I recently purchased a Sepcialized 2011 Tarmac s-works, new left over. It presently has a 54/39 s-works front chainring and i plan on chainging it out to a comppact 50/34 . LBS cant get the s-works compact 50/34 chainring and recommends i go with a SRAM red compact 50/34. All other components are SRAM red . My question is will the SRAM front chainring effect the shifting quality ...Rear cassette , derailuers are all SRAM RED too. Cranks are s-works. Any tips, advice .....comments would be greatly appreciated. Thanks!
Compact cranks have a 110 mm Bolt Circle Diameter (BCD) and standard cranks have 130 mm. The smallest ring that can fit on a standard road crank is 38 teeth, and the smallest on a compact crank is 34 teeth. So, to go compact chainrings, you need a new crank.
A good compact crank like the SRAM RED will shift just as nicely as a good standard crank (probably as well as or better than the s-works, which will be a relabeled FSA or something), with the caveat that the front derailleur should be set up correctly. There are "compact specific" front derailleurs, but I never could tell the difference- normal derailleurs work just fine for me.
The Red chainrings have a reputation for not very good shifting, although I think they are ok but not great. But the Specialized chaingrings have the same reputation. So I am not sure you are losing much by going to Red.
If it was me I'd look harder for a compact spider to go on the Specialized cranks. Red is perfectly acceptable (I have one) but I like the Specialized crank design.