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Zwift Exaggerated Numbers

Old 06-25-21, 06:58 AM
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gthomson
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Zwift Exaggerated Numbers

I've talked with a few people and read a number of posts on Zwift Power and how it calculates power and speed and the accuracy compared to riding for real. In some cases I've read there can be up to a 15% difference. I have an Elite wheel on trainer and no power meter so I rely on Zwift power to calculate my numbers.

When I ride outside, I have no power meter or bike computer but ride with my phone and run Strava to record my numbers.

The difference between what Zwift says I'm doing and what Strava records is way off. My Zwift numbers are way higher than my real life numbers. Does anyone else notice this? Am I just getting a very inaccurate reading because of my equipment (or lack there of)?

I know there's a big difference between riding all out on Zwift with no cars, no stop signs, lights or people to worry about compared to outside riding but am i really going that much slower on the road?
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Old 06-25-21, 07:04 AM
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I have no personal experience with Zwift, but don't you have to input personal details like rider weight? THat could be a hilarious way to cheat... an average 190 lb rider tells the system they weigh 32 lbs, then the power output from a 190lb rider makes them into a TdF-Z champion.
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Old 06-25-21, 08:48 AM
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Originally Posted by ClydeClydeson View Post
I have no personal experience with Zwift, but don't you have to input personal details like rider weight? THat could be a hilarious way to cheat... an average 190 lb rider tells the system they weigh 32 lbs, then the power output from a 190lb rider makes them into a TdF-Z champion.
yes, you have to set up your profile with age, height and weight and sex which is used to determine how it calculates power and speed. Can you cheat? sure you can but this gets monitored for competitive races to prevent this.

Keeping in mind, it's a tool to help training for triathlons or races so cheating only hurts yourself
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Old 06-25-21, 10:16 AM
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they're both "guesstimate" algorithms, strava and zwift with no direct measurement, so that kind of difference doesn't seem surprising.
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Old 06-25-21, 10:59 AM
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Without a power meter, you may as well just make up a number.
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Old 06-25-21, 01:17 PM
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what are you comparing exactly? Zwift estimated power compared to Strava estimated power? why would either be correct?
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Old 06-25-21, 04:51 PM
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Originally Posted by gthomson View Post
but am i really going that much slower on the road?
Well, only your outside riding speed is real.

On Zwift they are not only estimating your power, but also your whole system CdA and rolling resistance. Zwift doesn't even try to make the bike frame and wheels reflect reality, they just pick some numbers for weight, aero, Crr. Then everybody with the same height and weight gets the same CdA. Can you replicate that outside with your position, clothes, and body shape being what Zwift uses?
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Old 06-26-21, 07:43 AM
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Originally Posted by gthomson View Post
I've talked with a few people and read a number of posts on Zwift Power and how it calculates power and speed and the accuracy compared to riding for real. In some cases I've read there can be up to a 15% difference. I have an Elite wheel on trainer and no power meter so I rely on Zwift power to calculate my numbers.

When I ride outside, I have no power meter or bike computer but ride with my phone and run Strava to record my numbers.

The difference between what Zwift says I'm doing and what Strava records is way off. My Zwift numbers are way higher than my real life numbers. Does anyone else notice this? Am I just getting a very inaccurate reading because of my equipment (or lack there of)?
As far as zPower estimation, I've seen people for whom their zPower numbers were off by 400% from the numbers they had when they finally got a power source. I have a powermeter on the road and a Kickr Core trainer for power on Zwift - for me it's not far off. The Zwift speeds get a little high if you're racing in draft events or riding with the pace partners a lot, but when I go on really good pavement my solo speeds aren't that far off between Zwift and real life for similar power.

