This article is copy of PowerTap blog post - https://www.powertap.com/post/blog-16-powertap-advanced-pedal-metrics-explained
HOW THE P1 PEDALS COLLECT DATA FOR THE ADVANCED PEDAL METRICS
Before we dive into these answers, 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 new PowerTap Mobile App.
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.
GETTING STARTED WITH THE ADVANCED PEDAL METRICS
To use the Advanced Pedal Metrics you will need two things, the P1 pedals with firmware version .24 or later and the latest version of PowerTap Mobile for iOS. Once the pedals are paired to your iOS device, start an activity and swipe right two times. To change between the different view simply tap on the graphic. All of the data is recorded each time you start an activity and is available for playback and analysis at any time after your ride within the app.
A CLOSER LOOK AT THE ADVANCED PEDAL METRICS
Within Advanced Pedal Metrics we are displaying three different views. The first, and likely most popular view, is the Force Vector view (see Figure 1), which represents both the angle of force and the amplitude of that force. Or simply, what direction you are pushing and how hard. There are two colors displayed in this view: black, which represents positive force and purple, which represents drag (fighting the opposing leg). Because we are showing only the force component of the pedal stroke the data being displayed is in Newtons, not watts, and the scale is 20 Newtons per pixel.
So what exactly is a Newton? A Newton is a measure of force and is equal to 0.2248 lb of force. The Vector Force metric gives a very detailed look at how you are applying energy to the bicycle. It can be used to improve how you are pedaling by giving insight into where you are wasting energy by fighting the opposing leg (purple) and next trying to improve the direction of the force you are applying so it is as in-line (tangential) with the pedal circle as possible.
Figure 1. Force Vector View.
The second view is the Polar Plot, which displays the shape of your pedal stroke where the distance from the grey circle to the colored line is the magnitude of force, either positive or negative. As with the Force Vector view, any drag would be represented by the color purple. In the Figure 2 below you will see that the maximum force is applied at 3 o'clock on that pedal revolution. The Polar Plot is a great way to get a good idea of where the highest force, as well as the highest drag are being applied.
Figure 2. Polar Plot View.
The last and maybe most visually appealing view is the Heat Map. The Heat Map is a color-based graphic that shows tangential force as a percentage of a maximum. If you imagine your pedal stroke getting hot when higher forces are applied to move your bike forward, that is what you are looking at in the Polar Plot view. Calculated for each pedal stroke, the breakdown of percentages is below. The thin portion of the circle represents any drag that is being measured.
Figure 3. Heat Map View.
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.