From Off-Road Magazine:
"When off-road, the absolute best available traction device available comes in the form of a spool. There is nothing that is stronger, lighter, cheaper, or runs cooler. In fact, a spool is not really even a differential, as there is no differentiation of wheel speeds with a spool. The lack of wheel-speed differentiation in a fixed-gear unit (i.e. a spool) is the very thing that makes it horrific to deal with on the pavement. Differentials were invented to allow for different wheel speeds when not moving in a straightforward direction (as in turning)..."
Read the full article here