The Strava estimates aren't great for power - but they're probably closer to reality, especially for uphill segments, than zPower estimate for most trainers.
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Old 06-26-21, 06:45 PM
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Originally Posted by spelger View Post
what are you comparing exactly? Zwift estimated power compared to Strava estimated power? why would either be correct?
Well why would they calculate those numbers if they had zero accuracy? In Zwift you can take your FTP test and presume that there was some validity to it?
I'm not disputing the question, but wondering why bother generating any data at all if nothing is real?
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Old 06-26-21, 08:31 PM
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Originally Posted by gthomson View Post
Well why would they calculate those numbers if they had zero accuracy? In Zwift you can take your FTP test and presume that there was some validity to it?
I'm not disputing the question, but wondering why bother generating any data at all if nothing is real?
Without a power estimate there is no way to simulate the speed of your avatar. Without an FTP figure there's no binning your race category nor ability to properly use zones for workouts. That it can be inaccurate due to deficiencies in your training equipment is not a problem that Zwift can solve (nor should they care to when there are hardware solutions), and at least no one has ever reported that it's not directionally correct, meaning the estimate goes up if you exert more effort and down when you exert less.
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Old 06-27-21, 11:10 AM
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Originally Posted by gthomson View Post
I've talked with a few people and read a number of posts on Zwift Power and how it calculates power and speed and the accuracy compared to riding for real. In some cases I've read there can be up to a 15% difference. I have an Elite wheel on trainer and no power meter so I rely on Zwift power to calculate my numbers.

When I ride outside, I have no power meter or bike computer but ride with my phone and run Strava to record my numbers.

The difference between what Zwift says I'm doing and what Strava records is way off. My Zwift numbers are way higher than my real life numbers. Does anyone else notice this? Am I just getting a very inaccurate reading because of my equipment (or lack there of)?

I know there's a big difference between riding all out on Zwift with no cars, no stop signs, lights or people to worry about compared to outside riding but am i really going that much slower on the road?
Conditions on Zwift are always pretty much perfect. Granted it rains
every now and then but the rain doesn't seem to have any effect on speed.
It's windless which I believe is ideal for average speed. A tailwind
never helps as much as a headwind hurts

At least on the Zwift routes I usually ride there are plenty of other riders
contributing to a substantial draft effect.

Before drawing any conclusions about whether Zwift speeds are unrealistic
I would try to make it as much as an apples to apples comparison as possible.

I'd use a TT bike both on Zwift and IRL. The former because TT bikes don't
partake of the draft effect, the latter just trying to duplicate conditions.
I'd try to ride flats for both trials or at least try to ensure the ascent/mile
was really close for both. For the outdoor trial you'd need a windless day
and no other riders on the course. For the Zwift trial I'd use a TT bike
hooked up to the trainer and try to hold my best aero position as I would
IRL. Of course both courses should be of identical mileage.

Under those conditions assuming identical power output on both trials, I'd bet your average speed on Zwift and IRL would be
really close. The biggest variable would be whether your position IRL would
have more or less drag then the position modeled in Zwift.

Last edited by roadie77; 06-27-21 at 11:11 AM. Reason: Left something out
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Old 06-27-21, 04:42 PM
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Originally Posted by gthomson View Post
Well why would they calculate those numbers if they had zero accuracy? In Zwift you can take your FTP test and presume that there was some validity to it?
I'm not disputing the question, but wondering why bother generating any data at all if nothing is real?
i think we can all agree that neither would be correct. but my point really was why bother to compare two completely different algorithms. from Strava:

Strava can estimates power using information about an athlete's weight, speed, and elevation change.
https://support.strava.com/hc/en-us/...for-Your-Rides

strava does not take road conditions into account.

Zwift may actually be more accurate since it is based on particular trainers:
https://zwiftinsider.com/virtual-power/

neither takes wind or temperature into account (guess zwift needn't do so). i don't know how temperature would affect this but i seem to recall others here on BF making that claim.

i do think it is fair to compare one vs a real ride though.

i'd love to know what my power numbers are in real life but a meter is not in the budget for me. i still enjoy my rides regardless.
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Old 06-28-21, 04:37 AM
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Although I can't provide anything technical 300w on the trainer feels harder than 300w in real life.
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Old 06-28-21, 12:24 PM
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Originally Posted by IntraVol View Post
Although I can't provide anything technical 300w on the trainer feels harder than 300w in real life.
It's weird. I can do VO2max and anaerobic capacity work on the trainer and hit my targets pretty consistently, but I really struggle with sweet spot and threshold intervals inside. I wonder how much is psychological. The intervals are long enough to start thinking about how long they are....
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Old 06-28-21, 01:20 PM
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Pretty sure at least some part is psychological. I seem to be able to hold power better while participating in an event or race, than when just riding around or doing intervals.
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Old 06-28-21, 01:34 PM
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Originally Posted by atwl77 View Post
Pretty sure at least some part is psychological. I seem to be able to hold power better while participating in an event or race, than when just riding around or doing intervals.
Same with me. I think it's motivation not to get dropped and/or to get in that peloton to make life a bit easier. I've done a few CRIT city races were I was going all out and achieved a new FTP and a time trial where I was cooking it (for me at least).

