The ZS-2 is of the same ZeroStack design which features very low profile aluminum cups that "hide" down in the head tube of the bicycle's frame, and are compatible only with specifically machined head tubes. The ZS-2 uses sealed cartridge bearings for solid durability. All 1-1/8" ZS headsets are compatible with Cane Creek Interlok spacers. Zero Stack (ZS): Offers a low stack height, the security of bearing cups. and utilizes bearing cups that are hidden inside the head tube.