News: Comparing power measurement technologies
The power transmission path from pedal to road.
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 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:
- We don’t deal with the debate on where in the drive train is the ‘best’ point to measure power, or whether any point is better than any other, or what are the differences in power measured at different points. That’s a debate for other places.
- We don’t consider accuracy, or measurement complexity, or calibration procedures, as these are largely dependent on how a technique is implemented by a particular product.
- We don’t consider the weight added to the bike and rider by a power meter that uses a particular technique, as again that depends on the implementation.
- We don’t discuss whether the characteristics in this table are important or not. That’s up to you!
- Some of the information in the table is just our judgement – you may have a different opinion.
|(See below for some explanation of these)||Rear
|Measure all effective power?||Yes||Yes||No||Yes||Yes||Yes|
|Measure left and right separately?||No||No||No||No||Yes||Yes||Yes|
|Measure negative push down separately?||No||No||No||Yes||Yes||Yes||Yes|
|Measure pull up separately?||No||No||No||Yes||Yes||Yes||Yes|
|How long to move between bikes (mins)||<1||<1||3-10||>10||3-10||1-3||0|
|Move between different types of bike?||No||No||No||No||No||Yes||Yes|
|Measure cadence versus crank position?||No||No||Yes||Yes||Yes||Yes||Yes|
|Measure torque versus crank position? ||No||No||Yes||Yes||Yes||Yes||Yes|
|Measure pedal angle versus crank position?||No||No||No||No||No||Yes||Yes|
|Measure force distribution across pedal?||No||No||No||No||No||Yes||Yes|
|Detect standing or sitting rider?||No||No||No||No||No||Yes||Yes|
Notes (from the numbers in square brackets [ ] in the table):
- The Bottom bracket technique can only measure torque applied via the left pedal and crank, so all answers for that technique are for the left side only.
- The Pedal technique is assumed to be sensors inside the pedal axle.
- ‘Torque’ is force trying to turn a wheel or axle. Low torque at high speed can give the same power as high torque at low speed.
Some explanation of what is meant by the characteristics listed in the table above:
There’s a lot of exciting stuff going on here in Brim Brothers HQ at the moment, and I hope to be able to bring you some announcements and information within the next few weeks. Stay tuned.
(Edit 26apr11: Changed table entry for force distribution across a pedal for pedal based measurement.)