Great workout tool and worth the money in my opinion. For the monthly subscription fee I can get in as many workouts as I want, whenever I want.
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Old 06-28-21, 03:54 PM
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Originally Posted by spelger View Post
i think we can all agree that neither would be correct. but my point really was why bother to compare two completely different algorithms. from Strava:

Strava can estimates power using information about an athlete's weight, speed, and elevation change.
https://support.strava.com/hc/en-us/...for-Your-Rides

strava does not take road conditions into account.

Zwift may actually be more accurate since it is based on particular trainers:
https://zwiftinsider.com/virtual-power/

neither takes wind or temperature into account (guess zwift needn't do so). i don't know how temperature would affect this but i seem to recall others here on BF making that claim.

i do think it is fair to compare one vs a real ride though.

i'd love to know what my power numbers are in real life but a meter is not in the budget for me. i still enjoy my rides regardless.
It depends on the trainer. If you've a high quality fluid trainer that's been mapped by Zwift, your numbers are pretty close. Other trainers vary wildly from individual trainers of the same model, and even worse, many people ride on unsupported trainers as "Generic", including on spin bikes where they can turn the resistance to zero..... I've seen riders with heart rates below my easy ride sustain 400W for an hour, when their average outdoor riding pace (at higher HR) is slightly faster than my easy RUNNING pace....... and then they got a smart trainer with a power meter, and their sustained power was closer to 100W than 400W.

Our online race team kind of insisted they check their setup out after an ex-pro couldn't get past them to do a pull during a team time trial.

That being said - if that's what you've got and can afford, go for it. There aren't many races that include zPower estimated power riders in the results, but there are some - and free-ride and group ride to your heart's content.
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Old 06-30-21, 06:00 PM
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I can throw an n=1 data point in here.

When I started on Zwift I had an SRM that was a wired version, i.e. not ANT+. So I had to wait until Zwift had my fluid trainer in their database. Once that happened I started riding, using calculated Zwift power (aka zPower).

I noticed right away that my SRM powermeter read much higher than what Zwift was crediting me for. For example, in this picture, my phone/Zwift shows 104w but my SRM shows 155w:
https://2.bp.blogspot.com/-XSnnFLgK09...0/DSC_0744.JPG


This was with a CycleOps Fluid2 trainer. The tire slipped at higher wattages so I had to really tighten the roller, increasing drag significantly. If I left it looser so it'd slip over, say, 800w, then the numbers would be more accurate. But if I wanted to do big efforts, Zwift was not accurate. Zwift can't accommodate a higher roller tension - there's no coast down or other calibration for "dumb" trainers.

I replaced the Fluid2 (it leaked - the 3rd one for me over 20 years) with a Kurt Kinetic Road Machine, another dumb trainer. It's known to be more durable and more accurate. With the roller relatively cranked down it was much closer, within 20-30w at that 150w range. zPower was still shorting me, just not as much.

I finally upgraded the SRM to wireless (aka ANT+) so now my Zwift data pulls off my SRM powermeter. When I went to wireless I got a nice little bump in power on Zwift because I was finally credited with the power I was putting down.

So that's zPower vs SRM, at least on a trainer with the roller at max pressure.

Strava, on the other hand, can only roughly guess your power. For example, it doesn't know if you're drafting, or if the wind is favorable. It just knows your ground speed, your weight, road gradient, and based on that it guesses your power. On Strava, in races, I appear to be an absolute monster, going 27 mph or whatever, averaging 400w. What Strava doesn't know is that I was doing 27 mph while averaging 175w sitting in a pack of riders.

On long, steeper hills, Strava is actually pretty accurate. It's because the wind suddenly becomes less significant at low speeds, steady power means it isn't trying to calculate acceleration power numbers, and the grade makes power output significant (so it's closer in its calculations). A slow, steady drag up a hill should give you pretty good idea of your power range. I won't say that it's accurate to, say, 10w, but if it says 200w, you're in that range, not the 100w or 300w range.
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Old 08-03-21, 12:02 AM
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Zwift uses your power out to determine your speed. It also assumes you have the optimal aerodynamics and smooth roads. At the same power output, Zwift would net me at least 2+ mph faster (~10%). The bonus seems to be more at generous at lower power. One thing that I haven't been able to reproduced outdoors is keeping the pedals spinning. The comparison is for shorter durations at around 15-20 minutes on a flat run. I wish I can climb as well in real life as I did in Zwift (switched to RGT for better road feel and customized routes).

