Wednesday, March 14, 2007

Can O' Worms (Part IV)

So, I was unable to stop myself yesterday and I continued on with some of my investigations. Yesterday's ride was done with all PMs in place and I just rode around on the CT however I felt like I wanted to at the time. While letting my mind wander, I realized that the reason the SRM creeps higher on me is due to heat. That is, the BB itself heats up during a trainer ride. So, how much? And, can I correct for it?

Well, the how much is certainly negligable. We're taking about a 3.5w drift over the course of twenty minutes. That's not enough to effect anyone's training.

As for correcting it? Well, yes. You can see by the second graph that you can create a simple linear regression based upon the torque values calculated by WKO+. If you then recalculate the watts based upon torque (WKO+ calculates torque for you whenever both Watts and Cadence are available), the difference goes away.

Miraculously, when you compare the predicted SRM watts to the Ergomo (used only as a comparison here), you see the two line up very well in terms of consistency. Yes. The SRM is reading higher. But, that's not my point as I can correct for that by adjusting the Ergomo's K-Factor upwards.

This also means that SRM users should follow the advice provided by the manufacturer. That is, perform the Zero Offset (ZO) anytime the ambient temperature changes as well as at the beginning of your ride. And, I would add that you also consider performing the ZO about 20 or 30 minutes into your workout as well.