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Chainring Decoder

Once upon a time, chainrings were only a little complicated. You typically had 4 or 5 holes of some common measurement apart. Teeth were approximately the same and if they fit, they were fine.

But with newer cranksets and chainrings, there are more options, and I see people get confused. I am sure there are some more systems out there than I'm listing, but here are a few:

Conventional Bolted Rings

These have been used for decades. The chainrings use some number of bolts to mount to the spider of the crankarm. The critical dimensions are the number of holes and the diameter of a circle that passes through them all. If your old one is not labeled (or the crank isn't) you need to measure and that can be complicated. 

Measure BCD on 4 Bolt (Source: Wolftooth)

4-bolt patterns are easy, whether symmetrical or not. Just measure across an opposing pair. In theory, you're measuring the centers of the holes, but edges on the same side is usually easier. That distance IS your BCD.


5-Bolt
But for other patterns, you need to be a bit more creative. You typically measure adjacent holes in this case and either lookup or calculate the conversion. As usual, one of the best resources is the late Sheldon Brown's site. Here is a cribsheet for common BCD and how to measure between the holes and convert to BCD. The good news is that most brands have common BCD options so if you measure 'close enough' you'll find the right answer.

For single chainrings, obviously one measurement is enough. But for 2x or 3x (double or triple) chainrings, it is possible that all rings do not share the same BCD. The inner on a triple is almost always smaller than the outer ones.

Note: The BCD on the crank will limit your options on how small the chainring can be. So if you are looking for a lower gear, and can't find one, check the possibilities. You may be asking for something completely unavailable or one that is only made by third parties ('first parties' like Shimano and SRAM often won't push the spec because they had options when they designed the crank)

Direct Mount Chainrings

Variations of Direct Mount rings
(Source: Wolftooth)
Most newer designs use some flavor of 'direct mount' ring. These do not bolt on to a spyder but rather are mounted 'directly' to the back side of the crank arm. There is usually some form of spline to take the turning torque and bolts or lockring to hold it in place.

The most common types I see today are SRAM, but even these come in two flavors:

3-Bolt. These were the earlier design which is till widely in use. But SRAM appears to be phasing them out. They were common on MTB parts but occasionally seen on road and gravel groups too, particularly 1x variants. They use 3 T-20 Torx bolts in an asymmetric pattern to retain the ring.

8-Bolt. Originally seen mostly on road bike parts, but appears to be the new SRAM standard. They use 8 T-20 bolts to retain the ring.

People are often confused by the lobes/notes of the spline as bolt holes. See the photo above to see which are actually bolt holes when counting.

Note that these chainrings often require an 'offset' to get the chainline correct. For example, on DUB Wide 1x cranks, a 3mm offset is common. Check your old rings or the crankset instructions to determine the likely correct answer.

DM+BCD?

Some cranks may require you to understand a direct mount AND a BCD. For example, SRAM 2x road cranks often use a removable spider that uses Direct Mount to attach to the cranks and provide a common hole pattern for the chainrings. This allows them to swap a power meter in place of the static spider.  This technique can also be used to adapt a DM crank to run conventionally bolted rings, should you want to.

Tooth shape

While the tooth shape used to be of little consequence when selecting chainrings, it has become a factor with newer designs.  First off with a 'narrow-wide' tooth pattern, which was designed for better chain retention on 1x systems at the expense of not being able to shift well (not needed on 1x). In this design, alternating teeth are wider to fill the space between outer shain plates and narrower (like a conventional ring) to fit between the inner plates. In the example here, the shape is pretty clear to see. Sometimes, particularly on black rings, it's harder but you can feel it. Make sure to align your chain correctly when putting it on. Note: Some rear derailleur jockey wheels also have this pattern.

With 11 and 12 speed systems, we started to see special chain designs (Flattop in road 12s for example) that demand customized tooth shape as well. Look for this compatibility when selecting.

You can read more about chainrings at:

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