As I carry out cycle coaching with power on a daily basis, power meters that are not only accurate but also offer relevant measurement, make the difference between looking at true values in total confidence or just having power data that is consistent over time that could possibly under or over report.
Power meter users will have one power meter attached to their bike almost all of the time. The limiter to that is that the data can feel relevant, but there is no way to truly test the accuracy of that power meter until you put it up against another power meter on the same ride, measuring the same pedal strokes, time for time.
To test multiple power meters against each other, there are some relevant key differences to test for:
Where the strain gauges are
What type of force it is
How is the crank velocity/ cadence measured
External forces that may be mistaken but the strain gauges as leg forces
Long term degradation of materials the strain gauges are fixed to
The 2 power meters on test; the Verve InfoCrank and the Garmin Vector 2 offer quite different technology to each other.
Here is a brief breakdown of the Garmin Vector 2 technology:
Pedal based strain gauges in pedal axles of left and right pedals
Force measured at first point of contact with rider
Force measured at 13 times per
Crank velocity/ Cadence measured through accelerometers
ANT+ messages transmitted through pods plugged into pedal axles
Measures Torque Effectiveness and Pedal Smoothness
Measures Power phase
Standing and moving force
Inside/ outside of the pedal force
The Verve InfoCrank technology stacks up like this:
2 Load cell strain gauges that convert force into electrical current, in each crank arm (4 in total)
Force measured inside solid aluminum crank arms at closest point to spider
Force measured at 256 times per second (at 60 rpm)
Crank velocity/ Cadence measured through crank arm position taken by force measurements
ANT+ messages transmitted through pods on inside of crank arms
Measures Torque Effectiveness and Pedal Smoothness
Measures Tangential forces only (no side or radial forces)
You can read more on the technology here: https://www.vervecycling.com/dig-deeper/
DURING THE TEST
Once both power meters where fitted and linked individually to 2 separate Garmin ANT+ head units with updated relevant settings to crank length it was time to test on a chilly 3°c February morning .
First impressions out the door, I had expected to see some differences, but straight away there was a clear 30 watt difference between the 2. This continued through the whole ride and by the end, there was a 39 watt difference between the 2 power meters on average.
So in this situation which power meter is giving the most accurate reading? There are several reasons why the InfoCrank is giving a stable reading (and also why it only needs to be calibrated once) :
Because the infoCrank strain gauges are build inside a solid crank arm, this means zero flex. The only time there would be flex is if the crank arm was bent and you would know about that if it happened!
Temperature difference occurs when a hollow crank arm or pedal axle expands or contracts with temperature difference. This expansion or contraction is often mistaken for leg forces by power meters bonded to hollow surfaces that can flex.
Degradation of materials:
Through time, hollow axles and crank arms degradate. This again is another reason that Verve use a solid crank arm, because there is no flex through degradation over time.
POST RIDE ANALYSIS: KEY DIFFERENCES
Those 2 points cover the external factors. So what are the other factors that could contribute to the differences? Once uploading both files from the Garmin head units to WKO4, some of the difference started to become clear.
One of the most noticeable differences was with short, hard acceleration and with soft pedalling.
This can be seen in the power balance scatter plot below at shows Effective force x crank velocity, with the soft pedaling circled in red and extra acceleration that the InfoCrank picked up on, circled in green.
The chart below shows the rolling power through time of the Vector 2 and the InfoCrank together. Again, the acceleration and deceleration difference can be seen (click on the image for more detail).
The most likely reason for this is the extra times per second that the InfoCrank is recording for, so it picks up on more detail.
Of course, force is only half of the measurement of power. Cadence or Crank Velocity is the other half to this.
The main differences in technology in measuring this is the Vector 2 uses accelerometers and the InfoCrank uses the 256 times per second it measures force to know where the crank arm is.
So why would this potentially give a different measurement? The answer is road vibration. Accelerometers can measure road vibration as well as cadence. So this can have an influence on cadence accuracy, which in turn contributes to the overall power reading.
The below chart plots the cadence on the test ride from both power meters (click on the image for more detail):
The extra acceleration points with cadence on the Vectors is most likely from road vibration.
VERDICT AND RECOMMENDATIONS
In conclusion, my curiosity on Verve’s claims has been satisfied as to what are the differences in technology, why they are relevant and which is the most accurate power meter and you need to understand the why to come to that conclusion.
I would recommend the InfoCrank, but I would still certainly recommend the Vector 2 also.
As to who I would recommend each one to depends on your needs for functionality.
There is no doubt the Vector 2 offers some of the best function in terms of swapping easily between bikes.
And if you calibrate regularly (I recommend before every ride), you still have a power meter that will satisfy most basic needs for a measurement tool that should offer consistency of tracking performance improvement, even if that is consistently off what you are actually doing. It still gives you an idea of your improvements.
Also, the extra data that the Vectors shows is very handy to give coaching feedback on, with power phase and inside/ outside of the pedal force.
I would recommend the InfoCrank more to someone who wants the most accurate power meter and accuracy of power acceleration detail. The InfoCrank is a fantastic genuine upgrade if you already have a power meter, say a single sided model and want to upgrade to having the most relevant accuracy.
In : Product Tests