Hi,

I'm considering swapping out the middle chainring from my triple - from a 42 to a 39.

Because it's the middle chainring will I need to adjust the derailler at all?

Thanks in advance.

Probably not if it is shifting well now. If you have STI shifting you will benefit from replacing the middle ring with one designed to work in a triple with ramps and pins.

You will not have to do anything with the front derailleur as it's position is defined by the big chainring, not the middle or granny. Your shifting from the granny to the middle and the middle to the big ring may be a bit more sluggish since a 39T chainring intended for a double won't have the shifting enhansements provided on your 42 unless you get a 39T ring intended for a current 105 triple.

But make sure you match the old chainring spacing. It's possible to insert a "different" chainring and have it a little farther out --especially if you're talking about a 10 speed setup and an older 39, and end up with a gap just large enough for the 10 spd chain to spin freely in between them. Been there. Not cool.

Is there anyway to check if a chainring is intended for a triple or double? I've had a quick look for what's available and I haven't seen any mention of it. I'll probably be buying from somewhere such as wiggle.

If it doesn't say it's probably for a double. You haven't said what brand of crankset you have. For smooth operation it may be best to buy a ring of the same brand. There is always someone looking to trade a double for a triple. See if you can make a trade, but include the bottom bracket with the crankset.

Crankset brand is Truvativ. Will go with a 9 speed setup.
BCD would be 130.

CRC seem to have a much better range of triple chainrings available than other places.
I'm trying to work out why this one is so cheap, even though it is Sora.
https://www.chainreactioncycles.com/M...?ModelID=38171

I'm not sure what the BCD on that chainring is and the Chain Reaction site doesn't specify. RSX and I believe Early Sora was 110 mm.

Checking the model number with Shimano it looks to be the newer 9 speed Sora with 130 mm BCD. *shrug*