I just replaced the chain on a used Trek Cobia. I didn’t have any problems with the old chain in terms of shifting.

I took off the old chain and counted links, to my surprise it was the length of a brand new KMC chain that I used to replace it. I put on the new chain, uncut and found that out of the saddle pedaling when in the middle chainring caused the chain to move around enough that it jumped onto another chainring.

I took off the chain, used the big chainring/big rear cog method (not through the derailler) and this warranted cutting off only about 5 links. So I did that. Now, when I back pedal, the chain is slack enough that it hits the chainstay. Overall the chain just looks too long.

What am I doing wrong here?

Thanks