Why no VO2max (biking) without power meter?

This has to do with how FIrstbeat calculates VO2 Max for cycling. Here is some good information stright from them: www.firstbeat.com/.../

Here is a vo2max bike datafield for your Garmin watch that dont need a powermeter, I dont know how accurate it is apps.garmin.com/.../d7ec9b1b-8353-42e9-a3d3-c8ac641a2dba

Thanks! But its value is not shown in the Garmin Connect graphs over time afterwards - or is it? 

Thats correct, I can only find the value at a bike activity, You can se my bike ride to work this morning. And also during the ride it display the vo2max on the Watch https://connect.garmin.com/modern/activity/3836541787

That's a bit of a pity but already great! Thanks a lot! Would be interesting to compare it for running with the value that Garmin/Firstbeat calculates. :-)

It is  impossible to estimate the power based on speed, gradient and weight. You need to take into account the wind, surface, tires and friction, wind resistance etc. 
ie.  Cycling on the same road with the same bike in the same direction and the same speed, you can be doing 300W going into the wind in an upright position or 150W tucked in and with the wind. 

Ps. VO2max is only estimated even with the power meter. You need a proper lab test. 

Hm... Also for running it is only an estimate, so I don't see why it shouldn't be possible to estimate for biking as well. Strava for example is estimating a power for biking.

I could guess that maybe the error margin is higher than for running / too high to be meaningful... is it this that you're saying?

This is very annoying for me. I mountain biked for over a month with a hrm tri hrm, for up to 6 hours a day and my training status is blank. I think i took 4 day off the entire time.  I have to go for runs if I want to use training load, which is a huge part of why I bought the watch. Unfortunately my knee is messed up and I cant really run so I'm screwed.  That or spend a huge chunk of change on a power meter that I don't want to put on mountain bike I use for Enduro racing. 

Did this change recently? Before I had training load, maybe I did enough runs to get a v02 estimate? Does anyone know the minimum number of runs? I see minimum 10m run with HR over 70%. I'll try twice a week runs and bike 5 days and see if it works. 

I have a "PowerCal" chest strap from PowerTap that measures Power from your heart rate.  They argue that it's possible to estimate power from heart rate during a bike ride, and it "seems" fairly accurate.  Most likely not like a DFPM, but close enough to generate VO2 estimates.  Also, if you don't want to invest in one of the >$2000 Direct Force meters, an outfit in Florida, (Velocomp LLC) makes a power meter that clips on your handlebars, and uses grade, speed, wind, etc quite well to determine power.  Tests have shown it to be very close to a Direct Force Meter.

Power for running is much less wind sensitive. When running in flat, hard terrain, your power usage pr. kg is almost entirely a linear function of velocity.