Saturday, October 27, 2007

Body -> Comparing Power Meters: Polar CS600 to PowerTap

This is one of those reviews that I really didn't want to write.

Mainly, because I knew it would be a bunch of work. However, seeing that I needed to ease back into training after the forest fires, I decided to take a short ride to do a bake-off between my two power meters: The Polar CS600 and the Cycleops Powertap.

Conventional wisdom would tell you that the Powertap would be more accurate than the Polar unit. I pulled my Powertap off of my racing biking and crammed it onto my commuting bike. This highlights the first issue: The Polar was a pain to set up. While I didn't like moving the Powertap, I really didn't want to move the Polar.

Moving the Powertap was a bit easier, but the biggest challenge is that my commuting bike is based around a mountain frame, and the spacing is 5mm too wide on the rear wheel. However, you can clamp hard and a road wheel will stay in a mountain bike dropouts. However, I rode a bit gingerly, not wanting to dislodge the wheel.

Although I did not have an ergometer to test them against, I could compare them against each other and against my perceived exertion. If they both measured the same, then I had confirmation. If they varied, then I would use my body to judge which was closer.

Once I got home, I dumped the data and parsed it into an Excel data file, and plotted the two meters against each other.

The first half of my trip was up a hill, and while I didn't grind it out hard, I did have a moderate pace that was above 150 watts for the most part. I immediately noticed on this section that the meters seemed very close. This was better than I expected.

Here is a chart of this section. Click on the chart to make it bigger.

The polar samples at a lower frequency. You can see that the power curves are simply smoother. However, for the most part, the Polar curve and the Powertap curve track nicely together. The TSS score from this data was close together, as were the calories burned.

Net-net of this section showed that if you are pedaling at a moderate pace to harder pace, the two power meters track nicely together. Now the Powertap samples twice as often, but in my mind you don't vary your instantaneous power that fast. So the higher sample rate is inconsequential. A bit like a Nyquist sampling test.

However, once I crested the hill, I had a long down section where I had no desire to pedal hard, due to the jammed in nature of the rear wheel. So, I was just pedaling at an easy pace. This is where the two power meters split. The Powertap had under 100 watts of power. The Polar, on the other hand, was showing 30% higher readings.

As you can see on the graph, this is reflected in the data. I don't need any rational on what was happening. The Polar unit measures vibration from the chain. As you put power on the chain, oscillations get faster. If you could hear the chain, it would sound higher. However, at easy pedaling, it is difficult to catch the note. So, I clearly believe the Powertap. It felt right.

So what are the take aways?

1. The Polar is surprisingly accurate. I thought it would be okay, but under moderate to high cycling loads, it is very close to the Powertap.

2. Therefore, as long as you are really working out, and keep the chain under tension, the two units will be very close. The only issue is if you are easy pedaling. However, unless you are pedaling easy all day, I don't see this as an issue. While I had a tracking issue, it only happened under a particular condition, which I don't do very often. If you are tourist that pedals at 80 watts, it will over read the power. However, if you are an easy tourist, I bet you don't care about power meters anyway. For me, this was a short bit of high biased data that doesn't ruin the whole data set. It is a "who cares?"

Now, in some sense. I have also seen reports that some people have removed a link or two to keep the tension a little higher, and this has removed bad data. I haven't tried it, so your mileage may vary.

However, I am pretty happy with the CS600. I bought it mainly for other things like the altimeter and the great pulse monitor. However, it does a very good job for power also.

1 comment:

DrG said...

Cool, thanks for the comparison.