crank options
I've been researching this topic as I spec out the silvio I hope to build in a couple of months.
Since there are a few related threads, I thought I'd summarize and tie them together. I've just read about it, so please correct my errors.
The philosophies for getting your low gears:
1- triple crank- you can spend all day in the middle ring with a close spaced cassette and go into the big ring or granny when you need higher or lower gears. Or you can go with a wide cassette and have the low-low combination for bigger climbs.
2- Wide double- wide span on the cassette to get the range of a triple with less hardware.
3- Harden up, who needs gears below 42-23?
Advantage of #1 is the narrower spacing on the cassette allows you better match load and cadence. I'm in this camp.
Advantage of #2 is reduced weight, cost, complexity,
Hardware options in order of smallest ring size.
1- Triple with 74mm bolt circle on the granny (Ultegra 6603, 105 5703, Tiagra 4603). These require the modifications in iow's thread you are reading now. Trim the chain suck tabs and get creative with spacers and shims.
http://www.cruzbike.com/74mm-bcd-inner-ring-silvio-2.0.
Positives- allows you to get a 24 tooth granny gear.
Negatives- chainline is a few mm further out, tight clearance, maybe derailleur capacity issues.
2- Triple with 92mm bolt circle on the granny (Ultegra 6703 dura ace 7803). This is addressed in fthills' vendetta build thread- includes modifiying a 92bcd ultegra granny ring. It looks like you trim the tabs and slide the crank on.
http://cruzbike.com/vendetta-unboxing-and-build#comment-17023
Positives- less modifications, Chainline isn't offset
Negatives- the only 92bcd granny available is 30 tooth.
3- Double- (both brands) You can get a double anywhere from 50/34 to 55/42 depending on brand.
Ivan went with Sram Force 53/39, and 11-32 cassette, with the wifli package (longer rear derailleur to handle the wide cassette).
post 16 in http://cruzbike.com/silvio-2.0-journey-build-and-ride-video
Eric Winn went 53/39 and 11-36 on the vendetta
http://cruzbike.com/mixing-sram-road-and-mtb-components#comment-22847
Postives- complexity, weight, cost (maybe). No modifications needed.
Negatives- front derailleur capacity limits the difference in size between big and small rings. The smaller your small ring, the smaller the big ring.
I have a 50/39/30 with an 11-25 cassette on my Rans V3. For the Silvio (when I get there) I like the 6703 92 bcd triple with a 12-27 cassette. It's relatively flat here, and I can put a bigger cassette on for trips to hilly areas. I like close spacing on my cassette so I don't have to hunt for the ideal gear, but that's a different topic.
A 30 chainring with an 11-25 sprocket would deliver the same 32.2 gear inches at the bottom end as Ivan's 39 ring and 32 cassette
A 30 chainring with a 12-27 or 11-28 would roughly match Eric's 28.6 gear inches.
A 30 chainring with an 11-32 cassette would match the 25 gear inches of 34 chainring with a 36 tooth cassette.
gear inches calculated using www.sheldonbrown.com/gears/ assuming 700c x 25mm
Tim
