By Caley Fretz
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For years, riders seeking a double crankset for their fat-tire rigs were somewhat out of luck.
Finding or making a double was possible. A few small manufacturers offered double sets while old five-arm cranks could be jury rigged or four-arm spiders filed to accept a 30t ring. But there was a blatant lack of interest from the “big boys” of mountain bike manufacturing in producing 2x.
In the last two years, however, that dynamic has changed. Shimano and SRAM now offer doubles with their new ten-speed groups, and FSA has their own high-end offering among others.
The 3D XC2 is Rotor’s latest entry into the world of double cranksets, utilizing hollow crank arms and their unique ovalized Q-Ring chainring design. The result is a light, stiff and efficient package, ready and worthy for anything from the World Cup race circuit to your favorite local loop.
Rotor’s 3D crank arms begin as a piece of extruded 7075-T6 aluminum before being CNC machined and hollowed out using Rotor’s ‘Trinity Drilling System,’ which bores three holes down the length of the arms. Rotor, a Spanish company, claims the process reduces weight without sacrificing stiffness. Burry Stander won the 2009, U23 World Championships on a set of XC2’s, and the same crank-arm technology is used by Thor Hushovd and Carlos Sastre on the road. Needless to say, if the 3D arms are strong enough for those guys, they’re strong enough for you.
The Rotor 3D is available with a steel or titanium axle, and the option of a double (110/74 BCD) or triple (104/64 BCD) spider. Singletrack.com wrenched and rode the double option, dubbed the XC2.
Ridden: Lord of The Rings?
Rotor’s ovalized Q-Rings take some getting used to. The theory is that during the comparatively “dead” points of any rider’s pedal stroke — around 6 and 12 o’lock — the effective gear drops, then rises again as the foot comes around to a more powerful position.
The rings have multiple bolt-holes, labeled the Optimum Position System (OPS), so riders can fine-tune where those high and low gear peaks reside within their stroke. From the first pedal stroke, the constantly fluctuating gear ratio is certainly noticeable.
My first ride impressions were not positive. Something about the Rotor rings seemed to bog down my pedal stroke at low cadences. Not good, particularly with the double setup lacking a bailout gear.
But, I had heard that the rings took some time to get used to, so I kept riding them. I also messed around with bolting down the rings in different positions, noticing a distinct difference with each change, and finally settled on a position that worked for me. I even put a set of Rotor rings on my road bike, to double my ride time on them. I now have about 800 road miles and 30 hours of trail time on the Rotor rings.
Long story short: I’m a convert, at least on the dirt.
Apparently, my legs adjusted. Now I hop on my cross bike – the last in my fleet without Rotor rings – and it feels odd. Am I faster with the Rotor rings? If I am, it’s not by any perceptible amount. I ride with a power meter on both bikes and haven’t seen any noticeable improvements. I just like how they feel now – my pedal stroke feels more round and even, and I tend to ride at higher cadences.
Not a fan of the Q-Rings? Not to worry, there are plenty of options available for 110/74BCD. 110 BCD is used on compact road cranks, and 74 is the same as a road triple’s granny ring. So finding a huge range of tooth-counts and brands in addition to those designed specifically for mountain bike doubles is not a problem.
The 40t/27t ring combo is superb. Low enough to get up and over just about anything, particularly with any of the new 10 speed groups that offer up 34 or 36t cassette options, and high enough for all but the fastest dirt road descents.
I raced the 3D a few times and always had the right gear for every situation. The 40 tooth big ring, compared to the usual 42 or 44, allows more big-ring time and less front end shifting (always a good thing in my book). Moreover, shifting on the funky ovalized rings is surprisingly good. The heavily ramped and pinned rings pick up and drop the chain on command every time.
The cranks themselves feel reasonably stiff, with perhaps a bit more flex than the SRAM double I was on last, and are pretty light to boot. On my scale they came in at 549g without chainrings. Build quality is spot on, and the versatility of the entire system is second to none. No stupid proprietary chainrings either. Graphics and shaping make for an eye-catching package as well.
Installation of the 3D’s is a bit trickier than the average crankset these days, but nothing a moderately equipped home wrench can’t handle (though Rotor’s literature still says to get them installed by a pro). The external bottom bracket installs like any other, and, as expected, feels super smooth out of the box. I got the steel-bearing version, though a ceramic option is available.
The cranks themselves require a set of plastic spacers and rubber O-rings be placed in the right order on either side of the bottom bracket; assemble them wrong and you’ll run into issues. But the included instructions are good — if you passed Lego’s in kindergarten, you’ll probably be OK.
After everything comes together, an 8mm allen key tightens a non-drive-side cap, cinching down the left crank arm and loading the bottom bracket bearings, similar to the cap on Shimano’s system. Careful though, the cap only needs to be a bit more than finger tight, ~1Nm of torque. Any more and you’ll feel the BB friction skyrocket.
With only a sliver a season on the 3D’s, commenting on long-term durability seems a bit premature, but,so far so good. The rings have whacked off a couple logs and rocks and remain solid and straight. If they spontaneously explode, I’ll be sure to post an update.