into the chain, too. You do that once a week and your chain will last a long time.

Oh - and buy a Park chain checker and use it once/week after oiling. Change the chain when it's well into the red.