Can O' Worms (Part II)
Yesterday, I set out to adjust both my SRM and Ergomo Powermeters so that they both at least showed similar readings. Without having the benefit of first calibrating my SRM externally (using the Mfgr protocol), I set out to just "eyeball" the differences.
The SRM allows for the adjustment of Slope - that is the response to torque. When you do the zero test, you are setting the Offset (or Y-Intercept). All of this comes straight from your Intro to Statistics class.
The Ergomo allows for the adjustment of K-Factor. Unfortunately, I don't know what K-Factor is. I think it's actually closer to being a Y-Intercept.
Yesterday's testing showed that my SRM consistently tracked higher across a range of wattages. However, the differences grew as watts increased. Thus, the slopes of the two devices are/were different. So, I began to adjust the SRM's slope in the hopes of finding something where the differences tracked evenly across various wattage ranges. This morning I continued this approach until I reached a trend in differences that was very close to zero.
However, this still left the ergomo consistently reading about 10w lower than the SRM. So, I began to adjust the K-Factor on the Ergomo by small amounts and repeat the protocol.
You can see by the graph above that I think I ended up being pretty successful. The difference between the two is 0.5 - 1.8w depending upon how you look at the data.
Another question, though, that I wanted to answer, was how much does a simple change in K-Factor effect watts? Well, I was able to gather a small set of data and do a simple analysis. The end result was that for me, increasing the K-Factor by 1 unit results in an increase of 3.6-4.2w for the cadence zones that I'm interested in (80-100 rpm). Obviously my sample set is very small, so your mileage may vary.
So, what have I really learned from all of this? Well, not much except that two of my powermeters now read very closely. Unfortunately, I don't have the means this moment to say that they are both reading "correctly". Nor am I all that worried about it either. Funny thing is, they're both on the same bike. So, why does it really matter? :P
I did notice that the SRM is quicker to record/capture spikes that occur when a rider quickly increases cadence. And, some of the differences are clearly due to the Ergomo's dependence upon the left leg's input. All in all, both of these seem inconsequential to anyone doing Time Trial riding. If you ride the track, maybe you'd feel differently.
One feature that I'm sure that everyone with multiple Powermeters would crave is having the ability to just raise or lower the displayed/recorded data if for no other reason than giving us a warm and fuzzy feeling when we switch bikes. Unfortunately, there isn't consistency among the manufacturers. So, this becomes such a difficult task (whether you own different brands of PMs or not).
Now, I've still got two more PM's to try and bring into sync. So, wish me luck!