Descend Upper Headset

It is the S.H.I.S. standard, and it reads in three parts. ZS means zero stack: the cup presses up inside the head tube and finishes flush with it rather than standing proud. 44 is the head tube's internal diameter in millimetres. 28.6 is the steerer diameter the bearing takes, which at the top of the fork is the straight 1⅛in section. Get those three right and the cup fits — our introduction to the S.H.I.S. goes through the system properly.

ZS44/28.6, ZS49/28.6 or ZS56/28.6 — the only thing that changes is the head tube's internal diameter at the top: 44mm, 49mm or 56mm. All three take a 28.6mm steerer, so that half of the code never varies. Measure the bore at the top of your head tube, or take it from the frame manufacturer's specification, and match the number. Stack heights are 9.6mm, 10.3mm and 13.5mm respectively, at 72g, 92g and 117g.

Because most modern frames are tapered, so the two ends of the head tube rarely take the same code — a frame might want ZS44 at the top and ZS56 at the bottom. Selling a fixed pair would force you to buy a half you cannot use. It also means a single worn bearing costs you one cup rather than a whole headset. Pair this with whichever Descend Lower your frame's bottom end calls for.

It is how much the cup adds above the head tube, which decides how much steerer is left for spacers and stem. A taller cup lifts the whole cockpit slightly, so going from the 9.6mm ZS44 to the 13.5mm ZS56 changes your bar height by roughly 4mm before you touch anything else. It is not a fitment question — the cup still goes on — but it is worth knowing before you cut a steerer, and worth accounting for with the Descend Headset Spacer Kit if you want the bar back where it was.

Yes. S.H.I.S. is a standard, and the two halves of a headset do not interact — each one only has to match its own end of the head tube and the steerer diameter at that end. So mixing brands works, as long as both codes are correct for your frame. Using the matching Descend Lower simply keeps the same angular contact cartridge bearings at both ends.

No, and it does not need one. A crown race is pressed onto the fork crown at the bottom of the steerer, which is where the lower bearing sits — the upper bearing runs on the straight 28.6mm section higher up, with nothing pressed onto it. So the crown race comes with the Descend Lower, not this half. If you are rebuilding both ends, buying the two together gives you everything.