PowerTap vs TrainerRoad Power Numbers

author : FitWerx
comments : 0

"I started out a couple months ago using TrainerRoad with virtual power, but recently got a PowerTap wheel, and I am in the process of trying to re-establish my power zones. It seems to me that I might have some calibration issues with the PowerTap.

For my previous 20 min power test using TrainerRoad, my 20 min test results were: Power 237 watts, HR 156, Speed 19.1 mph. I just re-tested with the PowerTap today, and the results were: Power 173 watts, HR 164, Speed 30.9.

As you can see, there are some pretty big differences there."

Answer from Dean Phillips
FitWerx

The PowerTap results are likely more accurate than the TrainerRoad results when it comes to measuring average power. It also looks like you used two different setups or trainers which won’t impact power results, but can have a big impact on indoor riding speed. Once you get an accurate powermeter on the bike such as a PowerTap, you’ll no longer even need to look at or record speed indoors since it ends up being too dependent on setup and conditions.

Use a PowerTap for Baseline Tests

I recommend using the PowerTap to measure your 20-min baseline power and any other testing going forward. The PowerTap uses strain gauges in the rear wheel hub to measure actual power being generated. PowerTap rates this accuracy at +/- 1.5% and in our experience checking various units, they’ll typically fall into this range.

TrainerRoad Assumptions

The TrainerRoad program is an excellent indoor training tool, and under the right conditions can create repeatable conditions for indoor workouts. The TrainerRoad program doesn’t measure power, but uses an assumption for the power required to ride the trainer model you’re using. In your case, the Kurt Kinetic Road Machine has been tested by TrainerRoad for power required at different rear wheel speeds. In a perfect setting, it will always require the same power on the bike to ride this trainer at 15mph, 20mph, 25mph, etc. However, TrainerRoad needs to use another assumption for tire rolling resistance which can vary significantly between different setups on the same trainer. Tire rolling resistance will vary depending on type of tire, inflation pressure, the press-on force of the roller on the tire, and even temperature which can change as the tire heats up.

I find that the following comparison makes it easier to understand how different tires and setups will impact required power in TrainerRoad:

Use a race tire on your bike and attach it to the trainer with as little roller press on force as needed to get the wheel to spin without slipping. Then get on the bike and notice how easy it is to ride at 20mph. Now get off the bike and install a thicker winter bombproof tire on the rear wheel and take it a step further by cranking down the roller as much as you can. Now get back on and try to ride at 20mph again. It takes a lot more effort doesn’t it? That’s because it requires more power to ride at that speed with the higher tire rolling resistance in the 2nd setup. TrainerRoad will tell you you’re riding at the same power output in both cases as long as it sees the same trainer wheel speed and trainer model, but reality may be a difference of 50+ watts between the two scenarios.

We asked Nate Pearson at TrainerRoad and got some great feedback. He’s documented a test showing the Virtual Power (VP) using Trainer Road being consistently 30 watts higher than the power on his Power Tap (PT) unit here: http://blog.trainerroad.com/kurt-kinetic-power-curve-virtualpower/

The Bottom Line

The bottom line is that TrainerRoad doesn’t claim to measure power, but has proven to show repeatable precision as long as you stay consistent in tire choice, psi, and number of turns on the roller pressing the rear tire. Nate Pearson has been a valuable resource for TrainerRoad users and is always quick to respond to questions. 

Extra Credit – Checking the Accuracy of a PowerTap

You can also measure your PowerTap’s accuracy with a “Stomp Test” if you’re up for it. To perform a stomp test, you’ll need a known weight that you can hang off the pedal while it’s in the 3 o’clock position. I recommend something in the 30-50lb range, and we typically use a 45lb plate that you’d find in a weight room - it’s the right size and easily fits over a Speedplay pedal. Get somebody to hold the brake levers down so the bike doesn’t roll forward when the weight is placed on the pedal.

  1. Change the PowerTap CPU to display torque instead of watts
  2. Hang the weight over the pedal at 3 o’clock
  3. Gently roll the bike forward and back until you get the highest stable torque value and record that value.
  4. For best results, repeat using 3 different cog sizes (smallest, middle cog, largest), and in both chainrings. In the end this gives 6 different gearing combinations.

Compute the real-life measured torque using the following formula:

Torque = (crank length mm) / (25.4 mm/in) * (weight lbs) * (cog) / (chainring)

Let’s take a set of data using a 175mm crank, 45 lb weight, using the 12t cog and 39t chainring

Torque = 175/25.4*45*12/39 = 95 in-lbs

If the Powertap CPU is displaying a lower value than 95, then it’s underreporting power. If it’s displaying a higher torque value than 95, then it’s over-reporting power. I recommend testing the wide range of gearing which will give high and low torque values, and also use cogs on both ends of the cassette since it covers the various stresses on the cassette while in different gears. A PowerTap unit reading too high or too low will quickly jump out using this protocol.


Dean Phillips is a co-owner of Fit Werx² in Peabody, MA.  Dean frequently writes tech articles for BeginnerTriathlete.com and is humble enough that he would likely never tell you (so we'll tell you for him) just how fast he is on a bike.  Dean holds multiple TT course records in New England, having broken records previously held by some of America's best pro cyclists, and he set these while being a father of three young children and owning his own business.  Dean knows speed and how to get the most out of his training time.

Rating

Click on star to vote
12948 Total Views  |  287 Views last 30 days  |  76 Views last 7 days
date: January 25, 2012

Author


FitWerx

Fit Werx offers the most scientific and complete bicycle fitting services in New England, the Northeast and beyond. Regardless of where you are from (Massachusetts, Connecticut, New York, New Hampshire, Maine, Rhode Island, Florida, Pennsylvania, Texas, Vermont, Australia, Macau...) a Fit Werx' bike fit is guaranteed to be worth the trip.

Author

avatarFitWerx

Fit Werx offers the most scientific and complete bicycle fitting services in New England, the Northeast and beyond. Regardless of where you are from (Massachusetts, Connecticut, New York, New Hampshire, Maine, Rhode Island, Florida, Pennsylvania, Texas, Vermont, Australia, Macau...) a Fit Werx' bike fit is guaranteed to be worth the trip.

View all 55 articles
 

    From the forums