PowerTap Advanced Pedal Metrics Explained
Pedal-based power meters have given cyclists new performance insights that before could only be assumed or speculated. While watts and torque are the standard metrics on which training with power begins, this newer power measurement platform brought with it new and interesting packets of data.
This data revealed such things as the rider’s dominant leg, how often they sit or stand, as well as the arc, or roundness, of their pedal stroke. Here at PowerTap, we set out to separate the useful data from the noise with our Advanced Pedal Metrics (APM).
Getting Started with the Advanced Pedal Metrics
The Advanced Pedal Metrics are available with the PowerTap P1 pedals paired via Bluetooth to an iOs device with the PowerTap mobile app, or via Rouvy virtual training on iOs, Android, or PC. The APM display can be viewed live or reviewed after an activity on your Rouvy Training Diary.
Note that when you create a PowerTap Mobile App account, a Rouvy account with the same username and password is also created to store and analyze data.
How the P1 Pedals Collect Data for the Advanced Pedal Metrics
Before we examine the data displays, first a little background on what is being measured by the P1 pedals and how we are able to accurately extrapolate those measurements into the metrics that are displayed by the PowerTap Mobile App. Let’s begin with tangential and radial power.
Tangential power is a measurement of “positive” power. For example, if you have your crank arms at 9 o’clock and 3 o’clock and you push straight down on the pedal at 3 o’clock, you’ll be using full advantage of the leverage from the crank arm. This force would be then be measured back to you as tangential power.
On the flip side, radial power is a measure of wasted energy. For example, if you have your crank at 6 o’clock and 12 o’clock and you push straight down on the pedal at 12 o’clock none of your work will be turned into forward momentum.
Figure 1: Advanced Pedal Metrics use pedal and crank angles to determine how much of a pedal stroke’s force is helping propel a rider forward and how much force is being wasted.
If we look back to basic physics, we will remember that Power has both a force component and a speed component. Each of the P1 pedals have four pairs of strain gages that are measuring the force applied to the pedal body. This is done in both the vertical and horizontal plane.
The speed measurement is being captured with a magnetic ring affixed to the pedal axle that has 20 North/South transitions and two tiny hall effects sensors that measure the transition between North and South on the magnetic ring. This gives 40 angular velocity measurements per pedal stroke on both the right and left sides. To sum that up, we are measuring forces vertically, horizontally and angular velocity 40 times per revolution.
That is a LOT of data, so how do you separate the metrics from the matrix?
A Closer Look at the Advanced Pedal Metrics
Within Advanced Pedal Metrics there are two separate options to simplify the data into visual displays: the HeatMap view and the Pedal Angle view.
The HeatMap view shows percentage of power left/right with tangential power represented in shades of red and blue, with red representing positive (tangential) force and blue as negative (radial) force. The darker the color shows the intensity of the forces.
The pointer in the example below shows the spot within this particular pedal stroke where tangential (positive) power is greatest. Finally, the shape of the HeatMap circle shows the magnitude of radial (wasted) power. In short, a very lumpy pedal stroke that deviates from the perfect black circle is very inefficient.
Figure 2: An instantaneous view of a HeatMap from a Rouvy training log. This example was recorded during a standing acceleration.
Figure 3: This HeatMap view is the average of the entire activity as viewed within a Rouvy training log.
Next up is the Pedal Angle view. Again, we can see the intensity of positive and negative forces represented in color and shade. Percentage of power balance and maximum tangential power is also displayed along with the measurement of pedal angle relative to the horizon at 12 different points per stroke.
Figure 4: This instantaneous Pedal Angle view shows significant left heel drop on the down stroke.
Figure 5: This Pedal Angle view averages the entire activity and confirms that this rider’s heel drop is persistent.
Within Advanced Pedal Metrics you now have a tool to see your inefficiencies and symptoms of other trends that could be concerning. For instance, if your heel is always at an exaggerated drop or lift at the bottom of your pedal stroke, a seat post adjustment or visit to a bike fitter may be necessary. You can also use the live display via Rouvy during indoor training to eliminate bad habits and train a new muscle memory.
If you haven't read Hunter Allen's piece on using pedal-based cycling power meters to improve pedaling, do yourself a favor and give it a read. In this piece he explains how the right and left legs interact with each other during pedaling, and gives a few examples of things you can try to help improve your pedaling - and as a result waste less energy or go faster.
At the end of the day, our Advanced Pedal Metrics are another tool cyclists can use to help make improvements, gain speed and get the edge over their competitors.