Bicycle Mechanics - 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.

My guess would be that if you have a cassette hub - which the shop will be able to see at a glance - then the freehub body and internal mechanism needs replacing (the thing that the 7 sprockets at the back are wrapped around). The slipping will occur when the pawls inside it jam up and don't operate correctly. This may also be the source of the noise you are getting - especially if it has had all of the grease washed out and is now full of crap. You can replace that part quite easily as a service spare. If you have a screw on block then you will need to replace the whole thing, again at a relatively low cost.

do you think i need a new cassette and chain ring to go with my new chain? oh, that noise keeps coming from my bottom bracket. give me my plan of action-LOL.

My first suspect would be the cassette. The fact that it's happening in your fastest gears (smallest sprockets in the back) would support that. Generally, if the chain gets worn past a certain point, it tends to start wearing the rear sprockets to match it. When you put a new chain on, it want's to skip, especially on the smaller cogs, since the load per tooth on those are higher. A new cassette should handle the skipping. As for the BB noise, that can be a tough one to pin down, as a number of threads here will testify too.

Hope this Helps....

Steve W
Who worked in the yard and is now has grass-colored feet

do you think i need a new cassette and chain ring to go with my new chain? oh, that noise keeps coming from my bottom bracket. give me my plan of action-LOL.

I just replaced the Rear Cassette, Chain, and Front Chain rings... although I replaced the Front Chain Rings and Crank Arms... was cheaper that way for some strange odd reason, but I did it for under 80.00
My Trek is a 7 Speed (rear) and a Triple (front)

The Chain was 15.00
Cassette was about 25.00
that leaves @ 40.00 for the Chain Rings w/ Crank Arms

All was Shimano.... rides great

Oh I switched over to the "dry Lube" for chain. (Finish Line Tephlon Dry)

Good luck to ya... iffin your hearing noise from the BB... I would have a Bike Shop look at it, or replace..

thanx everyone!!!

I agree with Mentor58. It's fairly common to find it necessary to replace a cassette after replacing a worn chain. The problem usually presents itself as "skipping" in the gears that you use most. My bet is that a new cassette will solve your skipping problem.

Noises that appear to be coming from the bottom bracket can be hard to pin down. The actual source of the noise can be coming from almost anywhere on the bicycle. The pedal/crankarm and crank/bottom bracket interfaces are common candidates. Some other common culprets are the saddle/seatpost and handlebar/stem/headset interfaces.