Monday, March 12, 2007

Can O' Worms (Part III)

Last night, I found what I was looking for.... The Computrainer has a Potentiometer screw that allows for adjustment of the resistance (thus allowing someone to raise or lower the reported watts). Now, be warned, if you move the screw and return the load generator to RacerMate, you will be charged for recalibration against their dyno. Fair enough.

The truth is, there probably isn't any reason to turn the screw anyway unless you're bothered by looking at results compared against another Power Meter and even then as you're about to read, it still might not matter.

So, this morning, I rode using the SRM, Ergomo and the CT. I didn't follow any protocol. I just rode the bike. In my haste, I forgot to perform the zero offset on the SRM. Well, I figured that since the bike hadn't been removed from the CT since yesterday, and the temperature is stable ... well, what would have changed to give me unexpected results? I did ride for 10 minutes in Ergo mode and then performed the RRC.

The results? Well, for some reason, the SRM was 12w higher than the Ergomo - the result from not performing the zero offset I presume. But, the slope still showed a positive bias meaning that I probably still need to lower it some more.

What about the CT? Well, the good news is that the slope of the differences between the CT and Ergomo was -0.004. In my mind, that's closer than anyone could hope. And, I feel vindicated in that the differences that I sensed between my CT and Ergomo were in fact real. More importantly, it shows that the differences are steady. The bad news is that my CT now reads (even after adjusting the potentiometer screw all the way left (raise watts) a clear 36-38w too low.

But, wait. Ergomo's and SRM's are supposed to read higher than a CT. In theory, drivetrain losses (chain, derailuer, hub, spokes and tires) will lose some power. So, the next question is exactly how much should I have expected to lose and are there things that I can/should do to the bike to salvage some of that loss?

I don't know the answer to those questions yet. If/When I find out, I'll write about it.