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Zwift, Tacx or Strava: who is lying?

by melliott
Wed, Jul 24, 2019 22:08
4

Well, not quite.

I am a big lad, 6ft 2 (1.88m), ~90kg and have been cycling for exercise and sport for about 4 years. Ex-rugby and football at a decent standard.

Decided to take my cycling a little more seriously with a view to look at racing and TTs, so I bought a turbo about a year ago. Having not used one before, I went for a Tacx Flow because it was cheap and could hook up to Zwift and RGT.

Made steady progress with my fitness and my FTP has risen from 280W to 378W. I definitely feel stronger and fitter, and this has rolled over into my on road performance where I'm getting close to some of the quick local lads' times and often averaging 3kph faster on rides than a year ago.

I've done some all out blasts, but Strava consistently rates my power output much lower, e.g. in the 180-200W region. Had a full on blast last night, heart rate in z3/4/5a region, fastest ever 60km and Strava says 182W average. I pedalled pretty consistently throughout with little coasting.

So where is the discrepancy?

It's my Tacx massively inaccurate?
Are Zwift's power curves for my turbo well out and I'm nowhere near 378W?
Is Strava really stingy?
Is the difference an artefact of cycling indoors versus outdoors?
Something else?

Any ideas greatly appreciated.

If you're new please join in and if you have questions pop them below and the forum regulars will answer as best we can.

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4 comments

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CXR94Di2 | 5 years ago
1 like

Tacx Flow, rubbish for power reading. Get a higher grade model or a set of power pedals and use these on both Zwift and outdoor. You will get accurate comparisons.

If you filter your longer Strava segments with weight, then if you're regularly in the top 10% , you are pretty quick.

I weigh 200+ and are satisfied with Strava top 15% on 1 mile hills. I dont really bother with short segments, 5-90 min segments give a better indication. But Strava is also full of club runs results, where a fast pack can be massively faster than an individual. Weather/windspeed /direction have a big influence too

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EddyBerckx | 5 years ago
2 likes

To get (reasonably) accurate power numbers inside and out you need a power meter on your bike -  nothing strava estimates is particularly accurate. It doesn't know the wind speed/direction/temperature/road condition/every little hill/your cda and so on. Your weight needs to be accurately set also I believe for it to even begin to estimate properly, and even that will change throughout the day.

 

While you can't trust the accuracy of your turbo too much, I reckon it'll give you a decent ballpark figure tbh. Those numbers are pretty good

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fukawitribe | 5 years ago
2 likes

Also...

  • Yes the Flow is not very accurate - it's a good value way into trainers with feedback but take the figures with a bag or two of salt.
  • It's not Zwift that calculates the power - that's just broadcast from the trainer brake unit.
  • Strava is not stingy as such, it's just that it's only a rough estimate based on insufficient data and it seems to get worse as the variation in the course increased.
  • Outdoor vs indoor power will almost certainly make a difference - typically the actual (accurate) outdoor power figures are higher than indoors for a number of reasons, Google is your friend there

Bottom line though is the training is having a good effect and sounds like you're enjoying the benefits. Don't sweat the numbers - although if you want an estimate then Strava on a time-trail effort (similar to what you posted) with little wind or elevation variation would be not a million miles off, or for outside, the classic 'ride up a hill, time it, weigh everything then calculate average power'  is a decent measure.

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Xenophon2 | 5 years ago
0 likes

Read this:

 

https://www.cyclinganalytics.com/blog/2018/06/how-does-your-cycling-powe...

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