calculating watts on a trainer
 

Hi
I just got a Tacx Flow trainer that measure Watts. Can anyone tell me just how it does measure Watts output.

It seems to me that it measures Watts more based on my cadence than anything else. It can't know what gear I'm in (presumably higher gear means harder pedalling means higher Watt output) so how can it accurately measure Watts...and how accurately does it?

thanks for any help.

I don't know about your meter, but PowerMeters use strain gauges and RPM.

Power is constant across gear/chain/belt connected shafts (neglecting friction), so you can measure the RPM and torque anywhere - at the ring gear, at the cassette/hub of the wheel, or at the trainer. Multiply the torque and the shaft speed and you have power.

Your sig is uncannily appropriate.

Check the answer in your other thread:

http://www.bikeforums.net/showthread.php?t=270448

You have no idea *how* much it's appropriate. Look it up and be ashamed.

I don't know much about your trainer, but it might be calibrated so's it knows that it takes X power to turn the roller at Y speed when Z resistance is dialed in. If there's no resistance adjustment, the #'s are even easier.


This gives me a flash of inspiration: Borrow a PT-equipped bike (wheel would be better) and ride your rollers or trainer on it. You should be able to find out power levels required to run it at different speeds - calibration, if you will. Fatal flaw: resistance may vary with temperature (or humidity, or phase of the moon, etc...)

Tacx trainers calculate power using a set of curves giving power as a function of resistance setting and roller speed. Most of these trainers allow for some calibration of the power calculation. Some people have done some tests with PowerTap and SRM, and found that in steady speed conditions, the calculation is pretty good (i.e. within 10% of that measured on PowerTap or SRM). See the Tacx forum, there are a few threads on the subject. There is a calibration procedure involving timing the spin-down from a specific speed; I don't know how much that helps accuracy.

Any transient measurements (i.e. accelerating or slowing down) will be wrong, since the calculation assumes steady conditions. So accelerations will be under-estimated, and decelerations over-estimated. The typical example of this is hitting a slope on a Virtual Realilty trainer: the initial reading is very high power, since you are going fast, and the resistance is high, regardless of the fact that your speed is dropping like a rock.

No matter how precise or imprecise it is, it is repeatable (as long as you maintain the same roller pressure, and the same tire pressure), so that you can compare your progress, e.g. measure how much HR you need for a given power output, and see the evolution with your fitness level.