A recent post to the Wattage list sparked my curiosity on whether our PowerTaps were calibrated. I knew there was a procedure to test it, but I didn’t realize it was so simple! So I finally tried it today. It’s also referred to as the “stomp test”. For you PowerTap users out there, this procedure only checks the calibration; there’s no way to calibrate it yourself. For that, you’ll need to send it in. But from what I’ve heard, Saris will usually cover it, if it is found to be way off.

The PowerTap calibration test is posted on the Training with Power FAQ. Basically, you put your bike in a trainer, lock-up your rear wheel (a toe strap cinched tight on the rear brake lever works well), set your PowerTap to display Torque, then apply a known weight onto the right crank-arm in the 3 o’clock position. After you record the number displayed, you can compare it to the result of this formula:

Torque = Weight(lbs) x Crank-length(mm) x 1in/25.4mm x Cog/Chainring

At first, I tried to use my bodyweight. But even though the bike was in a trainer, balancing on one pedal while trying to avoid any vertical forces, proved harder than I thought. At one point I almost fell over!?

Then came the revelation: The instructions say to “hang” a weight; so I was stuck into thinking that I had to elevate the bike on a table and put some weights on a string, or something like that. But since Heather and I both use Speedplays, all I had to do was take a 45lbs Olympic weight-lifting plate and place it over the pedal axle. Voilà! The plate hung perfectly vertical and didn’t touch the ground. Yet another benefit of Speedplays!

The results? Both Heather and my hubs are slightly on the low side, less than 3% off. Fortunately, they are both off by nearly the same amount, so swapping wheels as we sometimes do, doesn’t present any problems. Of course, when we swap wheels, we still have to zero the torque (which is different than calibrating a hub). I realize the instructions do say to use at least 50lbs, but I think 45lbs is close enough. Besides, the numbers I did manage to record while trying to stand on the pedal, were in line with the plate results.

All in all, it was an interesting exercise, but it won’t change the way I train with power. I’ll just have that piece of information tucked away in the back of my mind when comparing my numbers with my buddies!