Wednesday, June 23, 2010

Is your PowerTap lying to you?

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


Anonymous said...

You would be an excellent engineer. Nice excel spreadsheet.

Ed said...

I would have considered myself as powerful as Fabian and called it good.


David said...

Does it matter if you only compare your mtb no.s with mtb no.s and the road bike with the road bike?
As long as they are consistent their shouldn't be a problem... I have an SRM on my roadbike and a powertap on them mtb, and there is a 30w difference bewtween the two at full noise. It seems to be the same with everyone I know, the numbers don't match and usually the mtb is lower.

FixieDave said...

I'm with ed....

Kris Lunning said...

Not sure how much bearing I put on that test....but I will say that watts are only relative to the individual power meter......even with an SRM.....It works best to send them both in so they can be calibrated to match each other....or just adjust your zones for each bike if you like a mental challenge....scientific instruments are for laboratories not bikes. I understand and agree that I'm a slave to them but Its sometimes more of a pain than it is worth....they seem to F with our heads more than anything...WATTS the deal....BTW this comment has no emotional relevance...keep pedaling..

elscorcho said...

Dave, came across this post recently and since i've had similar questions just had to ask. When you went into the test mode, did you get the Test Mode readings for the other Trials? It seems like the Alpha 340 is reading near the 512 target but the MTB is higher. From what i understand as long as that reading is near 512 your pretty close.

Unknown said...

Dave, as someone who just sent his abruptly-out-of-calibration and three months-out-of-warranty Powertap back to Saris and is not too happy about the $350 price tag, let me say that your posting on this issue is far and away the best one there is on the internet. And Ed, your comment about blindly accepting your newfound equality with Spartacus... Brilliant!