Looking at doing a triathlon down in Deep Creek and the bike course has quite a bit of climbing, especially when compared to traditional bike course elevation profiles. The crankset on my bike is 110 bcd with a 52/38 setup, DA 7900 groupset. Cassette is an 11-25 so swapping it for an 11-28 is another possibility I guess. Anyways is there any reason I couldnt swap the small ring for a 34 and not encounter any problems or is the 52-34 transition just too great?
depends on your derailleurs. if you have a medium cage rear you are probably fine, but a short cage might not have enough throw to tension the 11/34 and still fit around the 25/52. also your front derailleur might not have a large enough inside plate to push the chain back out from the 34.
That front derailleur is designed for only a 16t difference between inner and outer rings. And even if the derailleur can be made to work, the shifting will probably be pretty rough, since the ramps on the 52 ring aren't going to be designed to pick up the chain from that low.
A wider range cassette or a smaller ring pair is a much better bet. Though putting on a bigger cassette might require a new, longer, chain.
David Millar at the Giro last year, SRAM XX 11-36:
Vinokourov on a 53/34 and 11-28 in 2010, Mt Zoncolan, Campy:
I have no idea for Shimano, but DA7900 RDs will take a 32T cassette w/ the B-screw jammed all the way in. Not sure if it can take up the slack of a 18T jump up front though...
I tried to do this same swap and it would shift up to the 52-chain ring but I had to hold the shifter in place longer then usual. The problem I had was the chain would fall off when I down shifted to the 34-chain ring.
Greasefoot: Chain catcher is definitely a must for these xxxtreme setups.
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If you switch the cassette and chainrings at the same time your low gear will be 34/28 vs. 38/25 so you will have essentially no difference in chain tension from now. And you probably don't care about using the 34/11 gear since you'll have near equivalent gearing with your large chainring in the middle of your cassette.
You definitely should get a chain deflector -- probably a good idea anyway since chain drop is so embarrassing.
34/25 and 38/28 are the same gear ratio.
I would switch the chainring. Cheaper and less issues with chain/cassette wear discrepancies.
