Guides / Process

What Makes a Hub Shell a Hard Part to Machine

September 15, 2026

A hub shell has one job that overrides everything else: hold two bearings on the same axis. Everything about how it is machined follows from that.

Concentricity is the whole part

The two bearing seats have to be coaxial. If they are not, the bearings are loaded against each other, the hub drags, and no amount of good assembly fixes it.

This drives the process. The two seats should be machined in the same setup wherever possible, from the same chucking, so the machine’s own accuracy holds them in line rather than the fixture having to. A shell where one seat is machined, the part is flipped and the second seat is machined from the other end depends on how well the part re-located, which is never as good.

On the drawing this becomes a runout or coaxiality callout between the two seats, referenced to a common axis. That single callout tells a machinist more about how to make the part than a page of dimensions.

Bearing seats

Cartridge bearings in bike hubs are usually 6802, 6902, 6903 or similar. The bore for each is a light interference fit, measured in single hundredths of a millimeter.

Things that go wrong:

• Anodize growth. A hard anodized seat can lose 0.04 mm on diameter, which is more than the whole fit tolerance. Mask the seats or machine an allowance, and say which on the drawing.
• Shoulder square to the bore. The bearing seats against a shoulder. If that face is not square, the bearing is tipped.
• Entry chamfer. Without one, pressing a bearing shaves the bore and the swarf stays inside.
• Surface finish. Ra 1.6 or better. A rough bore does not hold an interference fit properly.

Flanges and spoke holes

The flanges set the wheel’s bracing angle and the spoke holes carry the load into the shell.

• Spoke hole diameter is set by the spoke, typically around 2.5 mm for a 2.0 mm spoke, and it needs to be consistent
• The holes are usually angled or counterbored so the spoke head and elbow sit without bending. Angled holes need a 4th axis or an indexer
• Every spoke hole is a stress concentration. Chamfer both sides and do not leave burrs inside the shell
• Flange spacing and diameter should be dimensioned from the same datum as the bearing seats

A hub shell with 32 spoke holes has 32 chances to leave a burr. Deburring is a real line item and it belongs on the drawing.

Freehub bodies

The freehub body is the harder of the two parts. It carries the cassette, it takes the full drive torque and it has a spline form that is a defined standard.

• Spline geometry is fixed by the standard, HG, XD, XDR or Micro Spline. There is no room for interpretation, and the form has to be cut cleanly because the cassette has to slide on and off.
• Material choice is a real trade. Aluminum bodies are light and get gouged by steel cogs. Steel bodies resist that and weigh more. Some makers hard anodize aluminum bodies specifically to slow the gouging, accepting the fatigue cost.
• The drive interface at the hub end, whether pawls or a ratchet ring, has its own tight geometry and a surface finish requirement.
• Concentricity again. The cassette has to run true.

What this means for the quote

Hub shells and freehub bodies get quoted by shops with turning capability and driven tools, not by pure milling shops. That is a smaller pool than for a stem or a bracket. Expect fewer quotes and longer lead times, and expect a shop that has made hub shells before to price more confidently than one that has not.

Say on the RFQ that the part needs turning plus angled drilling, give the bearing part numbers and state the spline standard by name. That is enough for a shop to know within a minute whether the job is for them.

Frequently asked questions

Keep reading

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