Bicycle Mechanics - SRAM mountain double compatible with Rival BB, FD and brifters?
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12-03-11, 11:21 PM
Thinking of getting a CX bike equipped with SRAM Rival drivetrain. Lowest gear on it now is 38x26 (~39.5 gear inches) which isn't sufficient for me to tour with a trailer around the Rockies, one of the intended uses for this bike. I can swap chainrings to a 50/34, put on a mountain derailleur and 11-36 cassette, but this will only get me to 34x36 (~25.5) which might be enough, but just barely. Can I throw a SRAM mountain double on as well and keep the same bb, derailleur and brifters? If I wanted to make it a triple would I just have to switch the FD?
Basically, yes, but only with a SRAM 10sp rear derailleur. Leonard seems to only mention the XX, but Sram makes other 10sp mountain derailleurs with "exact actuation" technology, which ensures the same cable pull ratios as Sram road shifters. The 10sp X7 is the cheapest of these.
Sram doesn't make a brifter that will shift a triple. If you really need to run this bike as a triple you could do some crazy setup with a barend shifter in addition to your brifters.
Another crazy option would be to run a mountain double crankset. A 39/26 with an 11-36 cassette would get you 19.6 to 95.7 GI range. Running a 2x10 mountain crankset with a road fd might not work with a road derailleur because the chainrings are so much smaller. I poked around and I am not sure if you can run a Sram road shifter with a Sram 2x10 mountain front derailleur and Sram 2x10 mountain crank.
Also, why would you get a CX bike with a standard road crankset? 50x11 is still pretty fast if you can spin it. I wasn't aware that anyone sold a new cx bike like this anymore.
12-04-11, 06:49 AM
Front derailleurs have different cable pull for road and mountain. A MTB FD will not work with SRAM road shifters. If you go to a smaller MTB crank, try your road FD and see if you get adequate shifting.
I was the opposite. Flat bar SRAM shifters, road crank, tried to get MTB FD to work and couldn't. But that was because curvature was too tight on the FD. You may be fine with road FD to MTB crank.
12-04-11, 07:33 AM
You may be fine with road FD to MTB crank.
I can't say about SRAM but I know from personal experience that Shimano road front derailleurs work fairly well with their MTB cranks down to at least a 46T big ring. You do have to get the fd's cage low enough to clear the big ring by 2-3 mm as you would with any fd.
12-04-11, 09:03 AM
Thanks tons for the input!
To answer your question, the current crankset is 46x38 which seems like fairly average CX gearing, I mentioned 50/34 so I could get the 34 tooth small ring, but I do see SRAM offers a 48/34 with the Apex. Total newbie to both CX and touring though, so maybe I'm misinformed. I like the idea of a mountain double, especially if I don't have to change anything else, I can just swap cranksets before a tour (will SRAM mountain cranks fit road BB?), and I don't see the top end being an issue while doing loaded touring.
Interestingly, I noticed Salsa spec Apex brifters with the X7 FD on the Fargo 2 build kit. I wonder how well that works? Is there something different about the actuation of the Apex shifters that doesn't carry over up the line to Rival? I looked through manuals SRAM has online and couldn't ever find any info about how much cable pull shifters and FD use per actuation. Any idea where to find such info?
12-04-11, 09:45 AM
the gxp bbs are the same. the road and mountain double fds are interchangeable for the 10 spd ones. for example i have done xx shifters and red crank and fd
12-04-11, 11:12 AM
Great news reptilez! Thanks!
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