I want to replace the system on my mountain bike with SRAM parts. From my quick search on the internet Im finding that its hard to find parts if I want to build a 7-speed bike (3 x 7). Im not looking for the most expensive high end parts. I would like to build an entry level mountain bike. I already have SRAM MRX grip shifts but I would like to figure out the bottom bracket and crankset. Can anyone give me some pointers?