Power difference between Zwift and outdoor bike power meter

General Discussion
1

I’m curious about the difference between Zwift wattage and wattage readings on my outdoor bike. Outside I use a Specialized crank arm power meter. I see significantly higher watts outside. Is my road bike power meter flattering me?

2

hi @Gerry_Boyle

Welcome to the forum.

I would suggest putting the road bike on the trainer and compare the power using the tool in Zwiftpower.

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3

Thanks. I didn’t know I could switch to bike power meter. Will try that. Okay

4

Which trainer do you use?

Your power meter is probably more accurate than your trainer.

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5

you would record your bike power with the head unit and trainer power with Zwift. Then compare the two files using Zwiftpower.

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6

Tacx Flux. Works fine but doing intervals, for example, maintaining high end of required power is way harder than sustaining same wattage outside. Road bike restores my self-esteem!

7

Perfect solution. Thanks for the tip.

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8

I did some test on this for fun. My bike on power meter I did a hold of 225 watts, a number I could pedal consistently for a time. My trainer a Kurt was when calibrated was actually really close BUT was always higher than my bike on meter. Nothing really crazy higher but sometimes 20 watts higher. My take or advise I guess would be no matter what you use, make sure you compare the same devices always. I trust my bike on meter the most.

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I once did a comparison of Wahoo Kickr Bike with Favero Assioma Duo power pedals, installed on Kickr Bike. I did not find any significant difference. I was recording power from the trainer in Zwift and power from the pedals on my bike computer, then uploaded both to WKO5 software. However, one should note that most people have some degree of asymmetry between power which they can generate with left and right leg (part of it is pure power generation, but another part is resistance of the opposite leg), which can be 5% or more.
Another factor is how your power is averaged. Instant power may have significant spikes, 3 seconds averaging removes the spikes and calculates averages.
You probably can use your bike’s power meter in Zwift. If you ride the same route with power from your pedal meter, and then with power feed from your trainer, you will be in a better position to compare. Or do a dual recording as I described above.

10

Unless I’m doing a zwift race that requires me to use the trainer as the power source, I always use my crank based PM as the source for my power. It keeps the data more consistent when I’m switching between riding inside and outside.

The extra wattage outside could be a number of factors. Everything from calibration of your PM, mental factors that Zwift is actually harder than riding IRL, not having the motion of the bike and road underneath of you, etc. I’m sure the more you ride on zwift, the more those numbers will close the gap.

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Thankss, Jonathan. I do keep a bike on my trainer permanently. I think the motion may be a factor, staying in the saddle more on the trainer. May be, as you say, that just keeping cranking on the trainer and trying to get those numbers up, is the answer. Thanks.

12

Just curious, is it the same bike that you ride outdoors? Or is it a different make and model of bike which you permanently attached to your trainer? If so, small difference in bike efficiency and fit could account for differences.

13

Thanks for your question. I keep a Specialized Allez on my trainer. I ride a Specialized Roubaix expert outside. Roubaix is ultegra. Allez is 105. (My first road bike). Both are 54 cm. One difference may be that I tend to stay seated on the trainer because of the way the bike is locked in. Then again, I tend to stay seated when climbing outside anyway and find I can generate more power in that position. Still considering the question!

14

I think many of us produce higher sustained power outdoors, because we become limited by indoor heat stress, through a mix of insufficient power fans and not doing enough frequent z4+ indoors to acclimitise to the heat stress.

After a typical turbo break in the summer, it takes me approx 2-3 weeks for my turbo heart rate for the same ballpark z4+ power to drop to the outdoor equivalent.