Virtual power

MacBook Pro, Garmin Speed/Cadence and Cyclops II (virtual power)
Connected very easily
Rode 40mins this morning which I wouldn’t have bothered doing if Zwift wasn’t here
I think my Virtual power is a bit over estimated (fun - but feel I am cheating) - have other noticed this?

Genuine LOL… it’s the hottest topic of conversation on the forums and facebook pages. Like walking into a war zone and asking if anyone feels there’s a bit of tension in the air.

The virtual power is a major bone. People who genuinely enter the correct details seem to be OK, and some have made comparisons of their virtual power to power meters with fairly consistent results.

The problems seem to come when people enter incorrect details, maybe to use a trainer which isn’t listed, maybe to intentionally cheat. So there are people doing constant 5w/kg laps, unreal times and incredible power surges. This morning someone had the Orange (lap) jersey with a time of 1:28, which means they can ride at 200km/h.

So yes, it has been noticed :slight_smile:

Virtual Power is also highly dependent on how you setup your trainer. It is easy to show that resistance changes depending on tire pressure used AND how hard you turn in the roller drum.


  1. Increasing tire pressure all else held equal increases your required power output to maintain a given speed. I’m not sure if you’ve noticed that if you leave your bike on your trainer for months at a time (which is what I do with my ‘trainer bike’ when there’s snow outside) and you don’t keep your tires topped up, you find yourself hitting higher and higher mph for a given power output. Top up the pressure and that resistance is back. This is not to be confused with outdoor rolling resistance and tire pressure effects since the deformation of a tire on a trainer is NOT dependent on tire pressure like it is outside - outside increasing pressure creates less tire deformation and therefore less rolling resistance. On a trainer, the tire and drum are locked in place therefore increasing the pressure has a different effect - turns out it’s opposite of the outside effect.

  2. Turning the roller drum hard into the tire all else held equal increases your required power output to maintain a given speed. This is kind of the same effect as riding outside and being heavier or using lower tire pressure. The increased tire deformation yields higher rolling resistance.

The best you can do to make sure your virtual power readings are good is verify with whoever set the power curve that they did so using the trainer manufacturer’s recommended setup. Kurt has pretty loose specs calling for 2-5 turns based on getting the tire to stop slipping so I honestly don’t know how it can even be considered a valid machine for virtual power.