Ramp Test - Virtual Power on Kickr Snap

trainer

#1

Today i start my training plans for the upcoming race year. Yesterday, I compared the virtual power, against my Power2Max and it looks like the virtual power is somewhere around 30 watts off. I would just use power match, but i will switch between my road bike and my TT bike so i want to keep the value consistent on both bikes so virtual power makes sense for me to use for my indoor training. (Note that my TT bike has a left only stages PM).

With all of that said, i plan on doing the ramp test using the Kickr’s virtual power, but also track the ride on my garmin with the P2M, and then manually calculate my FTP value with that PM to use for my outdoor riding, and then use the virtual power for all indoor training regardless of bike.

Does this logic make sense, is there anything i am missing? Is it normal for virtual power to be that far off? Note that what i was seeing yesterday was like a 30 watt difference, i.e. steady 200 watts with virtual power was resulting in about 170 on the P2M. When i jumped up to say 240 watts that same 30 watt difference was still there so the gap seems to be consistent which i think makes sense.


#2

Just wanting to rule this out, but did you do a proper spin down of your Snap once it was warmed up (10 mins of riding) and a zero-offset of your P2M?


#3

Just for clarification of terms - the power number you get from the Kickr Snap is not virtual power - it is actual power.

Now that that is out of the way - yes it is possible to have that large of a gap between your power meter and your smart trainer. Particularly early generation Wahoo units, but also on lots of smart trainers, there are consistent accuracy problems. You should compare a variety of data points beyond just the 200-170 watt alias such that you can define the gap at various points in your power curve


#4

You will also need to take in to consideration the point where you are measuring the watts.
P2M = Where you put down the power, nothing that goes to waste
Kickr = The power has to pass by the pedals, the crank, through the drivetrain to the hub and thats where you will measure power. That way you will loose some watts and measure differently.


#5

i always do a spin down on the Kickr before every ride. To be honest though i never have done a spin down during the ride. I can give that a try and see though. The p2m auto calibrates anytime you stop pedaling for 2 seconds or more.

Quote from DC Rain Maker Regarding this functionality.
"The Power2Max also has auto zero enabled permanently, which means when you don’t apply force to the pedals for more than two seconds it’ll automatically zero the unit, such as when coasting. Note that their track editions have the auto zero option permanently disabled, requiring a manual zero.

In scenarios like long climbs, the unit will leverage temperature monitoring and compensation. This means that the Power2Max unit will compensate for temperature changes – which can otherwise cause drift in power value accuracy."

I will definitely do a zero offset on the p2m this afternoon before the ride, as well as attempt the spin down 10 minutes into the test and see if that gets the gap between the 2 any closer.


#6

in that case though wouldn’t you expect the P2M (crank based) to be higher? In this case it is 30 watts lower. I am going to give the calibration suggestions suggested earlier a shot and see if that gets the numbers closer together.


#7

I lost about 35-40 watts in my FTP going from a Kurt Kinetic using virtual power to a Kickr Snap…I think all the various ways to measure a watt vary alot more than people realize…I always want to ask the question what is the gold standard for measuring a watt anyway?


#8

I noticed quite a big drop when I went from using the power of the snap to using my 4iii left sided pm. I think the snap reads higher even after calibration.


#9

What’s important about power is that it is internally consistent and linear. Putting out 200w should be the same every time. Putting out 250W should be 25% more power then 200W. Putting out 300W should be that same amount of extra power again. This allows you to do structured training accurately and reliably.

What doesn’t matter, as far as TrainerRoad is concerned, is if your power and my power are consistent. Or if virtual power matches up with wheel power.