A History of Power Meters

A History of Power Meters

It’s hard to believe that cycling power meters have been on the scene now for over 30 years, but during that time they have revolutionised the way cyclists both train and race. Back in the late 1980s when SRM launched the first commercially available power meter, the cost was high and the demand low.

Over time however, as both the technology and the awareness of it developed, the demand increased and naturally the cost steadily decreased from the thousands to the hundreds of pounds.

Today the recording of power output takes its place alongside the numerous other data-sets (such as heart-rate, speed and cadence) as a way for cyclists to track their exploits. But broadly what measuring power offers that other ways of measuring rider-effort do not can be summed up in one word; objectivity.

Measuring power provides data that is immediate and direct, taken as a measure of the physiological process used to generate it, force through the legs. Other ways of measuring rider-effort – such as heart-rate – are useful as a way of quantifying rider effort, but arguably they are not as repeatable or reliable over time.

Many brands have entered the market over the last 30 years and novel ways of measuring power generated by the legs have sprung up. Instead of being measured by the deformation of the crank-spider, as in the case of SRMs, power can now be measured via the crank-arms, pedals, hubs, bottom brackets and even chains – there’s no denying that accurate measurement of power has become the cornerstone for every cyclist serious or merely interested about their own performance. 

So, for someone who wants to train with power, what are the options?  Here’s a selection of the common currently available approaches to measuring a rider’s power output.

Crank Spider: This solution to power measurement calculates the power output of the rider by measuring the amount of deflection occurring in the crank spider when the cranks are placed under load.

The obvious disadvantage to this approach is that only the combined power output of both legs can be measured, there’s no way to measure one leg independently of the other, and so any physiological discrepancies cannot be identified and eradicated. As mentioned above this approach was the one taken by trailblazers such as SRM, but it is widely acknowledged that this technology has been superseded.


Pedals – With pedal-based power meters, the power reading is taken directly as a product of the rider pushing on the pedals. At first glance this may seem ideal but the reality is there is a lot going on mechanically at the pedal/shoe interface and so the complexity of accurate force measurement taken here is very much multiplied.

There is also the obvious problem with pedals being one of the most exposed areas on a bicycle, especially off-road, where they can collide with the ground, trees and any other obstacle nature has thrown in the way. One hard collision and the impact will be felt most acutely in your wallet.

Rear Hub – Here the rear hub is what measures the power-output of the rider, by measuring the force by which the chain is being pulled over the rear cassette. This approach is hampered by the obvious fact of power lost through the drivetrain, which whilst not significant on a clean drivetrain cannot be consistently adjusted for unless the drivetrain always remains in the same state.

This type of solution is popular in mountain bike circles as it’s less likely to see power-spikes when the terrain – instead of the rider’s effort – produces a significant force through the crank-arms.  Such a solution would also of course be useless for competitive tandem racing where only the combined power output of both riders can be measured.

Crank Arm – Here the power-measurement is taken by measuring the deflection of the crank arm when placed under load. So, as the rider pushes on the pedals and propels the bike forward there will be a degree of flex in the crank-arms that can be used to calculate the force applied.

There are inferior single-leg powermeters that simply double the power numbers up, and double-leg powermeters that measure both legs and transmit the data to a head-unit. The single-leg solution is far from ideal unless the user simply wants power-measurement as a novelty. The double-leg solution really is the gold-standard since the highest levels of accuracy and repeatability are possible.

Double leg measurement also makes it possible to identify and correct any physiological discrepancies between the legs. This is the approach adopted by InfoCrank, which has the added advantage of housing all of the electronics internally within the crank arm well out of harm’s way.

So, if you are in the market for a powermeter hopefully now you have a better overview of what is available. Measuring power is the best way to understand your fitness and the fastest way for you to improve. But as we have seen, not all power meters are created equal so think and choose carefully. The Verve InfoCrank takes the most intelligent and no-compromise approach to measuring rider effort, whilst still remaining highly affordable.

Getting the point with Accuracy

Getting the point with Accuracy

In the world of competitive cycling and coaching many things are said about the accuracy of power meters. What is acknowledged by all is that knowing and training the power of the athlete is the single most important determinant for success.  The energy (watts) expended by the cyclist in driving the bicycle forward determines everything else.

But confusion runs rampant and logic is totally lost due to the misinterpretation of the simple word – accuracy.

Most athletes and their coaches try to explain accuracy in terms of consistency.  They say that the only thing that matters is that the results from their power meter are somewhat consistent.

