The virtual power meter

It's not all web design and Apps you know! One of my other passions is competitive road cycling. I dabbled as a teenager and have fond memories of riding a 40mile charity ride with an old school friend. I also did a very short stint riding with my University Team although got thourougly demotivated by the ability gap between myself and the core team members. And lets face it given the choice between the many other social activities whilst at University compared with hours on the road in appalling weather getting thourougly destroyed by much stronger riders, the cycling got forgotten. Roll forward 20 years, marriage, two kids and far too comfortable a life style and something needed to be done.

So back to cycling, however this time the bug caught hold properly and this year I completed my first season as a competitive rider at club level. I've had a great year, developed a fitness level to rival if not exceed that of my youth and there is still plenty of scope to improve. So how does that improvement happen?

In general terms improvement happens in the same way that you improve any skill or ability. Lots of hard work, hours riding on the road and on indoor trainers (Turbo Trainers), as well as cross training for general fitness for the truly commited. Now for me being very technically minded I like to see something tangible for my efforts. I need to see some raw data to analyse and digest to pin point exactly how I'm improving. In cycling this is most easily done using a power meter. Power meters come in a number of forms, crank based ones, wheel hubs and pedals. By far the most popular and accurate are the crank based units, but there is a problem. Cost. Even the most basic crank based power meter is over £1,200 which is a lot of money to justify these days. But where there's a will there's a way, a much cheaper way as it turns out.

As part of my training plan I have to complete a couple of indoor sessions each week. These are done on a Turbo trainer under much more controlled conditions than the open road. With careful research I selected a Turbo trainer that is known to have a fairly reliable and predictable resistance behaviour. So for a given speed ridden on the trainer the resistance is at a certain and predictable level. As power is a measure of how much energy is required to overcome a certain resistance, there is a direct correlation between the speed I ride on the trainer to the amount of power I am exerting. So if I can capture the speed I can calculate the power without the need for an expensive power meter.

As it turns out capturing the speed I am riding at is relatively easy, there is a pervasive tracking system in sports called ANT+ANT+ is a communications standard allowing various monitoring devices such as speed, cadence or heart rate monitors to transmit their readings to a paired reciever. These recievers generally take the form of light weight on-bike computers such as the Garmin Edge units. However as ANT+ is an open standard implemented by many manufacturers it's not just Bike Computers that can tap into this stream of tracking data. In fact the nice people at Garmin created a USB stick for doing just that. So getting the data as a live stream into a computer can be achieved. The next problem is what to do with that raw data.

Sports tracking is a big area online. There are many services that offer different views on how this should be done, with some being multi disipline, some being sport specific and others being GPS location centric. Also each vender of these monitoring devices usually has some sort of desktop App for tapping into the data and doind some sort of analysis. The problem is they mostly want to sell you expensive power meters rather than let you bypass that with some cobbled together virtual system. Of course the internet is vast and most problems have an Open Source solution. In this case the space is filled by a project called Golden Cheetah. This project does an excellent job of reading in performance data for cyclists and producing all sorts of analytics, reports, charts and predictions. Just what I'm after to understand my performance improvements.

The other great thing about Golden Cheetah is that it already has provision for getting Virtual Power from Turbo Trainers. The problem is that it can't do it from my particular trainer. Now this is a problem as each type of trainer has a very different 'power curve' with some being fairly linear and others being very progressive. My particular trainer, the CycleOps JetFluid Pro, has a progressive power curve, one that gets very much harder the faster you ride. This type of curve is supposed to simulate the sort of resistance that you get when riding on the road. Remember that as your speed increases wind resistance increases as a cube of the speed! None of the included virtual devices were close to my device. What now. Break out the code!

So right now its a real benefit being a developer as I'm able to grab the source code for Golden Cheetah and build it myself to add in the necessary power curve to represent my trainer. But it's never that simple. As it turns out Golden Cheetah is a pretty complicated beast. It is a cross platform product available for Windows, Mac and Linux. To achieve this it must use techniques and frameworks that support all platforms, in this instance the primary one is the QT User Interface Library. It also incorporates about a dozen other Open Source Libraries to achieve connection to USB devices, format graphs, talk to Google for Maps, export XML files and many other things. All of these sub-projects need to be obtained and built before you can go anywhere near building the actual program.

This didn't go as smoothly as I'd have liked. Golden Cheetah is a fairly old project, its been around a while and has been through many versions. This is good as it means there is a core of developers keeping the project going and it gets new features on a regular basis. However it also means that some of the libraries used are out of date as they have not been kept up to date in the project. This happens quite often, especially if there is no need to improve or tweak that part of the project. For the embeded developers this is not a problem as their build environments are built up over time. For me however this caused quite a few issues. Finding and compiling old libraries with new build tools is not always easy. In this case it actually took the best part of two days work to get all the components and get them all compiled and hooked together to build the project. It then took about five minutes to add my virtual power device!

But how do you represent a virtual power device in code? How does that specific speed to power curve get translated and distilled down to an equation? Now I'm no math geek, but there is a whole area of math that addresses this called polynomial equations. Essentially using polynomial equations you can represent curves on graphs. Better still given a set of data points on a graph you can derive a suitable polynomial equation. No idea how you do that, but the Internet knows and I was able to find an excelent tool to solve this particular problem that took my data points and spat out an equation in C/C++ format. But where did I get the data to build the curve?

Unfortunately the manufacturer was less than helpful in this area. Aparently the power curve for the trainer is proprietory and so could not be divulged. This seems crazy as there is an easy way to work it out. You stick a bike with a power meter and a willing test subject (my coach in this case) on it onto the trainer and then record regular speed and power figures across the range used for training. Of course this does mean you need access to a power meter, but borrowing one is a lot easier than buying one. So after a session of tourturing my test subject I had the raw data, the online tool gave me the equation and I could then build the program.

The proof is in the riding of course. My next training session produced a power trace and bingo the numbers matched up with the data gathered during the test session so I'm pretty confident that everything stacks correctly. Of course this is not as acurate as using a power meter, variance in temperature in the trainer and tire pressure, even cadence level during the session all impact on the true power levels I put out. But it is consistent enough and conservatively the margin for error is around 5%. So as long as I stick to this method I will see changes over time which will be a reflection of my improvement over the course of my training season. Power solved for the cost of a USB dongle (£25) and a few days of geeking out over some Open Source Code. Bringing the geek into cycling.

No test subjects were harmed for the purposes of this experiment, well not much at least. Consolation to the test subject as he did hit an impressive 700W peak power output during the testing.