I have a 2011 F4 with the standard double gearing (53/39) crankset. Two questions:-
1. Can I simply change the rings on the crankset to 50/34 (compact gearing), or do I need to buy a new crakset?, and,
2. If I make that change will I need to modify chain length or use different rear derailleur?
Thanks,
Damian.
Damian,
Thank you for your business, the F4 is a great machine, a workhorse in the F-series line. The standard 53/39t crankset uses a 130mm bolt circle diameter (B.C.D.) This size only allows for chainrings down to 38t, any smaller and the attachement bolts will interfere with the chain/ring interface. If you want to swap to compact gearing you'll need at least, the drive side crank arm and chainrings. You don't need a complete new crankset with bottom bracket and the left crank arm although it may be difficult to source just the drive side. The "compact" on the 50/34t crankset refers not just to the smaller rings but also the B.C.D. which is 110mm on the Shimano Ultegra cranks.
If the only change you make is to go to 50-34t then you'll also want to shorten the chain.
One option you might consider is a new cassette. The stock 11-25t cassette could be changed to a 12-28t or 11-28t and Shimano recently introduced a 12-30t option as well. Techically speaking the 30t exceeds the current "28t max" cog size on the Ultegra RD but it remains within the published capacity. It might be worth a try at your locall Felt Dealer to verify the function with a 12-30t cassette. That would be the most affordable alternative to get a lower gear range.
One change you'll see on Felt's 2012 models is the use of "performance compact" gearing with more models using the 52/36t chainring size on compact 110mm BCD. This has proven more versatile for many of our consumers. This option will be found on many 2012 Felt models and you'll see it being used from more brands in the industry in 2013.
-SD