Smaller chainring = lower derailleur (by how much?)

[zeroking17]

I'm thinking of getting a compact crankset (50-34) to replace a standard 53-39, and I want to be sure that I can lower the front derailleur enough before I plunk down the dough. The frame has a brazed-on derailleur tab.

I asked the framebuilder about this, and he thinks that the slot in front derailleur tab will allow me to lower the derailleur enough. (However, since the frame was built in 1998 -- before compact cranksets were prevalent -- he cannot absolutely positively guarantee that it will accommodate a compact.)

Can anyone tell me how much smaller (in millimeters) is the diameter of a Campy compact versus a standard Campy 53-tooth chainring?

[RPS]

A quick estimate based on the pitch diameters suggests that the difference in radius is about 6.06 mm. That's about .239 inches -- just under 1/4 of an inch. Of course that assumes the deraileur is adjusted to the same height over the large chainring.

I expect someone here will confirm these numbers soon.

[Dave]

The circumference is 2 x pi x R. Three fewer 1/2 inch pitch teeth equals 1.5 inches less circumference. 1.5/(2 x pi) = .239 inch or 6mm.

[zeroking17]

Thanks RPS and Dave!