So now you need to account for changes in moment of interia and rotational wind resistance of the wheel itself to accurately set up the formula. For simple linear offsets you could develop a procedure to do this without a power meter by timing a roll-down between 40kph and 30kph.īut then I thought some more and the picture is less clear - if I ride a 200g climbing rim with 32 round spokes a Tufo tub then it will come to a dead halt much more quickly than if I ride my old 1.5kg Aluminium Mavic Cosmic wheel with 16 bladed spokes. Because the difference doesn't seem to vary with speed, you need to tweak constant D only. The difference is because your setup (tyre, pressure, roller contact, machine wear etc.) is different to the original setup used to calculate constants A, B, C, D in the power equation. Now the interesting thing is that your chart shows a ~30W offset between calculated power and measured power. (This is also how an ibike estimates power, but in slightly different form to account for your weight, height gain, barometric pressure and wind speed). A-level physics suggests you would use a cubic function along the lines A*speed ^ 3 + B * speed ^2 + C * speed + D = Calculated Power. Then we come to the correlation between the Powertap reading and calculated power. Whether it's accurate, or has potential to be accurate Garmin watches for running show your pace for 1 mile, and the training program says run X distance at Y pace or time. But you know what - you could do the exact same thing with speed - this is how running training works after all. The only advantage I can see is that power allows you to design a training program around FTP, which simplifies parameterisation of training design. If I can do 40kph for 20 minutes on my trainer, I have improved if I can do 40.1kph. In terms of measuring training effect it's not really any better than measuring speed. I'm a bit mystified about what having a function which converts your speed on the trainer into power is really worth. My goal is to give you guys really accurate data so you can make a decision which Trainer is best to use. I suspect all trainers are not created equal. I'll also need to do this for just about every trainer we support. We're working on building a motor so we can make repeatable test and take out some of the variables. I wanted to publish a bunch of data at once on the site.but I should start blogging/publishing this. I hope to use the site extensively over the coming months, so keep the improvements coming! My HR drifts up quite a bit for workouts over an hour, so with Trainerroad I feel more confident that I am hitting my workout goals. I have used RPE and HRM in combination in the past, but the Virtual Power function makes workout calibration so much simpler.
Now, I was persuaded even without the graph, so I signed up without even seeing it.
I'm sure there are many considerations involved, but that graph should be somewhere on your site (if it isn't). Fatmantis wrote:Nate, that's a persuasive graph.
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