Park Tool FR-6 Freewheel Remover
Park FR-6 Freewheel Remover is designed to remove the four-notches freewheels from BMX and single speed bikes using the standard 1.37 x 24 tpi hub threading. (Packaged as FR-6C)
NOTE: Always hold the remover secure to the hub using axle nut or quick release skewer.
This tool has a tendency to pop out of the splines it's supposed to fit into--any tool with only four splines like this will do that. To counter this, clamp it face up in a vice. Set the wheel on it so the splines are matched. Put a quick release lever through this whole configuration and tighten it so you have just enough wiggle room to loosen your freewheel. Grab the rim and turn until the threads on the freewheel release. Loosen the QR as needed until no longer necessary.