Descend Lower Headset

It is where the cup sits. ZS is zero stack: the cup presses up inside the head tube and finishes flush with it. EC is external cup: it sits below the head tube, visibly proud of the frame. You cannot substitute one for the other, because the frame is machined for one or the other — an EC frame has no internal recess for a ZS cup to press into. The stack heights make it obvious: our EC cups add 13.4mm, the ZS ones between 2.8mm and 3.5mm.

Two halves, and both have to be right. The first part is the cup type plus the head tube's internal diameter, so ZS56 means a zero stack cup for a 56mm bore. The number after the slash is the crown race diameter your fork needs — 40 means a 40mm crown race seat, which is what a tapered steerer usually has at the bottom. The right cup with the wrong crown race size will not seat on your fork. Our introduction to the S.H.I.S. covers the system in full.

Match your frame's lower head tube bore to the first number and your fork's crown race seat to the second. The options are ZS44/30, ZS49/30 and ZS56/30 for 30mm crown races; EC44/40 and EC49/40 for external cup frames on a 40mm crown race; and ZS56/40, ZS66/40 and ZS66/46 for the larger bores. Weights run from 40g on the ZS44/30 to 100g on the ZS66/46. If either number is uncertain, measure it or take it from the frame and fork manufacturers' specifications — this is not a dimension to estimate.

It changes the second number, not the first. A tapered steerer is 28.6mm at the top and wider at the crown, so the upper cup takes a 28.6mm bearing while the lower has to match whatever the crown race seat actually measures — most commonly 40mm, which is why several sizes here use it. The 30mm and 46mm options exist for forks that do not follow that convention. The crown race is included with this lower cup, and only with the lower, so the size you order has to suit your fork. Measure the seat at the fork crown rather than assuming from the fork's model name.

It is how much height the cup adds below the head tube, which feeds into your front-end height overall. The EC44/40 and EC49/40 add 13.4mm because the cup hangs below the frame; the ZS options add between 2.8mm and 3.5mm because they sit inside it. This is not something you get to choose — it comes with the standard your frame was built around — but it explains why two headsets quoting the same bore can look very different once fitted.

Only if the upper bearing is also worn. The two halves are independent, and replacing just the lower is a common repair — it is the end that carries most of the load and takes the worst of the weather. When the top does need doing, the Descend Upper covers ZS44/28.6, ZS49/28.6 and ZS56/28.6, and uses the same angular contact cartridge bearings as this one.