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Accuracy of Zwift power/speed

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Accuracy of Zwift power/speed

Old 11-28-16, 03:53 PM
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BigPoser
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Accuracy of Zwift power/speed

So I have this trainer, Elite Qubo Power Smart B+ Trainer, and have been using Zwift with it. I don't have an actual power meter, but it's coming. Just curious about the speed and power measurements that I see on Zwift. When going up the hills on Zwift it usually shows something like 4-5mph, but when doing something very similar on the road it would be more like 9-10mph. Hell it'll show 3mph once in a while which makes me laugh a little.

The same goes for power, since it's an estimated power curve that Zwift calculates for my specific trainer.

Just curious if others have noticed similar and wondering about it.

Thanks in advance.

Brandon
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Old 11-28-16, 05:57 PM
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Speed in Zwift is meaningless. It is easily overestimated by 3-5 mph. Same goes with any virtual power.
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Old 11-28-16, 06:24 PM
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For me, same SRM inside & out, riding on a Kickr, on flat "Ground" it's somewhere around 2mph. My power at 18 mph outside nets me about 20mph on Zwift. Rough estimate as it's not really a big deal.

Lots of tailwind on the island.
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Old 11-28-16, 06:28 PM
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Athens80
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Originally Posted by BigPoser View Post
...When going up the hills on Zwift it usually shows something like 4-5mph, but when doing something very similar on the road it would be more like 9-10mph...
Originally Posted by Xherion View Post
Speed in Zwift is meaningless. It is easily overestimated by 3-5 mph. Same goes with any virtual power.
Okay, Zwift shows 4-5mph, and Zwift overestimates by 3-5mph, therefore BigPoser is really doing -1 to 2 mph on those hills.
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Old 11-28-16, 06:40 PM
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Zwift shows the value of aero carbon. My Zwift bike is lighter, my Zwift 808s are more aero (and they never cause any handling problems) and my Zwift avatar rides more aero than I often do.

All the same, yeah, it does seem like I'm doing 1-2 mph faster in Zwift than I do IRL at the same effort on the flat. I put my Zwift rides in my training log, and I record mileage, but I discount the Zwift miles to be as low or lower than the same effort would have gotten me outside.

I zero out the climbing too. But I don't discount miles on Zwift climbing courses. I'll record the full miles heading up to the Watopia radio tower. Climbing seems slower in Zwift than IRL.
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Old 11-28-16, 06:45 PM
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One more - I forgot - Zwift has no stop signs, cars or bad roads to slow me down. That makes a difference over the course of a whole ride.
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Old 11-28-16, 07:06 PM
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Originally Posted by Athens80 View Post
Okay, Zwift shows 4-5mph, and Zwift overestimates by 3-5mph, therefore BigPoser is really doing -1 to 2 mph on those hills.
Exactly. He should be lucky that he doesn't roll down the hill backwards.
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Old 11-28-16, 07:12 PM
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As a reference, I did the Z-Wheeler challenge last week. 64.2 miles, 3:00:28 ride time, 1017 ft. 172 watts normalized power (reading from PowerTap P1 pedals), 169 watts average. My speed was 21.4 mph. There is no way in real life 172 NP/169 AVG would net anywhere close to 21.4mph, even on a pancake flat, zero headwind day on a road bike. I did the ride solo as well, with little/no drafting.

Last edited by Xherion; 11-28-16 at 07:20 PM.
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Old 11-28-16, 07:14 PM
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I find that the Z power is inflated by about 20% over my actual power meter.
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Old 11-28-16, 07:24 PM
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I don't know but i used switt today for the first time and averaging 30+ mph for a few minutes was hard but doable. Hills at 20mph too.

I've seen posts on strava that average 25+mph average for an hour plus and real rides are usually around 16-17. It could be an elevation thing but it seems like a big difference to me.
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Old 11-28-16, 08:30 PM
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Man you guys are fast! When it's flat on Zwift I'm cooking along at only 15-17 but am working much harder to maintain that, than I would be outside going 18-20. At least my legs are telling me that. Maybe it's the trainer making me work harder? That's fine as I got the trainer to get a good workout. It's just strange to me.
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Old 11-28-16, 08:50 PM
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Originally Posted by BigPoser View Post
Man you guys are fast! When it's flat on Zwift I'm cooking along at only 15-17 but am working much harder to maintain that, than I would be outside going 18-20. At least my legs are telling me that. Maybe it's the trainer making me work harder? That's fine as I got the trainer to get a good workout. It's just strange to me.
Well, On my trainer with my quarq power meter, which is very similar to my other quarq, which is very similar power to my power tap, if I hold 250w for an hour it means 19mph average. If I do that on the road, which around here is about 60ft of climbing per mile, I should be averaging 21-22mph. So my speed on the trainer is less. However I felt like Swift may have been optimistic. That being said, I was using a Kicker power trainer (the $1,000+) and it seems like they don't use an actual power meter but rather an algorithm of sorts? I really need to dig into that a bit further.

I found that going over 1,000 watts on the Kicker is nearly impossible. I can do that on the road bike on demand.

And yes, riding indoors is harder than outdoors. The same effort feels harder.
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Old 11-29-16, 09:23 AM
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I don't look at the speed in Zwift.

I did the FTP test to set my FTP then I do the workouts based on that number. If it's wrong it's wrong, but it's consistently wrong so I'm still getting the same benefit out of it that I would if it were correct.
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Old 11-29-16, 09:53 AM
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if its a dumb trainer its about the power curve model, and also its variance to your actual trainer

if its a smart trainer, i think theyre often known to vary non-insignificantly from power measured at crank/chains/hubs

further, relating power vs speed on flats, it depends on the aerodynamic model compared to how aerodynamic you are in the real life

and lastly, on hills, i think by default zwift uses some sort of hill-assist / 50% difficulty type thing to lower work needed to get up the hills

so yeah lots of ways to go wrong. tho overall speaking, with my smart trainer, i found the overall speed per perceived effort pretty good approximation. wont have real life power meter until later to compare. but in the end, its all about training and work done, isnt it...not speed simulation
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Old 11-29-16, 11:07 AM
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Originally Posted by redfooj View Post
but in the end, its all about training and work done, isnt it...not speed simulation

Exactly! I'm not too worried about it really, just curious since I'm new to virtual training.
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