Changing 53/39 chainrings to 50/34

[Alex21224]

I'm looking to change a Shimano 105 53/39 crank/chainring to a 105 (or higher spec) 50/34 chainring. Can anyone please tell me what is involved with this? Do I just need to change the chainrings, or am I going to end up needing to change pretty much the whole drive train? Thanks.

[cyclerUK]

A 50T would fit where the 53T is but the 34T won't fit instead of the 39T.
It's to do with the "PCD" (pitch circle diameter) of the chainset.
ie the mounting holes for the rings are on a different radius for the small ring.
You will need change the right crank or change the chainset as a whole.
The chances are your PCD is 130mm whereas you need 110mm for the 50/34 chainrings.

See:-
http://techdocs.shimano.com/media/techdocs/content/cycle/SI/SI_1M30A/SI-1M30A-003-ENG_v1_m56577569830702207.pdf

The other thing you will have to check is if the front dérailleur is also compatible with the smaller ring?
Probably is but some aren't.