Sunday, December 13, 2009

Training: SportTracks Power without Power

As a follow up to my previous training post on crit vs. TT SportTracks Workout Analysis I thought I'd post on yesterday morning's annual SIR Peak TT. However, this time I've put an angle towards a new plug-in I discovered via thebikegeek who has done an excellent post on how to make the best use of SportTracks with Garmin here

The plug-in I'm talking about is the rather excellent 'Power' plug-in, available here. This takes all the inputs of your GPS data (i.e. elevation, speed etc.), combines them with the inputs you've put into SportTracks about your weight (bike + person + equipement), your position on the bike as well as your tyres (giving a rough aerodynamic / rolling resistence factor) and then searches for the nearest weather station to get the latest head / tail wind information. Add all these things together with some fancy formula and you can get an approximation of power to analysise after your ride, all for free and with no power meter attached to your bike. I'm pretty blown away that some nutter actually wrote the script for this to happen and publishes it for free but hey the internet is a wonderful thing!

Of course, in an ideal world I'd have a wireless SRM but in the interim I think this is a pretty good solution. If we say there is probably an error margin of +/- 10% then that's not too bad. Of course for group riding it's not going to take into account drafting etc. but you can't have everything. I'd love to test the plug in by having an actual power meter on the bike to compare with, maybe sometime in the future.

For the record, the winner's SRM data looked something like this:

Average speed: 23.0kph
Average power: 425W
Norm power: 431W
Average power excluding descent: 436W

And the time's looked something like this:

1) Colin - 18.23 ***NEW COURSE RECORD****
2) Ed - 20.35
3) Da Skunk - 21.14
4) Mark B - 21.34
5) Bemet - 21.41
6) James R - 22.38
7) James Mac - 22.50
8) Bo - 23.10
9) Dave - 23.17
10) Wim - 23.20
11) Peter - 24.02
12) Paul - 24.50
13) Keith - 26.40

So, I came in 2nd, 2 minutes down on Colin's quite amazing time. I was very pleased to have knocked best part of a minute off my PB largely due to being able to give Mt. Austin my full effort (thank you '29 rear cassette). The SportTracks plug-in data came up with 350 watts as my average power and I was on a 20.4kph average speed. If Colin's time was 10% quicker then you'd have thought that given we are a similar weights my time would require 90% of the power but 350 watts is only 82% of 425. I don't know what the exact relationship is between power and speed, other than on flat TT's it's generally a curve that says the faster you go the more expodential power you will need. Another way to put it is that it's a law of diminishing returns. On a hill climb aerodynamics are less of a factor so perhaps the 'power without power' figures are a little low, or within the error margin. Whatever, I'm pleased I have a new set of numbers to look at for these sort of efforts that certainly say more about the effort than speed, and that put the magnitude of Colin's ride into perspective.

Oh yes, and on a lighter note here's a picture of us waiting for the coffee shop to open after the TT (something not quite right about that...)



  1. Update on the above. Colin sent me through some of his historical power data for this climb and in amongst it were quite a few times in the mid 20's and guess what, all his average powers were in the 340s-360 range, with 350 looking about right. This is more like +/- 3% and certainly within the realms of different weather conditions etc. on the day that will affect the speed to power relationship, not to mention the differences between Colin & I's input variables of weight and so on. Perhaps this plug-in is more accurate than I thought?!?

    Another thing to note is that Colin's figures excluding the descent usually add 10 watts to the average. I can therefore say with a reasonable degree of confidence that my maximal 20 minute sustained power is around 360watts. Divided by my weight of 80kg that puts me at 4.5 watts / kg. According to the following
    that is Cat 1 material. Colin's 5.3 suggest UCI Di.3 material!

    Geek out...

  2. Just to avoid any confusion the above plug-in is called GPS2PowerTrack and the forum for it is here:

  3. Yes, you are right that you cannot linearly compare speed and power. You'd need something more like a third or fourth order exponential equation. For example, the bright lights at Cyclops have calibrated their Fluid2 Trainer so that a 265W effort will yield 20kph speed, while 24kph will require a whole 405W. Obviously, the exact relationship in your case would require an analysis of road resistance, wind, etc.

    Nice to see that the output values of the plug-in are reasonable.

    Remind me not to challenge you to a hill climb TT.

  4. Just realized those speeds above should read mph, not kph.

  5. What's interesting though is that the cyclops figures are based on riding on the flat where wind resistence is the biggest thing restricting your speed. But when the speeds are down at 20-22kph, as in this case you'd have thought aerodynamics would be less of a factor and that power would give you more 'value for money' in terms of speed. Colin's figures would suggest not. Perhaps aerodynamics is more important than you'd think, even at this speed. Some phd student must have studied this?!?

  6. The power analysis values in Sporttracks from my group rides are typically around 20% higher than my solo rides for the same speeds, and presumably I'm taking some pulls in those group rides!

    Another tool you can use to validate the power data is this one:

    Use the defaults for everything but slope, speed, cadence, crank length, and of course weight. I think this site was developed by that phd student you are referring to!