Chain slipping?? please help
 

If I get out of the saddle and apply more than normal force to the crank, I hear a "clunking" noise and it feels like the chain is slipping.

Does this sound correct? What should I do to fix the problem? I recently bought and installed a new Shimano chain for an 8 speed cassette.

Thanks!

You may need new cog/cogs/cassette. If you use the chain too long you will wear out either 1 or 2 or more cogs and even 1 or both chairings. The chain is the cheapest part of the transmission.

+1 on TLCFORBIKES said.

+10 more. If it's new problem that came on with the installation of a new chain keeping an old cassette, the first thing to consider is that the cassette might be worn. This is especially true if you replaced the old chain because it was running unsmoothly or skipping.

You can do a rough check either of 2 ways.

If you still have the old chain stretch it out against a 12" ruler and see how far beyond the 12" mark the corresponding point lines up. If more than 1/8" beyond the mark, odds are that the cassette is toast.

To check the cassette for wear directly, shift into the problem gear, and use a small screwdriver to gently lift a link from the sprocket at the center of the wrap (roughly 9:30). a new chain on a good cassette will barely lift away, but with a worn cassette it'll lift quite a bit, and you'll see the chain shift from both sides to create the slack.

One last note. It might not be the cassette. Often chains will slip on chainrings, though it sounds and feels like they're slipping on the cassette