I have a bike with a 46/36T crankset (FSA Gossamer Cross BB30) and I'd like to change the small chainring to an even smaller one.

​Googling around ​suggests that the smallest chainring I can get for a 110mm BCD crankset is 33T. Is that right?

Furthermore, it seems that I won't need to replace anything else at all, not even the chain. I run an Ultegra RD-6800-SS rear mech with an 11-28T cassette and all the specs seem to check out:

  • Max front difference: 16T (in my case, 46-33=13).
  • Total capacity: 33T (in my case, (46-33)+(28-11)=30).

So just swap out the chainring and good to go, or am I overlooking something? (Never done this before.)

Finally, how do I make sure that the chainring I buy is compatible with my drivetrain? It seems that the BCD (110mm) is all that matters, so something like TA 110 PCD Zephyr would work?

P. S. As I am looking at a picture of the Zephyr chainring, I notice that it says "9/10 speed". What does that mean exactly? (My drivetrain is 11-speed.)

P. P. S. This page lists what looks like the same product as compatible with 8/9/10/11 speeds, but with the following cryptic remark: "For 10-/11-speed drives only with 1 or 2-speed crank handle!" Are they simply trying to say it's too thick for a triple chainset or does this refer to something else?

TA 110 PCD Zephyr

  • Simply reaching the chainring may be a task. Probably easiest would be to remove the crank arm. But a 9-10 ring may be too wide for an 11 chain. (I lost track of the details about the time that 7s came out.) – Daniel R Hicks Aug 9 at 11:36
  • @DanielRHicks: Thanks. That's what I am wondering too. This page lists what looks like the same product as compatible with 8/9/10/11 speeds, but with the following cryptic remark "Note: For 10-/ 11-speed drives only with 1 or 2-speed crank handle!" Are they just trying to say it's too thick for a triple chainset? – NPE Aug 9 at 12:02
  • Depending on the length of your chain you might need to remove two links. If the derailleur is taking up as much as it can at 36/11 you might go slack at 33/11. – Ross Millikan Aug 9 at 16:53
  • @RossMillikan: Oh yeah, I haven't thought of that. Thanks! – NPE Aug 9 at 16:57
  • Random bit of trivia: In some systems (at least Campagnolo at the time 10-speed come out) the chainrings are the same but the thickness of crank spider varies. So it's the crank that also determines spacing between rings. I think road triples went out of fashion before 10-11 speed became common. – ojs Aug 9 at 18:55

Most drivetrain setups have a 16 tooth difference between large and small rings.

  • Near compact: 52/36
  • Compact: 50/34
  • Sub compact: 48/32

You are trying to get a close to a super compact on a 110mm BCD and will have a chainring tooth difference of 13.

That means the rear derailleur will easily accommodate this as you have calculated.

The front derailleur will also handle the chaining size difference.

Chain length is set on the large chainring and largest cog sprocket, so you should not have to shorten the chain (assuming your chain is correctly sized to begin with). You will be within the derailleur total capacity so it will take up the extra slack.

If you do have too much slack in small ring and smallest sprocket, check the chain is sized properly.

  • If they added 50 links they would be within the capacity but the derailleur wouldn’t take up the slack. Chain should be assessed once the new ring is installed – Swifty Aug 9 at 17:45
  • What does 'if they added 50 links' mean? – Argenti Apparatus Aug 9 at 17:47
  • It’s hyperbole to picture how the chain could be too long for the smallest gear – Swifty Aug 9 at 17:51
  • Well, yeah. If the chain is the wrong length there will be an issue. I made a small edit '(assuming your chain is correctly sized)...'. I guess the chain could be too long, but as chains are sized on the biggest ring and sprocket it seems unlikely. – Argenti Apparatus Aug 9 at 17:53
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