Interbike 2012: StageONE Add-On Crank Based Power Meters Explained
Stages Cycling’s StageONE crank based power meter was announced just prior to Interbike as a more affordable way to use power for your training. With typical crankset-based power meters topping $2,000, Stages’ $699-$950 offerings certainly look tempting. We met up with them at Interbike to get a little tech breakdown – here’s how it works:
They start with a new crank and surface prep the crank arm. Just looking at it, one naturally wonders why they can’t just sell the unit and let anyone stick it on their own cranks. The main reason is that the location and orientation is critical to proper performance.
The other reason is that they do a good bit of surface prep to ensure the unit won’t come off your crankarm. Once it’s cleaned and ready, the strain gauges are attached directly to the crank arm, which are then wired to the circuit board. All that’s covered by a custom plastic cover that fits the profile of the specific crank it’s going on.
The circuit board has a lot going on. There’s an accelerometer that provides info about rotational velocity correlated with the strain measurements to determine both cadence and power. That means you no longer need a cadence magnet strapped to your crankset. It’s also what puts it to sleep or wakes it up, and they say it wakes and knows what’s going on within one second.
Another cool feature: It communicates power data via Bluetooth Smart (4.0) and ANT+, meaning it’ll send power data to your iPhone 4S or newer (and any other phone that’s using the latest Bluetooth standard) without it needing a 3rd party dongle.
It’s calibrated for temperature by using an active temperature compensation that measures both outside temp and the temp of the electronics and continually zeroing out the calibration to keep it spot on. To set it up initially, you only need drop the left crank to 6 o’clock and use your cycling computer to zero reset it. For absolute accuracy, you should do that before each ride, but they said it’s not super critical since the unit is constantly self-calibrating for temperature anyway.
The StageONE only measures the left leg’s power, but it does it for the full 360° and then doubles the number. They’ve tested that against a left/right system and shown that power data averages out virtually identically between.
With different cranks getting the same basic unit, how does it know what you’re doing? They calibrate the strain gauges to each different crank arm by putting a known load on the arm to get both zero and a fixed number, then it knows the load range. For example, they check the strain on the crank with no load and with XX number of pounds. They wouldn’t disclose what that number was, but it gives them a low and high number. With that info, they have a strain curve (or line) and can figure out any amount of force (aka “power) within that range.
The only limitation seems to be that you’ll need to be running an alloy crankset from Shimano, SRAM or Cannondale…or willing to mix and match an alloy non-drive crank arm with your carbon setup. They’re not mounting it to carbon cranks.