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Power Meter Accuracy – the Zwiftpower Resource – Watts Occurring Information

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A dual power recording

The objective of this article is to bring to your attention a resource that sits within Zwiftpower. It is the analysis section.

For those of you who want to settle an argument at the IRL or virtual cycling cafe, you can spend hours searching on Zwiftpower. Here you can find the dual power recordings of the top racers and see how different smart trainers are recording power compared to their power meter cousins.

As you may know, in the top level races, Zwifters are required to record their power from a separate power meter. E.g. An Elite Direto XR connected to Zwift and a pair of Garmin Vector pedals connected to a Garmin head unit. In Zwiftpower, you can see these recording and therefore compare on power meter/smart trainer with another.

However, it isn’t that easy. Even through Zwiftpower is created on a database platform, Zwift have not opened up the tools where we can search for say a Tax Flux 2 S vs a Quarq power meter (if a rider has recorded dual power with those two models). However, what we can do is to search in the rankings section…

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…and then click on the analysis tab for each rider…

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Click on the analysis tab to see the dual analysis

Here we have looked at a Wattbike Atom against a power meter. The reason? Well, on Twitter one esports team owner levelled a claim against this smart bike. Here is the data from a rider on Zwiftpower using the Atom:

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This is the side by side watts comparison
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This compares the two with percentage difference

What can we see. In this case, the power meter is based at the pedals. We know that loses at the drive-train equates to pedal recorded power will be higher than at the ‘wheel’ in reality. In this case, the rider either has a drive-chain that is seriously in need or replacement, or calibration of the power meter needs carrying out (the Wattbike does not need calibration). There is an obvious comment, in this case, that both devices may be off and triple recording is needed? NB It is easy to mis-calibrate Favero Assioma pedals by entering the incorrect crank length in the settings of the device.

For comparison, here is the dual recorded data from another rider:

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Two different devices dual-recording

Here we have a rider using an Elite Direto XR and a Quarq power meter. In this case, the smart trainer is reading higher and considering what we’ve said above regarding drive-train losses, it seems that calibration needs some further attention. Please remember, a good twenty minutes of solid riding is needed before calibrating a smart trainer.. Calibrating straight after powering-up the device will mean over-reading. We are not saying that this is happening here. The percentage differences are illustrated below:

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Bearing in mind that most power meters and smart trainers are accurate within a tolerance of +/- 2% (1 and 1.5% in this case)

Currently I am personally using an Elite Direto XR and Garmin Vector pedals. The readings in my case track closely.

As may have read here before, I’m not a fan of comparisons. It’s like the dog show world. Everyone has the most beautiful dog until they enter it for a show and then every dog owner from second down thinks that the judge is an idiot, or the event is fixed! Or, if the dog keeps losing, they buy another and another until one wins! It is human nature. What makes it worse in the world of power meters is what follows.

In the world of reliable measurements, there is usually an internationally recognised pound, or kilogram or whatever. Let’s call it the Delta. A watt, as we know, is a calculation and it is down to the manufacturer to ensure that what they produce and measure is within their accuracy claims. To the best of our knowledge, there is currently no internationally recognised machine that measures a watt (or multiples of) to the internationally recognised measured watt. We stand to be corrected.

What this means in practice is that each manufacturer makes a device and tests it’s calibration to their own machinery. As you will know, in a manufacturing process, there are so many variables, from the materials used, the replication of build, the replication of testing and the reliability, maintenance and testing of the test equipment. Then we have the owners’ manual conundrum. How many read it, digest it and apply it? How many calibrate and re-calibrate (in the detail and context of the devices that they have) correctly? There is good news. From the very same top rankings list is this data:

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The power number of another top Zwifter eSports racer
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The same data in % terms

And if you want triple recording:

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Power numbers and three sources
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% tracking of the three devices

After reading this, you may stray into that rabbit hole that is smart meter and power meter number comparison and we wish you ever success. Please remember, there is no delta – no solid reliable ‘this is the one to compare everything else to.’ Some will say that have it. Behind every product are solid reliable people called engineers. There are also marketing and sales teams who are rewarded on results and who are fighting to the survival or a company and all that work there. Sometimes they may get carried away with claims. We therefore recommend the following reading whenever you come across data https://timharford.com/books/worldaddup/

If you have never come across Zwiftpower, here is a link https://www.zwiftpower.com/rankings.php

Zwift themselves have a list of approved smart trainers and devices for eSports eracing – this in itself provides you with all you need to know about the best choices of trainer to provide you with relavitely accurate data for training and racing. After all, how can you progress in your training if you are never sure that your power meter is reporting your power numbers accurately? To me, it’s just a waste of money buying something with toy-town numbers! You can find the Zwift list behind this link https://support.zwift.com/en_us/direct-drive-trainers-B1oH2meS

Lastly, I see a lot of rot written on forums about ‘what is the best trainer?’ It’s usually what someone else owns. Someone who may not have ridden that many. Beware of biases (we all have them and they are many). For me, take the Zwift list seriously and then buy the smart trainer with the largest flywheel!! I’m serious! The flywheel makes a huge difference. It’s the difference between fighting all the way around on your pedal-stroke and having that feeling when you are out on the open road, with good tarmac and you can get over a gear and fly along. Small fly-wheel (or virtual fly-wheel) smart trainers can feel like the dead road surfaces that we have here in the UK.

You don’t do Zwift! Not an issue: Dual Power Comparison Tool is something that was developed by the team behind RGTDb and many thanks to the Mighty Mike Lister for the heads up. This is where you can upload, say, your Strava record of a RGT activity (a recording of your smart meter data) and your Garmin activity (recording a power meter) and compare how accurately that they track each other.

And remember – wear a heart rate monitor – no-one can replace or recalibrate a heart. It’s your reliable source.

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