Power Meter and Turbo Trainer wattage differences

hf4z6sqo7vjngi · 11-10-2011 09:16AM #1

Just interested to see if anyone who owns a power meter has carried out a test to see how accurate the readings are on their turbo trainer when they compare it to the power meter?

I had a 60 minute easy zone 1 session focussing on a decent cadence this morning. The power meter is a power to max and the turbo trainer is a Tacx Fortius VR. Turbo ran for 5 minutes prior and calibrated to zero.
I have often heard that the turbo can over estimate power by as much as 10% but was surprised to see a difference of nearly 20%. Sucks to realise you are not putting out as much watts as you trained with last year before the power meter arrived:o

Any way the stats.
Power Meter: 124avg/125 np
Turbo trainer: 154 avg
Difference: 19.5%
Cadence Power Meter: 94rpm
Cadence Turbo: 89rpm

Beasty · 11-10-2011 09:22AM

I did a comparison last year - my Tacx Fortius was within 5% of the PowerTap (although the Tacx did do a lot more smoothing)

tunney · 11-10-2011 10:21AM

My Elite hydroforce with the power "measurement" was within 10% AFTER you held a constant wattage for 1-2 minutes, i.e. slow to respond. I found that once I started using my SRMs that on my intervals I was cheating. I was taking micro-breaks without knowing it. The Turbo never displayed it so I didn't know it (<0.25 sec breaks)

My computrainer (fycking heart rate gone again) is 7% off my SRMS. However I can calibrate my CT to match my SRMs, just need an hour and a helper (putting it off for the last year).

Turbos lie.

Murph100 · 11-10-2011 10:37AM

I managed to get my Tacx Fortius consistently reading within a couple of watts of a PowerTap, as Beasty says, the Fortius does a lot more smoothing.

To get it that close, you have to follow a set procedure, something like this :

Disengage turbo brake from wheel & pump tyre to 110psi ( I use the Tacx tyre as it slips less ).

Unwind the turbo brake adjuster and engage the brake. Turn the adjuster until the brake just touches the tyre. Disengage the brake and turn the adjuster wheel 3 complete turns ( can't remember if its CW or ACW, which ever makes it tighter on the tyre anyway, put some Tippex on the adjuster so you have a pointer ). Engage brake.

Warm up for 20 mins @ 140 Watts or more ( 5 mins is not enough ).

Do brake calibration ( Ensure you are sitting in the same position as you intend to train ! Keep the weight distribution the same. )

If you follow this routine, you should get calibrations values of 0.0 or 0.1 pretty much all the time. You're good to go if you do.

This all sounds very long winded, but in practice it take only 30 secs to pump the tyre and set the brake adjuster correctly.

hf4z6sqo7vjngi · 11-10-2011 11:29AM

Beasty wrote: »

I did a comparison last year - my Tacx Fortius was within 5% of the PowerTap (although the Tacx did do a lot more smoothing)

Smoothing would have a part to play alright but generally they seem to be just off, 5% is probably as good as it gets.

tunney wrote: »

My Elite hydroforce with the power "measurement" was within 10% AFTER you held a constant wattage for 1-2 minutes, i.e. slow to respond. I found that once I started using my SRMs that on my intervals I was cheating. I was taking micro-breaks without knowing it. The Turbo never displayed it so I didn't know it (<0.25 sec breaks)

My computrainer (fycking heart rate gone again) is 7% off my SRMS. However I can calibrate my CT to match my SRMs, just need an hour and a helper (putting it off for the last year).

Turbos lie.

Yeah i am sure tweaking the turbo i could get it closer than 20% but it would mean moving away from a 0.1 calibration to something like a 1.5 or something.
The fact is at 0.1 after calibration would mean a lot of people think they are hitting watts that they simply are not. As you said turbos lie!!

Murph100 wrote: »

I managed to get my Tacx Fortius consistently reading within a couple of watts of a PowerTap, as Beasty says, the Fortius does a lot more smoothing.

To get it that close, you have to follow a set procedure, something like this :

Disengage turbo brake from wheel & pump tyre to 110psi ( I use the Tacx tyre as it slips less ).

Unwind the turbo brake adjuster and engage the brake. Turn the adjuster until the brake just touches the tyre. Disengage the brake and turn the adjuster wheel 3 complete turns ( can't remember if its CW or ACW, which ever makes it tighter on the tyre anyway, put some Tippex on the adjuster so you have a pointer ). Engage brake.

Warm up for 20 mins @ 140 Watts or more ( 5 mins is not enough ).

Do brake calibration ( Ensure you are sitting in the same position as you intend to train ! Keep the weight distribution the same. )

If you follow this routine, you should get calibrations values of 0.0 or 0.1 pretty much all the time. You're good to go if you do.

This all sounds very long winded, but in practice it take only 30 secs to pump the tyre and set the brake adjuster correctly.

I may have not made it clear in my OP but my calibration was at 0.1 (always check before each ridE) and still 20% off so i am just raising awareness for those without a PM that the numbers a turbo puts out should not hold much weight.

Murph100 · 11-10-2011 11:44AM

Gimme a break, it was just before my second & most important espresso !!

Your warm up time of 5 mins is not enough, if you did a calibration after 20 mins I betcha you'd get a substantially different result.

Have a look on the Tacx forum, its a bloody mess I know, but there are some good posts about this.

Madilyn Long Banker wrote: »

I may have not made it clear in my OP but my calibration was at 0.1 (always check before each ridE) and still 20% off so i am just raising awareness for those without a PM that the numbers a turbo puts out should not hold much weight.

canhascheez · 11-10-2011 11:45AM

Madilyn Long Banker wrote: »

The fact is at 0.1 after calibration would mean a lot of people think they are hitting watts that they simply are not. As you said turbos lie!!

