Guides / Design
What a Chainring Drawing Has to Get Right
September 14, 2026
A chainring is a flat part, which makes it look like the easiest thing on a bike to machine. It is not. The tooth form has to work with a chain that was designed around a different manufacturing process, the bolt circle has to be located precisely and the offset has to put the teeth on the chainline.
Start with the interface
Before any tooth geometry, the ring has to bolt to something.
Bolt circle diameter and pattern. The BCD and the number of bolts are set by the crank. The dimension that matters is not the diameter on its own, it is the position of each hole relative to the ring center. Use a positional tolerance referenced to the center bore, not five separately dimensioned holes.
Direct mount. SRAM three bolt and Shimano direct mount interfaces replace the spider entirely. The interface is proprietary geometry with tight fits, so a direct mount ring is a harder part than a bolted one, and the drawing has to carry the interface dimensions exactly.
Offset and chainline. The distance from the mounting face to the centerline of the teeth. This is what puts the chain where it needs to be. Boost spacing moved the rear hub from 142 mm to 148 mm and the chainline out with it, typically to around 52 mm from a non-boost 49 mm, which is why boost specific rings exist. Get this dimension wrong by half a millimeter and the bike shifts badly across the cassette.
Tooth geometry
Standard profile. Teeth sized so the chain rollers seat correctly, with the tooth thickness under the chain’s inner link width. Straightforward to machine.
Narrow wide. Alternating thick and thin teeth, thick teeth filling the wider gap between outer link plates. This is what holds a chain on a 1x drivetrain with no front derailleur and no guide. It also means two different tooth widths alternating around the ring, each with a tight tolerance, because the whole function depends on the wide tooth being a close fit.
Practical implications:
- Tooth width tolerance is usually tighter than anything else on the part
- The tooth flanks need a good finish. A rough flank wears the chain and is noisy
- Deburring matters. A burr on a tooth flank will be felt by the rider
Ramps and pins. On a multi-ring setup, shift ramps and pins on the inner face lift the chain from one ring to the next. These are machined features with real geometry, not decoration, and copying them badly gives a ring that shifts worse than the one it replaced.
Where the machining time goes
A chainring starts as a plate. The steps are roughly:
- Rough the outside profile and the center bore
- Machine the mounting face and the offset step
- Cut the tooth form, usually with a small end mill following the profile
- Cut weight relief pockets and any ramps
- Deburr, which on a ring with 34 teeth is not a trivial step
- Finish
The tooth cutting dominates. Each tooth has two flanks and a tip, the cutter is small and the path is long, and on a narrow wide ring the machine is alternating between two profiles.
Material and finish
7075-T6 is the standard. The teeth are thin and loaded directly by the chain, so alloy strength is doing real work. Expect the darker, less even anodize that comes with 7075, and do not expect it to match a 6061 part in the same color. See anodize color matching.
Hard anodizing a chainring is a judgment call. It resists chain wear, and it also reduces fatigue life at the tooth root. Most makers stay on Type II and accept that rings are consumable.
A chainring drawing checklist
- Tooth count and pitch, 1/2 inch by 3/32 inch for derailleur chains
- Tooth form, standard or narrow wide, with both widths toleranced
- BCD, hole count and a positional tolerance referenced to the center bore
- Center bore size and tolerance
- Mounting face flatness
- Offset from mounting face to tooth centerline, with tolerance
- Ring thickness
- Ramp and pin detail if any, fully dimensioned
- Material and temper
- Finish, thickness and whether dimensions are before or after it
- Deburr requirement, stated explicitly
Send that with a STEP file on the RFQ and a shop can quote it without a single clarifying question.
Frequently asked questions
Which BCD should a new chainring use?
It depends what it bolts to. Common bolt circles include 104 mm four bolt and 64 mm four bolt on older mountain cranks, 110 mm and 130 mm five bolt and 110 mm four bolt on road, and 94 mm and 96 mm on various Shimano patterns. Direct mount interfaces from SRAM and Shimano bypass the bolt circle entirely and are increasingly the default on 1x.
What makes a narrow wide ring work?
Alternating tooth thickness that matches the alternating inner and outer link plates of the chain. The wide teeth fill the wider outer link gap. It is a tight fit, which is why the tooth width tolerance on a narrow wide ring is much tighter than on a standard ring.
Can a chainring be machined from 6061?
It can, and budget rings are. 7075-T6 is the normal choice because the teeth are thin, highly loaded and wear directly against the chain. The cost difference on a part this small is not large.
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