A spool is just a solid diff. It applies power evenly to each tire via the dogbones/CVDs. Here is a spool (They are also sometimes called straight axles): http://www2.towerhobbies.com/cgi-bin...&I=LXDGD6&P=ML You can see where the dogbones go into the ends of the spool. The gear goes on the spool and is where the shaft gear goes to spin the spool, which in turn spins the tires.

Here is a normal diff: http://www2.towerhobbies.com/cgi-bin...?&I=LXJHM9&P=7 This is limited slip. It supplies power to the tire with less traction.


Off topic, but I locked the diffs in my TXT-1 because I use it as a rock crawler. This is fine because I want max power to all the wheels all the time, plus, I am never really going full throttle and I am also not on a high traction surface, like concrete or carpet. High traction surfaces can add stress on your drivetrain. When locking a diff or installing a spool, you lose turn radius, however, doing it to a drifter is fine because you are trying to break traction. You also almost always install PVC/ABS drift tires and don't have to worry about putting added stress to your drivetrain. I hope this helps.