Correct size of front derailleur

[GixxerBoy]

Hi Guys,

I have an Ultegra 6800 groupset with 170mm 52-36 crankset and I need to replace my front derailleur, but I have two options to choose from: 34.9mm or 31.8mm.

Why two sizes and which one is the correct size?

Your expert advice is much appreciated.

Cheers,
Kaveh

[biker jk]

I presume your front derailleur is attached with a band to the seat tube. If so, there are two diameters to choose from to match the diameter of your seat tube. Just grab a ruler and measure the diameter of your seat tube where the front derailleur mounts.

[GixxerBoy]

Thank you BikerJK, I'm glad you've mentioned it as I just realized that I need a braze on front derailleur!

Thanks for your help.