Machined Hub Shells and Freehub Bodies
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, including why it needs a turning center with driven tools rather than a milling shop, and why the quote pool for hubs is much smaller than for a stem.
What this covers
- Front and rear hub shells
- Freehub bodies, HG, XD, XDR and Micro Spline
- Ratchet rings, pawl carriers and drive rings
- End caps and axle conversion hardware
- Pulley wheels and bearing carriers
Interfaces you do not get to choose
- MTB rear spacing148 x 12 mm Boost, 157 mm Super Boost, 142 x 12 mm
- MTB front spacing110 x 15 mm Boost, 100 x 15 mm
- Freehub standardsShimano HG, SRAM XD and XDR, Shimano Micro Spline
- Common cartridge bearings6802, 6902, 6903
- Spoke hole sizeAbout 2.5 mm for a 2.0 mm spoke
Full tables in the standards reference.
Material choice
What each option actually buys you on this part, rather than in general.
| Material | Why it gets used here |
|---|---|
| Aluminum 6061-T6 | Standard for shells. Light, machines cleanly, anodizes evenly. |
| Aluminum 7075-T6 | Freehub bodies where steel cogs gouge the splines, and high load shells. |
| Steel or stainless | Freehub bodies that must not gouge, ratchet rings and drive components. |
Where the tolerance belongs
These are typical working ranges, not promises. Tolerance these dimensions and leave the rest at a general tolerance, because every step tighter than you need buys machine time you will pay for.
| Feature | Typical | Why it matters |
|---|---|---|
| Bearing seats | +0.000 to +0.010 mm | A light press fit. State the bearing part number and work to the bearing maker's recommended fit rather than guessing. |
| Coaxiality of the two seats | Runout callout, common axis | This is the part. Machine both seats in one setup so the machine holds the line, not the fixture. |
| Bearing shoulder squareness | Perpendicular to the bore | An out of square shoulder tips the bearing. |
| Spline form | Per the standard | HG, XD, XDR and Micro Spline are defined geometries. There is nothing to interpret. |
| Bearing bore surface finish | Ra 1.6 or better | A rough bore does not hold an interference fit. |
Finish
Mask the bearing seats or machine an allowance. A hard anodized seat can lose 0.04 mm on diameter, which is more than the entire fit tolerance, and the bearing then goes in crushed or not at all.
Realistic quantities
Small. The constraint is finding shops with turning plus driven tools and angled drilling, not batch size. Expect fewer quotes and longer lead times than a milled part.
What goes wrong on this part
Failure modes specific to this component, collected so you can design them out rather than discover them.
- Flipping the part between bearing seatsIf the second seat is machined after a re-clamp, concentricity depends on how well the part relocated. It never relocates as well as the machine holds.
- Thirty two chances to leave a burrEvery spoke hole is a stress concentration and a burr trap. Chamfer both sides and put deburring on the drawing.
- Angled spoke holes need a 4th axisSay so on the RFQ. It changes which shops can quote.
- Aluminum freehub bodies gougeA known trade against weight. Decide it deliberately rather than discovering it in warranty.
What to send
A drawing with these on it gets quoted in a day. One without them gets quoted with padding in it, or gets four rounds of questions first.
- STEP file plus a PDF drawing
- Bearing part numbers, not just bore sizes
- A coaxiality or runout callout between the two seats
- Spline standard named exactly
- Spoke hole count, size, angle and counterbore
- Which surfaces are masked during anodizing
Printable version: the RFQ checklist.
Go deeper
- Process
Machining Bicycle Hub Shells and Freehub Bodies 101
Bearing seats, flange geometry, spline forms and concentricity. Why hub shells need turning and milling together, and what that does to the quote.
- Design
Tolerances on Bike Part Drawings: What to Specify
Steerer bores, bearing seats, chainline and bolt circles need tight numbers. Most of the drawing does not. How to tolerance a part without paying twice.
- Finishing
Type II vs Type III Anodizing for Machined Bike Parts
Decorative or hard anodize? What each does to color, wear, your bore sizes and the fatigue life of a structural part, plus what to put on the drawing.
Other parts
Stems and faceplates
Two clamp bores against fixed standards, a faceplate that has to close evenly and a rider on top of it.
Chainrings and spiders
A flat part with the tightest tolerance on the site. Tooth width decides whether a 1x chain stays on.
Cranks and pedal hardware
Fatigue critical, envelope fixed by the frame and the rider. The part where anodize choice changes test results.
Brackets, mounts and hardware
The long tail. Low part cost, fixed interfaces, and finishing batch minimums that dominate the quote.
E-bike drive hardware
Machined like a bike part, loaded like a light vehicle part, regulated like an electrical product.