Anybody clever enough to work this out?
Are power curves for trainers accurate? Yes and you can find multiple examples from people who have compared power readings calculated from speed (speed meaning the number times your wheel goes around within a certain amount of time since you aren’t actually moving) on a trainer versus readings from a power meter. In fact a good formula can result in “calculated power” being within a percent or two of “recorded power.” Considering that different power meters can vary by this much, or more, if you already have a trainer you actually already have a power meter – you just need to know the speed to power conversion formula.
Two main factors determine accuracy: 1) the formula you are using; and 2) controlling the variables still present even when using a trainer. The variables are things like clamping pressure, tires, and tire psi. If you keep those variables consistent from ride to ride by always clamping your bike in the trainer the same way every time and keeping your tire pressure consistent, and you use the same formula each time, you have an accurate way to measure your progress even if your formula isn’t technically accurate. If it’s off by 25% for your first ride it will still be off 25% for your 100th ride so you can still measure progress. If you also have an accurate formula you can compare yourself to others if that is important to you.
The formula: You might be able to find the formula for your trainer online. If you can, and use the same formula each time to determine your power, then you can train with power. The published formula for my cheapo mag trainer was pretty far off from what I ended up measuring with a power meter. If you want something more accurate for you, your tires, your setup, etc. then find a power meter to borrow or rent. Do a trainer ride, download your speed and power data into excel, put it in a scatter chart, and have excel calculate the power curve for you under the “trendline” options in excel. Generally you want a third order polynomial, but some trainers, like mag trainers, often have a linear “curve.” You can look at the scatter chart and probably figure out what kind of curve you need and excel can calculate each one for you pretty much instantly.
Once you have your formula you need a way to look at it. Here are some options:
Option 1 – Trainerroad.com: As long as you have an Ant+ speed and cadence sensor and an Ant+ USB stick for your computer you can use this service which provides a lot of great information, in a very usable and stable format. To give you an idea of stability, my wife accidentally unplugged my very old laptop that I was using during a workout. It is so old the battery holds a charge for about 10 seconds. When I restarted the laptop and opened up my workout it was still there recorded as if I had just stopped and saved my workout. They have grown quickly and have a lot of plans for the future to make their service even better including adding things like group rides over the internet. I was looking at Tacx VR trainers and computrainers when trainerroad.com went live and I really lost interest in looking at other options once I started using it. If your trainer isn’t on their list the developers are very responsive and will probably add it for you and if you can borrow a power meter and tell them what trainer you are using they will likely calculate the power curve for you. Plus you get a ton of workouts to do in a way that makes your training progress much more objective and the Sufferfest video integration is very cool. Many of the workouts have on screen tips as you do a workout and there are “plans” available and being developed that pick workouts for you to do on a schedule. In my opinion this is by far the best option out there for indoor training even if you have a power meter and especially if you want to use calculated power from a trainer's power curve.
Option 2 – Paper and Pen: Use your formula and calculate power for speed increments like 5 mph is X power, 10 is Y power, 15 is Z power, etc., write it down and put it on your stem or somewhere else where you can see it. Just look at your speedometer and your chart and you know how much power you are generating (unless you can calculate third order polynomials in your head on the fly in which case just memorize your formula.)
Option 3 – Excel: After the ride download your speed information, assuming you have something that allows you to do that like a Garmin, and calculate the power for each speed recorded. You can then use excel to create charts, calculate intervals, or whatever. Of course that is all after the fact rather than real time and what a pain in the ass.
Option 4 – Golden Cheetah: Golden cheetah has some power curves for a few trainers in the software. If your trainer is on their list, and you can figure out how to get it to work, it will let you display a ton of data on the computer in real time while you train. The downside is you can’t add your own power formula and there are only a few trainers listed. The other big downside is the complexity of the software and the lack of any Ant+ interface that lets you know if you are paired correctly. But it is also extremely customizable and you can create your own workouts to do. You can even use Google streetview to re-ride rides that have GPS data. Then you can do post ride analysis of your data for hours to figure out how to get better (instead of riding more to get better).
Option 5 – Sporttracks: Sporttracks has two plugins you can use. One is TrainerPower where you can have the software, after your ride, calculate the power for the ride. You can use one of very many power curve formulas already in the plugin and can add your own custom formula if you think yours is more accurate. Once you apply the plugin to your ride the software treats the power data exactly the same as if it had been recorded with a power meter and provides a wealth of information for post-ride analysis. There is also now a live recording plugin that works with Ant+ devices including power meters to show your data live as you do the ride. According to the developer of the live recording plugin on the Sporttracks forum a release that allows the plugin to integrate with the TrainerPower plugin is imminent so you can show real time calculated power. If all you want is to see your data on the screen and then use SportTracks to analyze it post-ride then this looks to be a very good low cost option (as soon as the TrainerPower integration is live.)
I hope this helps at least one person figure what options are best for them.
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