Before we even get to the power meter that lies within, Rotor’s triple-drilled cranks are impressive in themselves. At 668g including battery and chainring, they’re surprisingly light and seriously stiff, with almost zero appreciable flex when riding hard or bracing one’s self on the bars and pushing down hard underfoot.
Available in 170mm, 172.5mm and 175mm (tested), and compatible with any mainstream BB standard, they should fit almost any bike. Much like SRAM, Rotor uses a single 8mm bolt to both secure and remove the crank arm from the spindle, making it a doddle to fit. A single AA battery slots into the spindle, and once you’ve paired the ANT+ sensor to your bike computer, you’re ready to go.
Once paired with our Garmin Edge 520, data was streamed seamlessly. However, our test crank stopped sending signals after a while, which meant we had to reconnect our Garmin daily to get it to recognise the crank – though the issue may have been with the Garmin.
Unlike Quarq’s chainring-mounted strain-gauge, the Rex only measures the power delivered by your left leg, rather than both. If one of your legs is stronger than the other, then this will give a slightly misleading representation. We often felt the power reading it provided seemed lower than we’d expect given our heart-rate and level of suffering, indicating a lower VO2 max figure than we’d normally expect.
My left leg is measurably weaker than the right, which probably explains this apparent under-reading. Nevertheless, a left-leg-only power meter still gives relativistic data – useful for comparing efforts between days as you train, even if it can’t be relied upon to compare total power output between different riders.
Rotor’s trump card is its oval chainrings. While it’s debatable as to whether they result in more power, they certainly deliver power to the rear wheel more smoothly, noticeably reducing wheel-spin on slippery climbs.
After the ride, Rotor’s Torque 360 software provides a graph of the torque supplied by your left leg as a function of the angle of the pedal. This can be used to reposition the chainring to best complement your natural pedalling profile.
Again, though, this relies on your legs being roughly symmetrical. If you’re still not sold, you can fit normal spider-less chainrings instead.
After putting it through the muddiest Welsh trails and plenty of unsympathetic jet-washing too, the Rotor has shown no reliability issues so far, remaining dry inside the battery chamber. If you’re sold on oval rings and want to get the most out of them, and have roughly symmetrical legs, this could be the power meter for you.
This article was originally published in What Mountain Bike magazine.