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I'm thinking of building a bike from scratch around a Commencal HT AM 2018 frame, which normally comes with a 32T front chainring (1x setup) and says it takes 34T max. However a 36T would be ideal for my riding style, and the difference in radius measured at tooth tips between 36T and 32T chainrings is less than 9 mm (by calculation, roughly 72.9 vs 64.8 mm). Given such a small difference, I think 36T would fit based on this photo, which features a 32T chainring (you can see more of this bike's photos here):

this photo

However, I have no practical experience, and it would be a pretty expensive schooling if I bought a good chainring and found it does not fit. Anybody knows whether this would work?

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    It's a question of the clearance between ring and chain stay. It would be close. – Daniel R Hicks Jul 8 '18 at 13:03
  • Why not visit your local bike shop and ask them do a trial fit with a 36. I am sure they have one laying around. As noted above and below, clearance to the chainstay is the main issue, especially when you account for any flex in the frame. – kickert Jul 9 '18 at 14:43
  • Thanks for all the answers! Given all the unknowns, I think the beast course of action is to do as kickert suggests and do a trial fit at one of the local bike shops. – Mick Jul 21 '18 at 12:18
  • As for all the mentioned factors, if I think about the flex of the bike, it's not clear whether it would worsen or actually improve the situation; it seems to depend on whether the chainstay is below the chainring centre (in which case the flex, which effectively moves the chainstay upwards, would bring them closer) or above it (then they would move further away). As for the cassette combinations, I analysed all currently available options, and chainring swap turned out to be the best solution in my case. I'd need a 10-11-12 or 9-10-11T smallest three cogs for the cassette to solve it for me. – Mick Jul 21 '18 at 12:34
  • @Mick you need to consider lateral flex not vertical flex. Under power (climbing or acceleration) or cornering, the chain stay can flex towards the chainring – Andy P Jul 25 '18 at 9:23
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I think you will have to go with Commencal's specified max chainring size.

Radius of chainrings at center of chain rollers is:

32t: 64.8mm

34t: 68.8mm

36t: 72.9mm

If tooth profile is the same, the tips of the teeth will be a constant height above the center of the rollers, so a 36t ring will be approximately 8mm greater in diameter than a 32t - as you calculated.

Doing a rough analysis on the photo you provided: the gap between 32t ring tooth tip and frame looks to be about the same as the depth of a tooth.

enter image description here

If we assume the tooth depth is the same as the chain roller diameter of 7.9mm, you only just have 8mm clearance. If the chainring does clear, any frame flex will likely result in the ring hitting the frame.

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    That's a terrifyingly thin chainstay right there too - any scratches and scrapes would weaken it over time. Best to not to. – Criggie Jul 9 '18 at 8:19
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    When stationary, the clearance might be fine, but once moving and under load the ring will certainly touch the stay. If the builder says 34t max. they have a reason. And some mud, a twig or a stone change a lot of things. – Carel Jul 9 '18 at 20:46
  • What about chainrings with offset teeth? One can then set it up in a way that moves the chainline further from the chainstay, thus improving clearance. Or, one can use spacers between spider and cassette (if it is not a direct mount system) – Grigory Rechistov Jul 9 '18 at 21:05
  • @GrigoryRechistov do such things exist? Of course, the whole chainring could be offset outwards using spacers. – Argenti Apparatus Jul 9 '18 at 21:06
  • @ArgentiApparatus just googled for "offset chainring" and here is stuff from at least one vendor: wolftoothcomponents.com/products/… There are 3 offsets: Standard: 47mm chainline, 6mm offset for cyclocross and road cranks Standard: 49mm chainline, 6mm offset for GXP and long spindle BB30 Boost: 52mm chainline, 3mm offset – Grigory Rechistov Jul 9 '18 at 21:14
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My answer is meant to look at your goal from a different angle. You say: "36T (chainring) would be ideal for my riding style," and this assumes the gearing range you want can be achieved with a 36 teeth front ring.

The same or similar range can be achieved by changing another component of the gearing system — the cassette. Namely, you can keep the 32 (or get 34) teeth front chainring but use a cassette with lower minimum and/or higher maximum teeth count. Especially if you are building from scratch and thus have a lot of maneuver in components choice.

Old-style 10-speed cassettes are 11-36, but now one can even have 11-42.

The 11-speed systems provide a huge number of options, starting from "normal" 11-42 to crazy 11-50. If you need even smaller cogs, there are 9-46 cassettes.

If you go to 12-speed systems, SRAM Eagle variants offer 10-50, Shimano promises 10-51 (not available yet) and Rotor just went crazy and announced 13-speed 10-52 stuff. Some of those are going to cost huge money, but there are variants which are relatively reasonably priced.

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