In this case, half right is totally wrong. It is logically impossible to know (as an athlete or coach) if your power meter is consistent if you do not know if it gives true results.  That is why the actual definition of accuracy is defined as having two parts:

  • Trueness.   – is the closeness of a measurement quantity to that quantity’s true value. In other words, it is the extent to which the measurement is error free.*
  • “Precision” –  is the degree to which two or more repeated measurements show the same results each time. Sometimes precision is referred to as the reliability or consistency of the measurement.*

It is possible if you have extensive testing facilities to determine whether your device shows consistent results, without necessarily ever showing true results.  But for the cyclist on the road and the coach looking at results, it is impossible.

With the advent of the InfoCrank, acknowledged as the power device that is both true and precise the whole time, there is a sea change about to happen.  The InfoCrank has been independently tested for both torque (measured 256 times each second) and for power (watts) which combines the torque and the cadence to a level of accuracy (both parts of the definition) that in simple terms, the number on the screen of the athlete is always the correct wattage number.

For the first time, leading federations and coaches are realising that they now have a tool to help with selection of athletes for teams. Because they have trueness and precision all the time on every bike, they can directly compare each athlete and make selections accordingly.

Many commentators may have assumed that this was happening before, but it was not the case.  Some federations knew that their devices were quite consistent, but not directly comparable bike to bike or rider to rider.  This has now changed.

However, some competitive amateurs also knew it, but not because of expensive measurement tools. Riders who use certain indoor spinning machines with a crank based power device go to extremes to mark their favourite machine. They have learned that their perceived effort differs machine to machine and has no relationship to the number being shown on the head unit.

A break-through is now easily predictable.  Over the next years, say leading up to the Tokyo Olympics, there will be very significant improvements in cycling performance and not because of the chemists and doctors.

With only consistency (at most) to go on, it was quite a random exercise to have anyone other than podium athletes really train at the specified level.  The data was only viewed after the event, so normally only averages were observed. As the cyclist changed bikes from the road to the track or vice versa, the numbers differed greatly as they did in different environments.

The Cyclist did not know as they rode their intervals how closely they were maintaining their workload and so nearly all riders, even with all the technology available, have been training on Perceived Effort with numbers to prove it. This is why well-known professional riders say they do not ride with their power meters. They are very well tuned to Perceived Effort and the power estimator does not add particular value to them.

Have a look at the 50 second excerpt below from a steady ride (on a real climb) where the athlete was attempting to maintain 250 watts.  The InfoCrank showed the right number each pedal stroke as it has been independently tested to do.

Getting the point with Accuracy 1

The other power device (claimed to be accurate to 1.5%) showed numbers that varied second by second in a 100 watt range, both negative and positive.  For the rider who wants to measure his effort, he truly has no idea what power is being ridden.

Many coaches might be satisfied with the uploaded data after the event, because after all the fluctuations, the average result is OK.  The rider, the one who is riding the effort in order to improve, never had any idea what pace he was riding and does not know until later.

No wonder that professional riders when asked what they want from a power meter, say simply, “ I just want the right number”.

The advances in cycling performance will come from a combination of things that follow from having true and precise numbers all the time.

Firstly, the Bio Feedback loop, (what the legs are feeling, what the head unit is showing and what the eyes are seeing) will enable the athlete to control their efforts more closely than just by using Perceived effort and power estimations.

This in turn will enable controlled breakthroughs in thresholds and improvements in maintaining them.  The coaches will be key here as they adapt from averages to true numbers.

The pedal stroke itself will come in for significant improvement as that same feedback loop uses the intra-pedal stroke accuracy to voluntarily improve individual muscle performance and train fine motor skills.   The biomechanics, the bike fitters, the physiologists will all be key here as they ensure that the rider and the machine are optimised for his/her event.

All that will need to have to happen for a quantum leap in cycling performance to occur is to listen to the riders – They want the right (true) number and they are getting the point!

Sources / References:

* Source: Boundless. “Basic Inferential Statistics.” Boundless Psychology. Boundless, 26 May. 2016. Retrieved 14 Sep. 2016 from https://www.boundless.com/psychology/textbooks/boundless-psychology-textbook/researching-psychology-2/analyzing-and-interpreting-data-30/basic-inferential-statistics-479-16744/

* Source: Boundless. “Accuracy vs. Precision.” Boundless Psychology. Boundless, 26 May. 2016. Retrieved 14 Sep. 2016 from https://www.boundless.com/psychology/textbooks/boundless-psychology-textbook/researching-psychology-2/analyzing-and-interpreting-data-30/accuracy-vs-precision-480-16745/

Measuring Power on the Bicycle

Measuring Power on the Bicycle

When our engineers were first contracted to build a dynamic calibration rig for a Sports Institute, they were very surprised to find that the so called “Gold Standard” of then current power meters had some serious flaws.

