Rotor 3D+ MTB Crankset

Two-by-ten drivetrains have exploded onto the scene since Shimano and SRAM added an extra sprocket at the back. Shimano were a little slow out of the blocks to offer a double up front (unless you wanted XTR) so companies like Rotor were quick to fill that void. Here we take a look at their 3D+ MTB Crankset with their elliptical Q-rings.

Two-by-ten has it’s benefits: namely less weight and a narrower Q-factor and chain line. And the Rotor 3D+ Crankset is certainly light. In fact, they were so noticeably light out of the box that it begged the question of whether they were going to be fit for use, or whether they were some over machined bling for show bikes only. Rotor have been able to achieve this light-weight via their ‘Trinity Drilling System’ which takes advantage of the increased axle-crank contact area of the BB30 axle. This allows them to drill all three holes from the same direction making for a lighter crank, yet maintaining stiffness, and it’s apparently stiffer than the standard 3D crank too. Talking of BB30, the ‘+’ in the name relates to the fact that this is the BB30 compatible model. You can also get it in a regular threaded, external BB.

The thing that makes the Rotor Crankset not just another pimpy set of pedal arms though is the Q-rings – elliptical shaped chainrings that claim to improve pedalling efficiency. They may evoke distant memories of the old Shimano biopace rings (if you are old enough to remember them!), but these are different, the ovalisation is designed to be at different point in the pedalling motion. The rings are super light too, machined out to the max, the outer being profiled, ramped and pinned to aid shifting. All bolts supplied are aluminium, including the crank bolt. The Chainring BCD (although not the more common 4-arm) is a pretty bog-standard 5-arm 110/74BCD which means if you don’t want to run Q-rings or need a replacement in a hurry, it’ll be easy enough to source at least something that will do to get you by if you're otherwise stuck.

On the face of it, the principle of an elliptical ring is pretty sound. In any pedal motion, there is the power stroke and the much talked about dead spot. As cyclists, we spend much of our time trying to pedal in efficient circles to overcome this deadspot. Rotor’s thinking is to let the elliptical ring do the efficient work for you by eliminating the dead spot, putting that otherwise wasted energy in overcoming it, back into forward momentum. The tallest and shallowest sections of the Q-rings reflect this, the high point being in line with the most powerful position (the downstroke), while low point is at the dead spot. A nice touch is that Rotor understand that not every one’s pedalling motion is the same and have addressed this by designing in orientation options into each chainring. There’s a ‘standard’ position and then the ability to alter the effect a couple of degrees either way depending on a rider’s pedaling technique.

the cranks do look sleek...

Installing the crankset was relatively straight forward, but requires a bit of tweaking time to get it spot on. Supplied in the box are a number of spacers depending on whether the bike is set up as a direct press fit or a standard 68/73mm shell. We fitted this to the Niner Air 9 Carbon longtermer, and with its modular BB system coming in as a 73mm shell, it required the use of some spacers. in this case it was crucial to actually miss out the wavy washer supplied with the crank (it is recommended to read both the Rotor and frame manufacturers BB fitting instructions). In all it was a bit trial and error getting the spacer combination right, but once set, there’s a neat threaded lock ring on the left crank to finely tune the bearing preload. A little care does has to be taken not to over tighten the micro bolt on the lock ring though.

Once installed, the cranks do look sleek. They retain a classic look as opposed to the more modern, industrial appearance of some cranks. No curvy cranks arms here, they are straight solid looking items, with 2 grooves running the full length of the arms. On setting up the shifting, a triple front mech is needed to accommodate the amount the chain moves up and down on each pedal stroke. A double specific cage is shorter than a triple and isn’t deep enough to avoid rubbing in some gear combinations.

Out on the tail, riding on the rings was pretty choppy for the first few miles, trying to pedal in perfect circles. It did just feel all wrong to begin with, but it doesn’t take long to adapt to a more relaxed pedaling style and let the rings work for you. Caution should be exercised as you will feel like you want to push a bigger gear, especially during the power phase. This has it’s benefits in that you’ll ultimately go faster, but break yourself in gently while you adjust to the mechanics of it all.

In fact, it’s common to experience some mild aching in the quadriceps, which according to Rotor, is not uncommon during the adaptation process. Their recommended adaptation time is approximately 200 trail kilometres (500km on the road). Riding them on a training week in Spain, this was achieved in no time and within 3 days, riding on Q-rings was normal. Climbs quickly became a boon, feeling able to turn over a bigger gear on shorter climbs, and on the longer climbs it was easier to hold a steady rhythm for longer. The advantages of the elliptical rings also worked in those uphill technical sections where that little extra output or grunt is needed to get up and over roots and rocks at slower speeds. With the ability to give that little extra oomph at slow speeds, these rings would suit SS applications well.

up there as one of the lightweight options, and pips XTR and XX...

The stiffness of the cranks was the opposite to what you’d expect given that they are so light. But it shouldn’t be a surprise that they’re incredibly stiff given the road versions have been used under Grand-Tour heavy-weights such as Thor Hushovd (although it’s worth pointing out he opts-out on the Q-rings) and David Millar (who does use Q-rings).

With the the cranks and chainrings coming in at 588g, the Rotor 3D+ is pretty much up there as one of the lightweight options, and pips XTR and XX relatively comfortably for weight. It’s certainly not lacking stiffness either, but at £540 you’re certainly paying for it! The 3D+ could well be the crankset for you if you’re after the ultimate light-weight build, and there’s the added bonus of buying crank-arms only if you don’t want to experiment with the Q-rings. In practice the Q-rings did feel to offer an improvement, giving an advantage over round rings. Rotor also offer their Q-rings for standard 4 arm chainsets so you can experiment with elliptical rings on your non R-tor cranks too. Overall it’s a fantastic, if seriously premium crankset.