Hi,
I’ve added Wahoo cadance and speed sensors to my non-smart spin bike on the IOS version of the app on my iPhone 13. The speed feeding to Strava from TrainerDay doesn’t reconcile with the speed on my bike - approx 50% higher from TrainerDay.
I note that the Wahoo app has the option of adjusting the wheel circumferance to calibrate the speed sensor. Can this be done in the TrainerDay app?
Thanks,
A speed sensor on a trainer bike makes no sense. If you are using ERG mode and put constantly e.g. 150W and you are on a small/easy gearing, Trainer resistance will be higher, than on a higher gearing. Because of that, speed of the wheel depends on gearing. And as I know, TD calculate virtual speed as you would driving on a flat good tarmac street outside, so it will differ what you see from your speed sensor and later in strava.
Sorry, didn‘t read that you have a non-smart bike. There the resistance is constant, and the power depends on gearing. So you can simulate with your gearing the „virtual slope“. So you can imagine, if you put same power output on a steep hill you are slower than you would be on the flat. It all depends on your gearing and the resistance of your trainer.
We don’t take speed from the speed sensor, we convert watts to speed. So in this case it is funny speed>watts>speed. So wheel size does not matter to our app.
As I am sure you are aware the speed of your wheel inside the house, maybe totally irrelevant to outdoor speed. Now with good fans inside if your heart rate is similar for speed indoor and outdoor that means your indoor speed is semi-accurate. In this case there is a problem with the virtual power curve you selected in our app. If you don’t care about power accuracy and only speed accuracy we have an adjustment you can do to lower your speed coming from our app.
But this will mostly affect higher speeds since it is drag coefficient. Getting the virtual power closer to accurate would be a more accurate method.
Thanks, that makes sense.
I don’t think this makes sense at all…
In order to calculate watts based on a known resistance setting of a dumb trainer, you absolutely have to know the circumstance of the wheel. A larger wheel will take more power to rotate once than a smaller wheel, purely by the fact that it has a greater circumference and therefore causes the flywheel to have more revolutions.
It strikes me that speed is irrelevant in turbotraining, and all the workouts are using power/watts as a target. This absolutely needs the wheel circumference, and indeed this used to be a setting in trainer day. No idea why you got rid of it.
No we never had wheel circumference… trust me on that one 
So technically you are right, with speed to power in a perfect world wheel size matters. Most are assumed 700, which in most cases is close enough… now different tire sizes and if you are riding old 26” mtb or a bmx sure it makes even a bigger difference.
But from a more practical sense dumb trainers to power are not very accurate from an TRUE watts perspective but when configured ok can be fine from a relative stand point. So you might have a 200w ftp instead of a highly accurate power meter of 220w ftp but true accuracy does not really matter from a training stand point indoors. You still train at a percent of your FTP which is factored based on taking a test with that trainer and our app, and do a spin down and pick the right curves. Now if you are trying to compare different apps, and indoor outdoor it can be different but high accuracy and consistency from a wheel on trainer is fairly unlikely. So yes from a purist stand point you are correct, but I would suggest purist should probably not ride wheel on trainers and expect accuracy without a quality power meter.
