newbie mechanic(changing cassette)
i have 3 rings in front and 7 rings in back. i've been told that makes it a 21sp bike??? anyway, i've put thousands of miles on a ???yr Balance brand mtn bike, and i've taken it to the local shop. @ 1st look, they told me i needed a new chain. now with the new chain, in my fastest gear, my chain/rings/wutever is now slipping. i'm 220lb, and took bike to shop to get rid of noise coming out of bottom bracket(i guess from my wgt). $120 later, the noise still isn't fixed, and now i'm slipping with a brand new chain.
question: is there a way that i can find out what kind of thing-a-ma-jig i need on the rear tire, to match my new chain, with what kind of tools? this particular bike shop has lost my business.