I currently have a 50/34 Fulcrum RS crankset.
Can I just buy the 39/53 and swap them out or is my spider too small?

For example, is it really true that I can buy both 39T and 53T for £98? http://www.i-ride.co.uk/fulcrum/road_sp ... nring.html

Fulcrum like Campagnolo are 110bcd with one being set at 113mm therefore the only chainrings that fit need this bolt pattern :
http://www.parker-international.co.uk/1 ... nring.html

The fulcrum 53/39 rings are made for a standard 135mm bcd crank and wont fit on the 110/113mm bcd compact crank. There are aftermarket chainring that will fit, like the TA rings linked to above. I put TA 52/36 rings on my Fulcrum RS compact crank and they have worked very well.

Thanks!