Somethings just not right!

(Gary Edge (Team Lard)) #1

I’m using a classic setup, + HRM, Speed and Cadence sensors plus my Garmin 810 as a comparison. Started my first session with my usual resistance set and everyone’s flying by me and I’m going up hills at about 4mph! Dropped my resistance setting and suddenly I’m going at 10+mph and staying with groups and other individual riders. However, my Garmin says I’m doing 17+ mph which certainly isn’t representative of a climb.

My other query relates to distance. At the end of my ride Zwift says I’ve covered 10 miles whilst my Garmin says its nearer 12. I take it the Garmin uses the size of the wheel that’s set (700c x 23) x by the number of revolutions to establish the distance (GPS obviously wont work)

The differential between the 2 systems is quite substantial so which one do I trust???

My question is…can the system be anything like accurate without using a Smart trainer?


(Paul Allen) #2

Zwift is based on your watts per kilogram more or less. So if your rear tire is spinning at 20mph and you are going up a hill in Zwift, Zwift will show you going much slower maybe even 5mph. The distance difference would also be explained in a similar way.

Just because your rear wheel is spinning at 17mph does not mean you will be traveling that speed all the time in Zwift. You could go slower up hills and faster coming down. 

(Gary Edge (Team Lard)) #3

Understand the logic but it just seems wrong that by dropping my resistance I can effectively trick the system that i’m going so much quicker.

Think i’ll stick with my Garmin and the data that provides.



(Noel Nunkovich *USMES*) #4


Changing your resistance setting “tricks” Zwift because you’re changing the parameters of the equation. With your setup, what you’ve essentially done is told Zwift “My trainer uses this certain power curve.” Based on that curve, Zwift has to make some assumptions, i.e. the curve tells Zwift that your trainer requires, say, 200 watts to spin the rear wheel at 20 mph. When you change your resistance setting you change that power curve but you haven’t told Zwift you did so. I believe that most of the trainers Zwift supports that have resistance settings are supposed to be set at a specific setting and left there for best data accuracy.

Also, for whatever it’s worth, your Garmin’s speed and distance data is not really any more accurate (or inaccurate) than Zwift’s in this regard. The mileage or speed from your Garmin is not realistic either if you turn your resistance to the easiest setting and spin along in your 50/11 gear at 26 miles an hour without breaking a sweat. Or, put another way, if you do a 60 minute ride with your Garmin at your easiest resistance and average 23 miles an hour, then do another 60 minute workout at your highest resistance setting and only do 10 miles, which one was the more accurate?

Ride on!

(Gary Edge (Team Lard)) #5


I’m using an unbranded non-smart turbo…how can Zwift set any type of power curve?

As for the Garmin’s accuracy,  I know that I usually average between 16-17mph on the road, and by using the 3rd resistance setting it gives me something like the same sort of average if I did a straight hours ride with a reasonable effort on the turbo. (Ok its not 100% accurate but its not far off)

Don’t get me wrong i’m not really arguing for or against Zwift just trying to figure out if to get real benefits you have to use a Smart trainer with this type of product.


(Noel Nunkovich *USMES*) #6


If you’re using an unbranded trainer that Zwift doesn’t specifically recogize, it’s kinda “all bets are off” in terms of accuracy. If you are using a classic/zPower setup you select the brand and model of trainer you have so that Zwift uses the correct power curve. If your trainer isn’t listed it’s all guesswork.

Smart trainers are mostly a good thing in my opinion. On the simpler side, they make “just riding” on Zwift more entertaining, engaging, and realistic. On the more complex side, they can greatly improve the quality of your training sessions if your doing intervals and the like. That’s true whether you are using it with Zwift or not. The downside is it requires software or something to drive it and the complications increase.



(Gary Edge (Team Lard)) #7

Thanks all for the comments and suggestions


(Alex Bojanic) #8

I just joined zwift and have basic “dumb” trainer with wahoo cadance speed sensor. While i understand comment about watts and smart trainer being a right thing, why if i have dumb trainer zwift at least shows correct cadance and apeed and ignora watts? Otherwise why i need ant+ sensors for speed and casance if they are ignored?

(Alex Bojanic) #9

Btw while speed is way off 7mp higher, cadance is spot on??

(Noel Nunkovich *USMES*) #10


I’m not entirely sure I understand your question, so pardon me if this doesn’t answer what you’re asking.

