Don't have a power meter? No Problem!
What is VirtualPower
VirtualPower turns your indoor cycling trainer into a power meter. Just hook up an ANT+ or Bluetooth Smart speed sensor and select your trainer from the list of power sources. When you start to ride, your speed will be converted to power based on the known power curve for the trainer.
How does this work?
The faster you go on a trainer, the more watts you put out. We've quantified this into a resistance curve specific to your trainer. This allows us to plug in your current speed and display watts. We've built this into our software so it happens on the fly and is seamless to you.
How accurate is it?
In our unscientific tests with our PowerTaps and Kurt Kinetic Road Machines we were within 3 avg watts for our intervals...so pretty darn accurate. You can get a run down on the Kurt Kinetic Virtual Power Curve here.
So you are saying that instead of spending $800-$3000 on a power meter, I can spend a few hundred on a trainer and $40 on a speed sensor and train with power?
Yes. Power meters are great to race and train with but if you're not quite ready to make the leap or just want to get a feel for training with power, this is a great option. Obviously this won't give you power data for your outdoor rides.
We think VirtualPower is a much better way to train than by heart rate or RPE. But when it comes down to it, a power meter will still be more accurate.
What affects VirtualPower?
With VirtualPower, we are effectively measuring power at the trainer. That means that any variance in the drive train between your pedals and the trainer can have an effect. When training with VirtualPower, make sure your tire pressure and tension against the wheel are the same each time you ride. This will give you consistent power readings.
Different tensions on the wheel affect rolling resistance and can skew your readings. The important thing is to have consistent and repeatable readings to train with. That's why having the same tension on your tire and the same tire pressure each ride is important.
Here's a good example. Below I have power from a PowerTap, which claims +-1.5% accuracy, graphed with Virtual Power. My setup was on a Kurt Kinetic Road Machine with a continental trainer tire at 120psi. I cranked down the rear wheel 1.5 times.
VirtualPower vs PowerTap (no offset)
You can see that VirtualPower tracks really well with power, but is higher the entire ride. This is due to rolling resistance (tire pressure, tire tread, tire tightness).
With my setup my y-intercept offset is -30 watts. If I lower my VirtualPower reading by -30 watts I get this graph.
VirtualPower vs PowerTap (adjusted offset)
Now you can see VirtualPower graphs really close to a PowerTap. These are even more compelling results when you take into account the PowerTap has a +-1.5% accuracy.
Do you really need to care about those -30 watts? Well that depends on how much money you want to spend. If you just want consistent power numbers between workouts, then VirtualPower is good for you. If you want absolute power numbers that you can use to compare to other people, then we recommend getting an ANT+ or Bluetooth Smart power meter.
Read more about getting the most out of VirtualPower
Does your trainer work with VirtualPower?
We have power curves for the world's top trainer brands. Start with your manufacturer to see if your trainer is supported.