# Why will some chainrings fit 7s-12s while others are meant for just 10s?

When looking for a chainring, I see that Shimano specifies whether it is for 2x or 3x. I'm guessing that the pins (protruding rivets) on the former will be on one side only, and on both sides on the latter.

They're also labeled "PCD", but the numbers suggest that it is an alternative way for saying BCD.

Then they specify whether it is meant for 11s, 10s, 9s, 8s, or 8s/7s. I'm guessing that the thickness of the chainring exactly matches the exposed width of the pin between the inner plates. In other words, it would be possible to use, for example, a 10s chainring with a 9s chain, but the chain would wiggle slightly side-to-side. It would not be possible to use a 10s chainring with an 11s chain, because the chain would not sit properly. It would remain "elevated" at a larger radius (until eventually eating into the chainring to sit properly).

This all makes more or less sense. The puzzle is that some noname brands indicate "Machined Alloy Fits 7 to 12 Speed".

Why will some chainrings fit 7s-12s while others are meant for just, say, 10s?

Context of the question: I'm trying to drop my larger chainring from 50t to 48t or even 46t. I'm partial to faster rpms, and at 90rpm a 50t will let me approach 50kph, and that's simply not useful as I'm not racing.

Based on this article, you can see that chains from 5 to 8 speeds have the same inner width (2.38mm), and chains from 9 to 12 have the same inner width too (2.18mm). The difference is in the outer width.

In 1x (single chainrings) transmission, only the inside width matters, since the chainring is not meant to derail. The same chainring can be used for 9 to 12 speed systems, and also 5 to 8 since the spacing is wider, but you'll have a bit of play.

For 2x/3x systems, you have pins, but also guides in the chainring that are meant to help the chain to derail. These guides are sized to match the outer width of the chain. You can deviate a bit, that being said: I've often read that a "1-speed deviation" is perfectly acceptable. Personal example: I'm running a 10-speed chain on 9-speed chainring, it works perfectly.

And then you have another aspects, which labeling strategy: Shimano design their components as part of a system: the specs they write are "describing" the other components of the system rather than matching the actual capabilities of the product. Other manufacturers are less conservative and will try to have the widest specs as possible.

• I'm still not sure. Maybe you're saying it implicitly. The noname cranks that claim to fit 7s-12s (straddling the 7s-8s boundary where the inner width changes) will be missing the guides, making at least shifting up / to them difficult, if not also shifting down from them.
– Sam
Apr 10, 2022 at 0:48
• @Sam well, the question is also how much you can trust what a noname manufacturer is claiming. Internet has enabled business models based on disposable brands. For instance, the contact address on Deckas' website is a hotmail address, not very trustworthy for me. That being said, from what I've seen with a quick search, they are mostly selling 1x chainrings (judging by the shape of the teeth or the mention "narrow wide"). 1x chainrings should be compatible with 7-speed chains (having a too narrow chainring may actually be on the safe side, as you need lateral play to shift gears). Apr 10, 2022 at 5:50
• A nice detail is that their website says "Deckas Bike Chainrings Factory" but has a big photo of coal-fired power station.
– ojs
Apr 10, 2022 at 8:37
• @Renaud fair enough... The question "what makes a chainring fit 7-12" reduces to the related "could a chainring be made to fit 7-12" and the answer to the latter is "very unlikely for 2x or 3x, possible for 1x". For the buyer it boils down to choosing between a name-brand not-an-exact-match-to-what-is-needed (or no stock) and noname may-match use-at-your-own-risk type of deal.
– Sam
Apr 10, 2022 at 15:23
• @Sam you got it. I would add two other intermediate alternatives: buy from a LBS, they would be able to test in their workshop (or by experience know what would work) before returning you the bike, or buy from an online bike shop that have a wider selection than what LBS can offer, and are more rigorous in their offering than Amazon or Aliexpress (which is not hard). Apr 11, 2022 at 5:47