Watts on rollers vs trainer

[Topangarider]

Hi- I recently did some rides inside on my tacx flow and powertap wheel and my zone 2 seemed to take much more effort than on the road - higher HR and perceived effort to achieve same watts. Then I put the bike on my rollers and averaged 25-30 watts higher with same/less perceived effort.
Any ideas why this is the case?

 

[SirLes]

can't answer your question I'm afraid but a question in return: How close is the Tacx Flow power measurement to the power tap one?

 

[Martin318is]

can't answer your question I'm afraid but a question in return: How close is the Tacx Flow power measurement to the power tap one?

I can tell you from past experience that unless you are regularly calibrating the two, a PowerTap and a Tacx Fortius can be out by as much as 25W or more compared to each other.

That may have something to do with it.

 

[Alex Simmons/RST]

Any ideas why this is the case?

http://alex-cycle.blogspot.com/2009/01/turbocharged-training.html

 

[Topangarider]

I can tell you from past experience that unless you are regularly calibrating the two, a PowerTap and a Tacx Fortius can be out by as much as 25W or more compared to each other.

That may have something to do with it.

Thanks. The Tacx power meter is useless. I only look at the powertap. I wish I had known how bad the power function is on Tacx before I purchased.

 

[Topangarider]

http://alex-cycle.blogspot.com/2009/01/turbocharged-training.html

Great article. Thanks!

 

[Tapeworm]

Thanks. The Tacx power meter is useless. I only look at the powertap. I wish I had known how bad the power function is on Tacx before I purchased.

If I calibrate the Tacx Fortius before each ride it usually runs within 0-5watts of the SRM for values up to 500watts. Above that it seems to get sketchy. Not that I am really ever above these values

 

[SirLes]

If I calibrate the Tacx Fortius before each ride it usually runs within 0-5watts of the SRM for values up to 500watts. Above that it seems to get sketchy. Not that I am really ever above these values

So does that mean any error in the Tacx is random and varies from ride to ride?

I didn't think they would be necessarily close to the true value but had assumed that the majority of error would be systematic, allowing for reasonable comparisons with previous rides in order to measure possible improvement.

If the random error is somewhere near 10% then it becomes pointless as the uncertainty would be greater than any likely improvement- at least in the short to medium term.

I have been considering getting a turbo with power measurement. Not to accurately measure my power (ie find the true value) but in order to (hopefully) see some measurable improvement to aid motivation on long winter evenings.

 

[Tapeworm]

Not random as such, given the Fortius derives it's reading from an interaction between the roller and wheel, hence things like temperature, tyre pressure, force of roller on the tyre etc will affect reading. So for a given effort the difference in avg values are either spot on or a few watts off. The better the calibration the more consistent the results.

Given that all power meters have a margin of error this needs to be considered for any power reading.

Personally, I consider power to be a guide, not a definitive.

 

[Topangarider]

So does that mean any error in the Tacx is random and varies from ride to ride?

I didn't think they would be necessarily close to the true value but had assumed that the majority of error would be systematic, allowing for reasonable comparisons with previous rides in order to measure possible improvement.

If the random error is somewhere near 10% then it becomes pointless as the uncertainty would be greater than any likely improvement- at least in the short to medium term.

I have been considering getting a turbo with power measurement. Not to accurately measure my power (ie find the true value) but in order to (hopefully) see some measurable improvement to aid motivation on long winter evenings.

Then don't get a tacx unless you will ride with same tire, same level of wear on the tire, same tire pressure, and you will leave your bike on the trainer.

 

[SirLes]

Then don't get a tacx unless you will ride with same tire, same level of wear on the tire, same tire pressure, and you will leave your bike on the trainer.

"Not random as such, given the Fortius derives it's reading from an interaction between the roller and wheel, hence things like temperature, tyre pressure, force of roller on the tyre etc will affect reading. So for a given effort the difference in avg values are either spot on or a few watts off. The better the calibration the more consistent the results.

Given that all power meters have a margin of error this needs to be considered for any power reading.

Personally, I consider power to be a guide, not a definitive." Tapeworm
(Don't understand how you quote two different posts!)

Thanks for both those comments. That is the sort of thing I was hoping/expecting for.

Not decided yet but I think many of the variables mentioned can be controlled to give an uncertainty that is acceptable. It does mean taking care when setting up the bike and recording the variables.

in simple terms it also gives another thing to look at and think about to take ones mind off the pain!