Comparing power measurement technologies

The number of techniques for measuring power transmitted to a bike has grown over the last year or two, with various power meter announcements and launches.  Obviously we think our cleat-based system has advantages over all the others (otherwise we wouldn’t be doing this) so I thought it might be interesting to show you how we see the comparison of the various techniques.  This might help you to understand how the different ways that power meters work can affect the way you use the power meter and the information you may be able to get from it.  This is not a comparison of real power meters.  It’s a comparison of the theoretical capabilities of the different ways of measuring power.  We’ve just considered what each measurement technique is capable of doing, without reference to whether or not any announced or available product actually implements all the theoretical capabilities.

The power transmission path from pedal to road.

Measuring power

Measuring power on a bike is not simple. It’s not like measuring heart rate or cadence or speed, where you can just observe and count events (a heartbeat, or a magnet passing a sensor).  To actually measure power you must measure force or torque, and to do that you must intercept the power transmission train somewhere on its path from the rider to the road.  You must measure the miniscule distortion in some mechanical part as the force or torque passes through it on its way from the rider to the road. So power could be measured at the shoe, the cleat, the pedal, the crank, the chain ring/spider, the bottom bracket, the chain, the cassette, the rear hub, the rear spokes, or even conceivably at the wheel rim.  Of course there are systems that claim to measure power without doing any of these things, but they don’t measure power, they estimate it by measuring other things such as air speed, climb gradient or chain vibration.  This article only includes techniques that actually measure power.

Making measurements at different points in the power transmission train provides different characteristics.  One difference is in how the sensors are physically inserted into the power transmission train. For example, a technique might require a special rear wheel or a special pedal. A second area of difference is in the detail of the information that can be measured there.   For example, some can measure power from left and right leg separately and some can’t.

Why is this useful information?

I hope this will increase understanding and discussion about the differences and limitations of the various techniques used for power measurement.  Power measurement is entering a new era with lots of new and different ways of doing it, so this is an attempt to give a clear overview of the characteristics of each.  Power measurement devices will enable measurement of many other aspects of a rider’s interaction with a bike as well as just power, so it’s important to know where different kinds of power meter could go in the future and which ones have inherent limitations.  One interesting fact highlighted by this table is that the closer to the rider the power measurement is made the more information that can be measured about how the rider interacts with the bike.

And of course this table is also useful to show you why our cleat based system is the best way to do it all!

The table

The table below compares the theoretical capabilities of seven different techniques for measuring power.  These seven techniques were selected because they are used in existing products or in systems that are known to be in development. A few points to note while reading this table:

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