I have a gen 1 Elite Drivo and it's on par with my Pioneer dual (verified against my Drivo) and Assioma dual (verified against my Pioneer but been to lazy to check against my Drivo).
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Old 08-03-21, 04:56 PM
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Originally Posted by spelger View Post
i think we can all agree that neither would be correct.
^This.

Both are "guestimates" rather than based on actual power data. With your setup, Zwift is basing it's power reading on the speed of your rear wheel (and some calculations it knows about trainer power curves). It can be anywhere from pretty close to a fair way off. It can also vary depending on tyre pressure, roller tightness, trainer age and wear, etc. It's a means t oget you riding on Zwift, but it's not real power data.

Strava bases it's power estimates on, ummm, I'm actually not sure. They can also be anywhere from close enough to wildly out. I find uphill segments are actually the closest as I guess it takes out the effect tof wind, drafting, traffic, etc.

But at the end of the day, if you want real power data, but a real powermeter.



Oh and as for your average speed in Zwift, their algorithm is very optimistic. It gives you a very good CdA, plus there's no stopping at lights, slowing for corners, etc.
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Old 08-13-21, 01:09 PM
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Originally Posted by gthomson View Post
I've talked with a few people and read a number of posts on Zwift Power and how it calculates power and speed and the accuracy compared to riding for real. In some cases I've read there can be up to a 15% difference. I have an Elite wheel on trainer and no power meter so I rely on Zwift power to calculate my numbers.

When I ride outside, I have no power meter or bike computer but ride with my phone and run Strava to record my numbers.

The difference between what Zwift says I'm doing and what Strava records is way off. My Zwift numbers are way higher than my real life numbers. Does anyone else notice this? Am I just getting a very inaccurate reading because of my equipment (or lack there of)?

I know there's a big difference between riding all out on Zwift with no cars, no stop signs, lights or people to worry about compared to outside riding but am i really going that much slower on the road?
Either get an accurate power meter, or just keep guessing. When I ride indoors I'm on my road bike on a Neo 2T, so I know it's very accurate. And my indoor power numbers are higher than the Strava estimates. Which is good.
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Old 08-15-21, 08:23 PM
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I just put a stages power meter on my road bike and I'm running around 20% less in the real world than I expected based on my tacx flow + Zwift. It was a bit humbling.
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Old 08-16-21, 02:53 PM
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Originally Posted by Canker View Post
I just put a stages power meter on my road bike and I'm running around 20% less in the real world than I expected based on my tacx flow + Zwift. It was a bit humbling.
So, most of my outdoor rides have lower avg power, mostly because I coast a lot more, brake for stop signs, corners, etc. - lots more zeros on the road compared to Zwift. For a given climb or sustained effort it seems pretty close though (using Kickr Core indoors, PowerTap hub outdoors)

With the stages - have you tried putting the bike with the Stages on the trainer and dual-recording the same ride? https://zwiftinsider.com/zwiftpower-dual-recording/
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Old 08-19-21, 03:24 PM
  #24  
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My saris H3 it's very close to real life when going up hill not drafting anyone etc.... say 10% grade over 5 miles = x mount of time etc... steep grade is mostly power to ratio. flats & down hill is more aero
and rolling resistance.

Down hill is a joke. zwift must give you unlimited gears. How on earth I can pedal at 50+ mph and also do 50rpm + on 15% grade ? That must a rear cassette 5 to 50.

On the flats zwift must assume the most perfect smooth road, zero wind, best bike, game must also assume I am in the best possible aero postion etc.. I am good 2 to 3 mph if not more in zwift vs real life then of course there's so many people you can draft in zwift unlike real life. At home you could be riding on the hoods are even no hands with very bad aero position while on the flats doing 25+ mph.
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Old 08-19-21, 05:08 PM
  #25  
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Zwift halves downhill grades even before adjusting further for lower trainer difficulty setting.
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