a full spool is one piece that replaces your carrier, holds the ring gear, and keeps both axles in a constant drive mode. a mini spool replaces the spider gears in an open carrier to keep both axles in a constant drive mode, but they're made from about 3 different pieces and are not as strong as a full spool. mini spools are recommended for dirt track applications mostly, where there isn't enough traction to break them. after running a minispool for a couple seasons, i understand why they're not the greatest for street/strip use.