Guide
How Is a Bicycle Hub Shell Machined, and Which Features Drive Cost and Tolerance?
A hub shell is the main body of a bicycle hub: the part that carries the spoke flanges, seats the bearings that run on the axle, and takes the interface for a cassette body, a disc rotor or both. It is turned and milled from a solid bar or finished from a forged blank, and a few features on it set most of the cost and almost all of the tolerance risk.
By Saola Manufacturing engineering team · Published · Updated
Short answer: machining a hub shell from bar stock is a turn-then-mill route in which the bearing seats, the flanges and the spoke holes decide the price and the rejects. The two bearing seats must stay concentric with each other, the spoke holes must sit where the wheel builder expects them, and the finish, usually anodize, must not close up the seats. Write those three things into the RFQ before anything else.
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What does a hub shell have to do, and which features matter?
It holds the bearings, carries the spokes and takes the drive and brake interfaces. Four feature groups matter: the bearing seats, the flanges, the spoke holes and the drive or disc interface.
A hub consists of an axle, bearings and a shell, and the shell can be one piece with press-in cartridge bearings or free bearings, while flange spacing affects the lateral stiffness of the built wheel, with wider spacing stiffer (Wikipedia, Bicycle hub, accessed 2026). For the machinist this means each feature has a different job. The bearing seats locate the rotating parts and must be round and in line; the flanges carry spoke loads and set the wheel geometry; the spoke holes must be positioned and chamfered so that spokes seat without kinking; the drive and disc interfaces must mate with parts from other suppliers.
Because hubs are built into wheels by people who did not make the shell, the dimensions that matter most are the ones another part touches. Hub spacing at the frame or fork is set by the frame and fork standard the bike uses, so spacing is a customer-owned requirement the shell has to meet, not a number a machine shop chooses.
Is a hub shell machined from bar stock or finished from a forged blank?
Both routes exist and are documented. Bar stock needs no forging tool; a forged blank puts the material where the loads are and leaves less to remove.
A granted patent for a forged hub describes one starting point: two die-forged components whose spoke flanges are formed in the forging, with the bearing seats finished by chip removal and the spoke holes bored afterward (US 9,370,968, published 2016). The same document says that in earlier hubs the hub body was extensively machined by turning and milling with chip removal, which is the bar-stock route.
A second patent shows a third route, forming the shell from thin sheet with the flange holes punched rather than drilled, and notes that conventional hubs until then were forged from a tube and finished by machining (US 9,238,385, published 2016). For a buyer comparing quotes, the point is that machining never disappears. Even a forged blank still needs the bearing seats, spoke holes and interfaces machined, so the real choice is how much metal has to be removed and who supplies the blank.
What does the machining route look like step by step?
A typical bar-stock route is: cut the blank, turn the outside and ends, bore the inside, cut the bearing seats, mill the flanges, drill the spoke holes, then finish.
The order matters because each step sets a datum for the next. Turning and boring produce the round features, so the bearing seats and the axle bore are best cut in one setup or in two setups that share a locating diameter. Milling the flanges and drilling the spoke holes then happen on an indexed fixture that locates from the same diameters, which is how the holes stay in the right angular and axial position relative to the seats. Interfaces such as a cassette-body thread, a disc mount or a lockring thread come late in the route because they depend on the finished seats.
None of this needs equipment beyond turning and milling, which is why a hub shell fits CNC turning and CNC milling cells well. The risk sits in the setups and fixtures, not in the machine.
Which features drive cost and tolerance?
Cost follows setups and inspection; tolerance risk follows the two bearing seats and the spoke holes. The table lists each feature with what to specify and how it is checked.
The table is qualitative on purpose. The patents and product pages describe the features but give no tolerance values, and the right fit for a bearing seat comes from the bearing maker’s housing recommendation for the bearing you selected, not from a general guide.
| Feature | Why it matters | Specify | Check |
|---|---|---|---|
| Bearing seats (two) | Finished by chip removal even on forged shells; set bearing life and wheel feel | Bearing size, housing fit class from the bearing maker, roundness, stop shoulder | Bore gauge or air gauge on both seats |
| Concentricity of the two seats | Seats at opposite ends of the shell must share an axis | One datum axis from the two seats, concentricity or runout to that axis | CMM or dedicated gauge on the first article |
| Flanges | Carry spoke loads; spacing affects lateral stiffness | Flange spacing, diameter, thickness, canting where used | Caliper or CMM |
| Spoke holes | Position and chamfer decide spoke seating | Hole count, pitch circle, angular position, chamfer, edge break | CMM on a sample from each lot |
| Drive and disc interfaces | Must mate with parts from other makers | Thread or spline form, length, position from a seat | Go and no-go gauge or mating part |
| Finish (anodize) | Adds a layer on seats and threads | Masked features, color limit sample | Seat diameter after finishing |
What should the RFQ and drawing say for a hub shell?
Give the seat fit, a common datum axis for the two seats, the spoke-hole position and the finish with its masked areas. Ask the supplier to show how those four are measured on the first article.
Start with bearing seat fit: name the bearing, state the housing fit the bearing maker recommends, and say whether the dimension applies before or after the finish. Then the concentricity of the two seats: call out one datum axis, tie both seats to it, and ask how the supplier will inspect it. A shell that measures correct on each seat separately can still be out of line, which is why it is a datum question and not a size question. Third, spoke hole position and chamfer, with the reference faces. Fourth, the finish. Name the anodize by specification and type; MIL-A-8625F (1993) covers six types and two classes of anodic coatings on aluminum and aluminum alloys. Anodizing grows a layer on the part, so say if the seats, the interface threads and any contact faces are masked or must hold size after finishing. The guides on masking for anodizing and dimensional change from anodizing cover the logic in more detail.
For appearance, a limit sample beats a color word; see anodize color matching. Ask for a first article report that includes both seats, the flange spacing and a spoke-hole position set, and agree what a lot sample is before the first order.
How is a hub shell handled on a Saola order?
Turning and milling 6061-T6 aluminum bar is lead-plant CNC work, but the CNC turning page describes its turned work as small bar parts, so a shell with large flanges is confirmed at drawing review. Anodizing is managed through partner plants and named as such on the quote.
Anodize is a managed process covered on the surface finishing page; if a design uses a forged blank, who supplies it is settled in the quote and the forging is not described as done at the lead plant. Send the shell drawing, the bearing part number and the quantity per year, and the quote will list which steps happen where.
Frequently asked
Do forged hub shells still need machining?
Yes. The forged-hub patent says the bearing seats are at least finished by chip removal and that spoke holes are bored afterward, so the machining content moves but does not go away.
Can a hub shell be made from one piece of bar?
Yes. The forged-hub patent describes earlier hub bodies machined extensively by turning and milling, which is the bar-stock route. The trade-off is more material removed per part.
Why are two bearing seats a datum question?
Because each seat can be round and on size while the two are out of line with each other. The drawing must tie both to one axis so the supplier can inspect alignment, not just diameter.
Does anodize change the bearing seat size?
It can, because anodizing builds a layer on the surface. State whether the seat dimension applies after finishing, or mask the seat.
Sources
- US 9,370,968 B2, Hub, patent published June 2016
- US 9,238,385 B2, Bicycle hub and method for the production thereof, patent published January 2016
- MIL-A-8625F, Anodic Coatings for Aluminum and Aluminum Alloys, U.S. Department of Defense, September 1993
- Bicycle hub, Wikipedia, accessed 2026