How closely does the size of a chainring guard have to match with the size of the chainring you use it with? Or, put another way, what are the consequences of using a 39 or 40t ring with a guard intended for a 50t ring?

Reason I'm asking, I've got a double crank I wanted to get rid of, but thought of replacing the outer ring with a guard and running only the 39t inner ring for a fixie/singlespeed setup. I happened to come across a chainring guard for cheap, but it's said to be for a 50t ring, and I've got no experience to tell me how big a problem that might be.

Thanks!

well, your existing crank has a 50 or 53T or something big ring on it now? picture that ring with the teeth filled in as a disk, and thats about what the chainring guard will be like.

I use a triple with a 50t outer, by having the chainring teeth ,
close to the outside diameter of the chainguard,
The Front derailleur adjustment is as close to best , as if the chainguard was Not there..

To up shift well, the outside plate of the FD has to clear the outside of the chain guard.

the compromise is an unknown shifting performance degradation, ... from this end.

I have used only friction shifting, for many years.. so how STI works with this kit change.. IDK


42t chainguards are common in cyclocross racing single ring setups, so nice ones are made.

I'd say converting to singlespeed like the OP is doing would cause a considerably worse shifting performance degradation...

so may still have scraping on the guard if cross chain situations are happening.
picking up the chain though there are no teeth..
wider gap adding spacers may have chain drop in the spacve.


blind guessing over a typewrier is continuing since i'm not there.

The purpose of guard ring is to protect the teeth on your chainring. Thus it needs to be just a bit larger that the chainring itself. If it is too large your chain might hit it if using a front or rear derailleur...but you said you are on a strict fixiedent diet so no issue...