Thursday, February 24, 2011

SRAM Dual Drive a failure?

I've heard horror stories of the unreliability of SRAM's Dual Drive product. This scares me b/c one guy completely lost all his low end gears midway through a ride:

I can't imagine what I'd do if that happened. That would be horrific. Stuck at the bottom of a mountain with failed gear unable to get up the hill with 6 more days of 100mile/day rides would be a huge disappointment if not impossible mission.

The unit is probably much harder to fix. The mechanics on the ALC ride wouldn't want to touch it. Perhaps the standard 22/42 compact double that the KMX X-Class offers would be the way to go.

Reading through the forums further, I found that the SRAM Dual Drive has a limit of 30T chain ring. That means I cannot combine it with the 22T from the X-Class setup. Let's see how the gear chart would look with a 30T ring up front.

Low gear is still 12GI's. That's far from the 8GI's I originally found with a 22T chain ring up front, but that's not so bad. 8GI's is ridiculously low. The lowest GI I've ever heard recommended anywhere was 10, but mostly 12. 8 is unnecessary so I'm not really disappointed.

Gear range with this setup is 12.1 - 92.5

After tinkering around with the charts, I found a new configuration that might work better. I used the smallest and largest chain rings available from Sinz (cheap chain ring supplier) and picked the exact middle number for the center chain ring and the chart came out like this.

GI range is 13.7-111. I can comfortably start in 2nd or 3rd gear w/o engaging the SRAM Dual Drive using the 41T chain ring. I expect to do most of my triking in this setup which would be ideal since the efficiency should be best for the SRAM Dual Drive in this configuration. It's very similar to my existing 7-speed setup.

I would engage the SRAM's high gear for fast flat land and switch to the 48T chain ring while in SRAM's high for down hills rides. As I'm walking myself through this, I find that there will be a lot of double shifting. Engaging the Dual Drive while in the 41T chain ring pops the GI's up by 36%. To get a smoother transition, I will have to downshift my cogs by one gear every time I engage the Dual Drive to a higher gear effectively going from 69.6GI to 80GI's. That's much better than going from 69.6GI's to 94.8GI's.

This gets really complicated! Let's see how a compact double crankset would work. The chart is the same minus the 41T chain ring.

The jump from engaging the SRAM Dual Drive is still extreme but that's because it's governed by the ratio of the internal gears. There's nothing I can do about that. As expected the GI range has also remained unchanged. It's easier to think about though. I'll spend most of my time with the 48T chain ring which would be used for flats and minor slopes both up and down with the Dual Drive disengaged.

The Dual Drive would only be put into high gear for extreme down hills. I will only pop the Dual Drive into low and bring the chain ring down to 34T during extreme climbs. There we have it.


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