Most people at the time thought that the main problem with power meters at the time was the sheer expense of getting the information, but the dynamic calibration showed that this was only part of the issue.

At the same time, the average cyclist was moving to cheaper alternatives, mainly because the industry mantra (faithfully reported by the journalists and believed by many academics) was that all devices were accurate to +-2%. Over the last decade, this oft repeated accuracy idea became an accepted truth.

Now the market is being flooded by cheap power devices, each one more brilliant than the last apparently and of course much cheaper. All of them apparently accurate to +-2% except for two outliers. The ex-Gold Standard and the original usurper (Power Tap) both claim higher accuracy.

The problems with the +-2% or better assertion are many but in a nutshell revolve around some simple concepts;

  • +-2% of what when and how often?
  • If they are not measuring crank torque surely at best they are mathematically constrained estimators (see the embedded GIF where you can see the load path that needs to be measured for crank torque),
  • All of them are measuring something that is NOT the tangential force – the force that drives the bike forward as a result of the energy (watts) employed by the cyclist.

InfoCrank started it’s life as a dynamic measurement tool and still is employed in that mode by some users including our own R&D teams. For example, some components companies use the InfoCrank to measure precisely the wattage lost or gained as they strive to produce new products. Some medical people use the InfoCrank to measure the success of knee replacement treatments and are producing the medical papers based on that research. A prosthetic manufacturer uses the InfoCrank to determine the energy lost with different prosthetic sleeves.

What is most interesting is that the average cyclist just assumes that +-2% is good enough for cutting edge performance. (That is, except for the top sports institutes and the leading coaches of the world). The reason is very simple. They all assume that +-2% means that the reading on the computer screen of the bike is within 2% at all times. Seen that way, 2% either side of true is not so bad and whilst precision has to be better, why pay for it particularly when you can get “close enough- good enough” for a third of the price?

The problem with this is that there is no evidence to back up the assertion believed by both riders and journalists that the cheap devices are in fact accurate to 2%.

When the InfoCrank is used in it’s former role as a dynamic measuring device and tests the cheaper pedal or hub based power devices, the results bear no resemblance to the marketing.

For example, in a recent test on the open road, a pedal based power device ($200 USD cheaper than the InfoCrank package) that claims to be accurate to +-1.5% was measured second by second against an InfoCrank. Both devices were zero’ed, even though the InfoCrank does not require it.

The test* was done on a climb (real road conditions) as steady as a trained cylclist could be. The target power was 250w and the achieved watts over the entire 45minute climb was 249watts, attained by watching the second by second watts on the InfoCrank.

The cadence and the HR were also measured and the separate computers recorded them. There was virtually no discrepancy in the numbers of these measures on the different computers, thereby proving that any differences had to be in the torque measurements.

What were the results?

  • The pedal based device showed the correct number 6.5% of the time.
  • It was within product specifications 41% of the time.
  • It was therefore outside specification 59% of the time.
  • The errors were a maximum of 36% and minimum of -23%.
  • Basically the results were randomly strewn across the entire spectrum, meaning that even when the results were true, you would have no idea that they were true.
  • The devices algorithm seems to control the error by about +-37% – a number that shows up over longer rides.

Coaches often advise cyclists to “smooth” these wildly oscillating numbers by using multi-second smoothing on their computer. The assumption behind this advice is that the numbers are actually measuring the correct thing and it is the cyclist whose power fluctuations is making it difficult to read.

Smoothing random numbers, even when constrained by an algorithm, still produces a random result. We followed the advice in a random segment to see the results.

In a 50 second segment, the 3 second smoothing meant that there were NO true results at all. (Compared to 10% in the non-smoothed results) The results were evenly dispersed from +-9% making it less wrong than the average of +-15%. Remember, that with random results, there is virtually no relationship between any one number and the true number.

Our experience in the short time that InfoCrank has been available is that more and more athletes and their coaches are getting fed up with numbers that bear no relationship to effort (energy) and that a sea change is about to happen in the world of bicycle power measurement, just as the biggest crank manufacturer brings another “me-too” +-2% power meter to market.

*The test was actually over nearly four hours, but included different elements, which did not favour the pedal based device. The 45minute climb was the most favourable segment for the pedal based device. This test produced similar results as all other tests on hub based and pedal based power devices.

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