Verve’s InfoCrank is technically dense, yet incredibly simple to use
Verve manufactures InfoCrank to the high standard required of a precision instrument.
By controlling all manufacturing processes, we set a standard of accuracy that’s not possible by adding a power meter to an existing crankset, The crank is a top-quality, conventional 2D cold forging.
The pockets are machined to a precision of five-100ths of a millimetre (0.05 mm).
The strain gauges are bonded into symmetrical pockets in the inner and outer faces of the crank. Variations between left and right cranks in a pair, or from pair to pair, are vanishingly small, amounting in no case to greater than a 1 watt difference as manufactured—eliminated in any case by factory calibration.
Our focus in manufacturing the InfoCrank is on accurate assembly of mechanical and electronic components into a precision load cell capable of reliable and stable performance even in a hostile operating environment.
Manufactured for Accuracy
Why do we say InfoCrank—an alloy crankset for a road bike—is a precision instrument? Because each crank, right and left, is designed expressly around a strain-gauge load cell.
A load cell is a transducer that converts a force into an electrical current, and is used for measuring instruments such as lab and industrial scales.
A single InfoCrank arm is so very accurate because the strain gauge measures only tangential force, or torque—the force the rider puts in that drives the bicycle forwards. The load cell design means that side, radial, and twisting loads that are exerted on the crank (but do not move the bike forward) are not measured.
Every load cell is individually calibrated and independently tested and externally certified. The certification tests proved an installed, operational InfoCrank had an average error of <0.2% in the zone where we ride most. The greatest error in 100 tests was 0.57%. No other power meter has achieved such high levels of accuracy.
A strain-based load cell’s accuracy is degraded by external temperature changes—a problem for cyclists who, as all-weather athletes, expect to be able to use their equipment in all conditions.
Temperature changes cause thermal expansion that can be confused with a load change and result in incorrect power readings, so power meters that use strain gauges must compensate for these errors.
InfoCrank overcomes these false readings by using the highest-quality strain gauges that are compensated for the thermal expansion of the aluminum crank. Additionally, the use of four strain gauges in each crankset in a full Wheatstone Bridge configuration inherently compensates for temperature, and makes the InfoCrank insensitive to temperature changes. The quality, match, and placement of the strain gauges with the alloy and the full Wheatstone Bridge configuration is the most accurate solution to temperature-corrupted readings.
Ride in any weather
InfoCrank measures torque with high accuracy. To calculate power, cadence must also be measured.
InfoCrank uses a reed switch, triggered by a fixed magnet mounted on the bike frame. This provides a very accurate trigger point. This is a binary process (on/off)—in the rpm range required for purpose (10—250 rpm), a simple count is all that is required to support InfoCrank’s high torque sampling rate and derive accurate, detailed power measurements.
The reed switch is also used to wake the crank from periods of inactivity. This consumes less power, which contributes to InfoCrank’s 500+ hour battery life, and has faster start-up and response times than other types of cadence sensor.
Accelerometers and gyros are improving, but in our view their cadence readings are not yet accurate enough to calculate overall power to the precision we require.
Verve continues to design and test alternative options to cadence magnets that would further simplify InfoCrank installation, and we hope to have a solution as accurate as magnets in the future.
Graphs display percent error in a single crank rotation based on sampling rate
It is not possible to achieve 0.5% accuracy in a power meter with fewer than 50 samples per rotation. InfoCrank’s 256 samples per second rate far exceeds this requirement even at high cadence.
InfoCrank measures torque at 256 samples per second, so at 60 rpm, it samples 256 times per rotation, at 120 rpm, 128 times, and so on. With a low sampling rate, important torque information is not recorded, especially during the torque peak of the rider’s power stroke.
This has been put to the test: torque data of a track rider’s session was collected at 480 samples/sec, using local storage on the bike. After download, it was plotted using a discrete Fourier transform.
The data was then plotted again using the simplified algorithm implemented within InfoCrank’s firmware—the waveform is identical to the Fourier plot.
A sampling rate greater than 256/sec does not increase precision, requires more compute power (so more battery power), and requires more bandwidth than is currently available to transmit reliably to the head unit with no data drop-out.
Both left and right InfoCranks include an ANT wireless module.
Communications to the bike computer are via the left crank, but both cranks are in continuous communication, creating a true bilateral power meter. If the bike computer supports bilateral features, left/right balance, torque effectiveness and pedal smoothness can be displayed, as well as total power and cadence.
The ANT+ Protocol sends and receives messages at a rate of 4 times per second.
Often, power data will be updated on the bike computer screen once per second, or three times per second. So, 256 messages are recorded per second, averaged into 4 messages which are sent to the bike computer. Those 4 messages are averaged to display one message per second, however when the data file is uploaded to your training software the 4 messages per second will be visible.
Verve uses ANT+ to transmit InfoCrank’s data because of its compatibility with bike computers and sporting software. It has a very low power consumption rate, which add to InfoCrank’s 500+ hour battery life.