For the past year I have been using a PowerTap on my road bike and my mountain bike and the watts have always “seemed” to be in synch. At least the numbers were close enough that I never gave it a second thought.
While riding in St George this past April I broke my PowerTap Disc hub and had to send it to Saris for the $350 full-overhaul special that includes a one-year warranty. Since I bought this wheel used I felt as if this was fair and Saris turned it around pretty quickly.
I didn't know that my $350 overhaul included new quad muscles and a new VO2 Max. During my first post-repair ride I miraculously turned into Fabian Cancellara and was seeing some impressive numbers on the screen...but the numbers were lying to me. Damn, it was fun putting out 260 watts at 120 beats per minute. Evidently the hub was not calibrated properly and the newly repaired PowerTap had to go back again. Bummer.
Last Friday & Saturday were my first rides following the second repair. Although the numbers weren’t quite Fabian-like, they were still seemed inaccurate. WTF? I needed to be sure. My brain needed to know for sure because seeing two separate sets of numbers, one from the road bike PowerTap and one from the MTB PowerTap, was now messing with me.
Dave Harris kindly steered me in the right direction and I rigged up my own test. By using a fixed weight and the following formula, you can test whether your power meter is accurate.
Measured torque = (weight in lbs) × (crank length in mm) × (1 in/25.4 mm) × (cog teeth/chainring teeth)
The test confirmed that the PowerTap Disc hub is still out of calibration and needs to go back to Saris for a third time since April. Power meters are finicky little gizmos in general and I knew this going into it but I really hope that Saris gets it right this time.
This exercise made me wonder how many people have PowerTaps that were out of calibration from the start. How many people think they are putting out 300w when they are really only putting out 250w or vise versa? I guess it really doesn’t matter that much as long as you only have one PowerTap and that is your only point of reference. I will freely admit that I am very type “A” and I want to know what the “real” number is.
Even if my test methodology is not perfect, it is obvious that one hub is way out of whack
My high-tech fixed weight system
The crank arm must be level
Don't drop the weight on your toe while conducting this test