The pack of 3 PTSF-55 Spike Foot Set for Select Tripods from Photo Clam is a replacement set that originally comes with the tripods. The spiked feet also include a set of rubber feet that fit over the spikes. Once the spikes are part of the legs, you can drive the feet into soft terrain or snow and the spikes stabilize the legs from moving around.
Compatibility
Tripods: PTC 3440PS, PTC 3440PL, PTC 3440PXL, PTC 4440PL, PTC 4440PXL, PTC 5440PL, and PTC 5440PXL