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Maybe not reliable, but it should be consistent. Tyre pressure is my bet.
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How unreliable is virtual power?
"FTP test on new years day : 262w
FTP test Monday(13th FEB) : 338w"The defense rests.
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Woefully innacurate, but pretty consistent.
I've used Virtual Power on rollers for about 18 months and recently switched to using a power meter. FTP has gone from 369w to 290w. It was clear from the start that VP was obviously a huge overestimate (I'm clearly not capable of 5.3w/kg) but 52x11 @ 95rpm produced a steady result in terms of both VP and HR. I always pumped my tyres to the same pressure right before each session too to make sure that wasn't a factor.
So it's a decent training tool I reckon, though it's a lot simpler now I'm using real power.
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@Dry
How unreliable is virtual power?
Here are some numbers;
FTP test on new years day : 262w
Manually adjusted FTP 2 weeks later as workouts were very easy : 275w
FTP test Monday(13th FEB) : 338w
Now, as much as I'd like to believe I'm pushing out 4.7w/kg I'm pretty sure I'm not. I know I tested badly on new years day, and I know I felt really good on Monday, but what else could be going on here? Could my turbo of lost some resistance in the past 6 weeks? Something else changed that could make this much difference? tyre pressure? It doesn't matter too much as I am only currently using power within trainerroad at the moment and won't be able to get a powermeter any time soon.