When using a “basic dumb trainer” and a speed or speed/cadence sensor, you are telling Zwift to derive your power output with zPower. What this means is that Zwift uses a predefined power curve for the trainer you selected in the setup. That power curve tells the software “this trainer requires X watts to go Y speed.” In other words, the trainer manufacturer or someone else has tested the trainer and provided a table of values that relate a given wheel speed to a certain power requirement. Zwift then uses that table in conjunction with your speed sensor to calculate your power output. The software sees a wheel speed of, for example, 20 miles an hour, looks at the power curve table, and sees that for this particular trainer that speed corresponds to an output power, for example maybe 200 watts. That derived power number is then plugged into the game’s physics engine and, along with the rider weight you entered, the type of bike and wheels you selected, the climb gradient of whatever section of course you’re on, etc is used to calculate your in-game speed. Because of the physics engine, actual wheel speed from your sensor and game speed are almost never the same thing.

Think about it this way: if you were putting out a steady 200 watts outdoors on a flat road with no wind, what would your speed be? If you were putting out that same steady 200 watts on that same flat road, only this time with a 15 mph headwind, would your speed be the same? How about that same 200 watts on a steady 6% climb? In the real world, these kinds of variables affect your speed on the road, so they affect your wheel speed. In Zwift, these variables (wind speed, grade, etc) only exist in the game, so your speed is only affected in the game.

Hope this helps.


(Alex Bojanic) #11

Thanks for your prompt and detailed response. I did understand from prevous posts process to calculate speed based on watts derived from the trainer. However, that is based on assumption thst zwift has information about the trainer being used. In my case it doesn’t, at least it is not listed. hence zwift does not know predefined power curve.
If this is a case would be better to have watts as N/A and display at least correct speed ( cadence is already correct).
It would just make all experience better. Instead now I am having watts totaly wrong (in my case with “unrecognized” trainer I am “averaging” 400+ watts :smiley: ) plus speed is also completely wrong too. I would be satisfied if at least i got speed right on a dumb trainer. It just doesnt make sense for me to buy new trainer of $700+ when i have existing one. And i just started using zwift few days ago.

I was hoping my initial experience will be better. Basicaly smart trainer or one zwift " knows about " is mandatory which i didnt get from

(Noel Nunkovich *USMES*) #12


Unfortunately if you have a trainer that Zwift doesn’t recognize there just isn’t much the software can do about it. It’s not possible for Zwift to use your wheel speed, and really you wouldn’t want that anyway. After all, if the software used your speed directly from your speed sensor you could very easily cruise up the virtual 13% grade to the Radio Tower on Watopia at 20 mph! That would be an unrealistic experience and would also quickly get you flagged as a “zPower Flyer” in-game.

About the best advice I could give is to either try and get more information on what brand/model your trainer is (*somebody* manufactured it, after all, and there are only so many companies that make bike trainers…) or just experiment and try to find a “known” Zwift trainer that comes close to the power curve of your existing trainer. My guess is that your trainer is actually a known trainer, just rebranded or unbranded for some specific supplier.


(Alex Bojanic) #13

Got it. But that is happening even now. I picked “other trainer” and with wahoo recomended ant+. Speed/cadance sensor i am constantly going 25-35mph regardles the grade and producing 350-400watts :smiley:

(Noel Nunkovich *USMES*) #14

That’s just down to having a trainer with a wildly different power curve. Try a few other dumb trainers and see if you can find a better match.

(Neil Chandra) #15

I just joined Zwift using a Minoura V270.  I have the magnetic resistance turned off basically.  I am using a triple-ring older 700c Specialized Roubaix bike on the trainer.  Which ring should I keep it on or does that even matter?  My Garmin will say I am going 18 mph (I am using Garmin ANT+ GSC10 sensor) but Zwift says 8 mph.  I thought I selected a flattish course so I am a bit confused.  Could you help me understand this better, so I can use it as intended and enjoy myself, instead of just seeing riders fly by me and covering less than half the “distance” I pedal?  Thank you!!


(Alex Bojanic) #16

It seems same problem as mine. Zwift works well with smart trainers and those “nob-smart” that are “known” to zwift. For all others is missmor hit. In my case i kept selecting differentnone listed and trying them out e.g. Comparing zwift speed and casance with actuall. Once i found one that is closest i stick to it. The power output seems to be similar to one i make riding real computerized teainers so i am ok now. For any more accurate or serius traimer u would need the smart type.

(Joe Ostervik) #17

My power output is way out of the park !! I wish I was a Olympic racer ! I have all the correct settings my weight, age, height, my correct Cyclops fluid 2 trainer, my speed sensor reads correctly, my cadence is correct. I did 30 min Zwift and then 30 min using my normal app with Wahoo and they were almost the same just off a little. The power on my sprints maxes out 1200 wt(100 cad in 22nd gear). My trainer is really tight one month in and already have to replace the tire. A friend said I should change my weight to a lower weight ?? That didn’t work ! Is there any more settings that I am missing ???