Saturday, 17 May 2008

Power meter comparison on a 2 hour ride.


Following on from my comparison between the PowerTap and an uncalibrated CompuTrainer yesterday I have now taken my tests a step further. Today's test was run having firstly calibrated my CompuTrainer properly according to the instructions using an adequate press on force and a proper trainer tyre inflated to 110psi and cleaned with isopropyl alcohol to avoid tyre slip.

The weather here in Northumberland in the UK was poor so, confined indoors, I rode a route on the trainer that I had previously prepared from GPS data and converted to a 3D course. The route is just short of 50 miles and at a steady pace I would be riding for a bit over 2 hours. The plan was to record the ride on both the PowerTap and the CompuTrainer and compare the data sets.

I was pleased with the results which showed what I had hoped, that is that the 2 units were very comparable in terms of data output. There were of course differences but these are almost certainly a function of the different ways in which the units operate and calculate power. I cannot of course be certain which of the units is "right" but that was not the purpose, the purpose was to find out whether the outputs were comparable.

The main observations I made were as follows:

1). The average and normalised power figures obtained from the 2 power meters were pretty much identical, as were the TSS and IF figures calculated. Looking at the overview graph below it is clear that the overall power figures are very similar for this 2+ hour ride, calibration makes a difference!

2). Looking more closely at the data above and for a 5 minute period (below) it can be seen that the PowerTap records a higher value when there is a short, hard, immediate change in power output, as might be the case in a sprint or other sudden power burst. The result of this is that the PowerTap records a higher maximum power for the ride, at 616W compared to the 565W recorded by the CompuTrainer. The following graph shows the recorded outputs during a 5 minute period including one sudden power output change.

3). I observed that during periods where the "speed" was high, riding in a high gear with a lower cadence the CompuTrainer recorded a slightly but consistently lower power figure than the PowerTap.

A summary of the comparative data as extracted by the TrainingPeaks WKO+ package is enclosed below which shows the remarkable consistency of the figures produced by the units. On the basis of the trials so far I have no hesitation in recommending this combination of equipment for indoor and outdoor training. Having spent a lot of money on this setup I'm happy with the data coming out, even though I'd like the numbers to be higher!

Next week I hope to carry out an FTP test, it will be interesting to see which unit gives me the higher FTP, I think on the basis of my investigations so far it will be the PowerTap.

PowerTap vs CompuTrainer Comparison.

PowerTap CompuTrainer
Duration 2:11:40 2:11:40
Work Done 1723kJ 1712kJ
TSS 139.1 (IF 0.797) 138.7 (IF 0.796)
NP 223 223
Maximum Power 616W 565W
Average Power 218W 217W
Peak 5s Power 530W 456W
Peak 10s Power395W 386W
Peak 20s Power318W 315W
Peak 30s Power314W 310W
Peak 1m Power287W 290W
Peak 2m Power271W 272W
Peak 5m Power248W 250W
Peak 10m Power242W 244W
Peak 20m Power231W 232W
Peak 30m Power226W 229W
Peak 60m Power224W 225W

1 comment:

  1. I was half expecting the CT to be a lot lower than the PT. CT watts seem to hard in comparison, but I guess that is the indoor vs outdoor phenomenom.

    The comparison is excellent for the CPs that matter.

    Interesting stuff.