"The 33"-Road Bike Racing - calculating watts
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I just bought a Tacx Flow trainer and it reads watts, cadence, etc.
How does it calculate watts? It seems to me that the wattage output I get is sometimes unrelated to the effort I seem to be expending.
For instance, I assume it measures watts by interpreting the effort to turn the roller compared to how fast I am turning it (or pedalling). However, it has no idea what gear I'm in ...will this effect the measurement?
I'd just like to know how Watts are calculated and how reliable the reading I'm getting is. As I said, it seems to be more related to cadence than 'effort' as far as I can tell.
02-19-07, 09:45 AM
Yeah, it's resistance and speed of the roller. The gear you're in has no bearing on how it would be calculated -- although it will affect the accuracy of the reading due to varying levels of efficiency in different gears.
02-19-07, 09:48 AM
It's a fluid trainer right?
If you're in a bigger gear, you're going to put out more effort provided you've got everything set up right...
I'll go out on a limb here and guess that it estimates your wattage based on how fast you're turning the drum, as the resistance is known - so the faster you turn against that resistance, the more wattage you're putting out.
02-19-07, 10:13 AM
unless it's one of those extremely expensive pieces of lab equipment used for indoor testing, id be very skeptical of the wattage readings of those indoor trainers. not to rain on your parade or anything. but i tried out two different indoor trainers that gave power readings, and they gave me an average difference of like 50 watts. and on top of that, when i got my PT i saw how truely inaccurage they were.
ur trainer is still prob good for setting levels to train at, but i wouldnt rely on it for your actual power output.
02-19-07, 10:35 AM
Here's the formula supplied by Kurt Kinetics for other trainers on the market. Unfortunately, Tacx is not one of them:
02-19-07, 10:51 AM
And, here's what KK does to come up with their power curves:
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