Bike Headset Types Explained: An Introduction to the S.H.I.S

ZS44/28.6, EC44/40 — headset codes look cryptic until you know the system behind them. The Standardized Headset Identification System (S.H.I.S.) turns headset shopping from guesswork into a two-measurement job. Here’s how to read it.

What is a Bike Headset Used for?

A bike headset is the rotatable interface between the fork and the frame’s head tube — it’s what lets your handlebars steer. The headset type determines how fork and frame connect and move relative to each other.

Before S.H.I.S., the market was flooded with headset designs in slightly different dimensions, and matching one to a frame was a genuine compatibility puzzle.

Parts of a Bike Headset

A modern threadless headset is a stack of parts sandwiched between your stem and your fork crown. Knowing what each one does makes the S.H.I.S. code — and any headset problem you ever have to diagnose — far easier to read.

The parts you actually shop for are the cups, bearings and crown race — the top cap, spacers and star nut rarely wear out. That’s why Funn sells the upper and lower halves of the Descend separately: most frames need a different design type at each end, and a worn headset usually only needs one end replaced.

What does S.H.I.S. Stand for?

The Standardized Headset Identification System gives every headset a common naming code, so cyclists, manufacturers and shops can all talk about the same part the same way.

The code is built from three parameters: the design type, the outer diameter, and the inner diameter. Together they define whether a headset fits your frame and fork.

What are the Different Types of Headset Bearings?

The first element of the code tells you how the headset mounts in the frame:

IS (Integrated System)

Bearings sit directly in the frame — no cups at all — for the lowest stack height and the cleanest look. The frame itself forms the bearing seat, so it demands precise manufacturing. Funn’s version is the Descend Integral.

ZS (Zero Stack)

Bearings sit in cups that press almost entirely inside the head tube, keeping the profile low. It’s the common choice on modern mountain bikes — see the Descend ZS.

EC (External Cup)

Bearings sit in cups that protrude outside the frame, like the Descend EC. The traditional style: durable, easy to service, and able to house larger bearings.

Decoding the S.H.I.S. Code

Take a common code from the Funn Descend range: ZS44/28.6 | EC44/40. The part before the bar describes the upper headset, the part after it the lower.

What is ZS44/28.6?

ZS44/28.6 is the upper half: ZS means Zero Stack, 44 is the outer diameter in millimeters (the head tube bore it presses into), and 28.6 is the bearing’s inner diameter — matching the steerer tube where the stem clamps.

What is EC44/40?

EC44/40 is the lower half: EC means External Cup, 44 is again the outer diameter at the bottom of the head tube, and 40 is the bearing’s inner diameter — sized for the crown race seat at the base of the steerer.

Head Tube Sizes Explained

The head tube is the short frame tube at the front of the bike that the fork steerer passes through. Its internal bore is what decides the first number in your S.H.I.S. code, so head tube size and headset size are really the same question asked from two ends.

Head tubes come in a handful of standard bores, and most modern mountain bike frames are tapered — a smaller bore at the top than at the bottom, so the fork can be wider and stiffer where the loads are highest. That’s why the upper and lower halves of your headset are usually different sizes.

A tapered head tube almost always pairs a 44 mm or IS upper with a 49, 52, 56 or 66 mm lower. A straight head tube keeps the same bore top and bottom — usually 34 mm or 44 mm — and takes a straight 28.6 mm steerer throughout.

Choosing the Right Bike Headset Type: A Measurement Guide

Check the Manufacturer’s Specifications First

Your bike or frame manufacturer usually lists the recommended headset size. Start there, and use your own measurements to confirm — when the two agree, you can buy with confidence.

Frame Head Tube Inner Diameter

No documentation? Measure it yourself. Start with the inner diameter of the head tube at both ends — this sets the headset’s outer diameter, and it will land on one of the standard bores in the table above.

Then match your measurements against the S.H.I.S. frame-fit table:

Steerer Tube Outer Diameter

Next, measure the fork’s steerer tube — the upper section for the upper headset size, and the crown race seat at the base for the lower.

Modern MTB forks come in two shapes. A straight steerer keeps one diameter — usually 28.6 mm — top to bottom, common on traditional and entry-level bikes. A tapered steerer widens from 28.6 mm at the top to typically 40 mm at the crown, trading a little weight for noticeably more stiffness and steering precision — which is why performance bikes have adopted it.

Design type plus the two diameters gives you the full S.H.I.S. specification — and with it, the exact upper and lower headset for your bike.

Which Funn Descend Headset Do I Need?

Once you have your code, matching it to a part is a lookup. The Descend range covers the upper and lower ends separately, so you can mix the two design types your frame actually calls for:

All three run angular contact cartridge bearings in AL6061 cups. If your new stack height changes the fit under the stem, the Descend Headset Spacer Kit covers 2, 3, 5, 10 and 15 mm in nine colours.

Descend Integral Headset

A complete IS kit — headset cap, crown race, top and bottom angular contact bearings — in IS41/28.6 and IS42/28.6 upper sizes, both with IS52/40 lower.

VIEW DESCEND INTEGRAL

Bike Headset FAQs

What does a headset do on a bike?

It lets the fork rotate inside the frame so you can steer. The bearings carry the steering load while holding the fork firmly in the head tube — which is why a worn headset shows up as knocking under braking or notchy, stiff steering.

How do I know which headset fits my bike?

Check your frame manufacturer’s spec first, then confirm with two caliper measurements: the head tube inner diameter at each end, and the steerer outer diameter at the top and at the crown race. Those four numbers give you the complete S.H.I.S. code.

Are bike headsets universal?

No. Design type, outer diameter and inner diameter all have to match, and the upper and lower ends of the same frame are frequently different sizes. A ZS44/28.6 upper will not fit a frame that needs IS42, even though both sit at the top of the head tube.

Can I replace just the headset bearings?

Usually yes. Headset bearings are cartridges that lift out once the fork is dropped — on an IS frame they come straight out of the frame, and on ZS and EC frames they come out of cups that stay pressed in place. Replace the crown race at the same time if the lower bearing was rough, and check the cups for pitting before reusing them.

Do you tighten the stem or the headset first?

The headset first. Loosen the stem pinch bolts, tighten the top cap bolt in small steps until the play disappears but the bars still swing freely, then align the bars with the front wheel and tighten the pinch bolts to torque. Tightening the stem first locks the steerer and stops the top cap from preloading anything.

Conclusion

S.H.I.S. turned headset compatibility from a guessing game into a code you can read: design type, outer diameter, inner diameter. Two caliper measurements and you know exactly which headset your bike needs.