See the A/R/T forum

tunney · 11-10-2011 11:47AM

Furthermore unless you use a trainer tyre if you do intervals then the heat of the tyre will change and that will change the rolling resistance and thereby the wattage.

ideally you are recalibrating multiple times during a session but I believe that the tacx doesn't really allow this easily.

hf4z6sqo7vjngi · 11-10-2011 02:15PM

Tacx forum......shudder:) I will mess about with it next time to see if i can get the turbo to match closer to the PM.

The Tacx is not great for calibrating multiple times during a session as you have to come out of the session and then go back in and start a new session once the calibration is complete.
You would spend more time on/off the bike than actual training:)

Murph100 · 11-10-2011 02:44PM

Yeah, that forum is some beaut alright :rolleyes:

Multiple calibrations would drive you nuts, thats why I just do the one after 20 mins an leave it at that, I sometimes check the calibration at the end just out of curiosity, and it seems to hold, but definitely it aint an exact science.

Madilyn Long Banker wrote: »

Tacx forum......shudder:) I will mess about with it next time to see if i can get the turbo to match closer to the PM.

The Tacx is not great for calibrating multiple times during a session as you have to come out of the session and then go back in and start a new session once the calibration is complete.
You would spend more time on/off the bike than actual training:)

Numbfeet · 24-04-2013 11:07PM

Just a quick query relating to power. I run a powertap on a cycleops built rear wheel. My power on the road seems to err on the high side compared to on the turbo trainer with a similar effort and HR. For instance doing 100% FTP intervals on the road seems far less taxing than on the turbo. I have a tacx booster turbo. To confirm my suspicion, i did a cp20 test on the turbo..283w avg for 20 min, but 2 weeks later during a local league race, the power stats showed a max avg 20 min of 312w, a big difference considering i wouldnt have maxed out. Im thinking it might have something to do with the turbo rolling resistance, it seems quite high even on the big bike cogs (and the lowest turbo resistance setting). Has anyone had a similar problem??
D

kumate_champ07 · 24-04-2013 11:29PM

the cooling effect of cycling outside would be the cause of that. cars usually perform better with an engine cooling mechanism
you will also put out less watts if you use no fan indoors.

kumate_champ07 · 24-04-2013 11:31PM

also when youre actually racing you will push yourself more, its difficult to 'race' on an indoor trainer

Numbfeet · 25-04-2013 12:00AM

Thanks! The cooling effect did cross my mind, and I don't use a fan indoors, though I did try out the back garden on a cool day. It makes sense but it means resetting the power on the garmin depending if I'm training in or out.

Beasty · 25-04-2013 08:14AM

Do you exclude zeros in your power averages? - this could make a massive difference, as usually there is no let up on the turbo but plenty of freewheeling opportunities on the road (even more so when riding in groups - I find it a lot easier to record higher power averages even including zeros when riding in a group or racing)

Lumen · 25-04-2013 09:14AM

The heat stress of using a turbo indoors provides excellent adaptation for racing in Ireland.

hf4z6sqo7vjngi · 25-04-2013 09:23AM

Beasty wrote: »

Do you exclude zeros in your power averages? - this could make a massive difference, as usually there is no let up on the turbo but plenty of freewheeling opportunities on the road (even more so when riding in groups - I find it a lot easier to record higher power averages even including zeros when riding in a group or racing)

+1 on this, make sure the head unit is set to ZAP to include zeros as excluding zeros could easily overstate by 20w+ onto a spin outdoors with free wheeling etc.
I would expect to see performance decline by up to 5% indoors though, this is depending on what your "pain cave" setup is:) Get yourself a decent fan!!

Numbfeet · 25-04-2013 09:34AM

Not sure about that, ill go into the settings(forerunner 910xt). It should be on the default settings. Thanks

tunney · 25-04-2013 09:41AM

Madilyn Long Banker wrote: »

+1 on this, make sure the head unit is set to ZAP to include zeros as excluding zeros could easily overstate by 20w+ onto a spin outdoors with free wheeling etc.
I would expect to see performance decline by up to 5% indoors though, this is depending on what your "pain cave" setup is:) Get yourself a decent fan!!

More NZAP and NP are very similar in my experience.

Numbfeet wrote: »

Not sure about that, ill go into the settings(forerunner 910xt). It should be on the default settings. Thanks

Default is the "pussy mode" (NZAP)

BennyMul · 25-04-2013 10:07AM

stupid question, why would you care (apart from simple knowledge if this is the case stop reading

) if the Turbo power is greater or less than the PM,

should you not be using the PM for monitoring power?

tunney · 25-04-2013 10:08AM

BennyMul wrote: »

stupid question, why would you care (apart from simple knowledge if this is the case stop reading ) if the Turbo power is greater or less than the PM,

should you not be using the PM for monitoring power?

Peoples power outputs on the turbo and on the road can vary ALOT but using the same power meter.

BennyMul · 25-04-2013 10:12AM

tunney wrote: »

Peoples power outputs on the turbo and on the road can vary ALOT but using the same power meter.

I agree but then why introduce anothere variable in to the equation and really cause confusion?

Do you recomened dual testing Road\Turbo to set zones for each ?

tunney · 25-04-2013 10:27AM

BennyMul wrote: »

I agree but then why introduce anothere variable in to the equation and really cause confusion?

Do you recomened dual testing Road\Turbo to set zones for each ?

Who is introducing another variable? Its not a choice, its a fact.

BennyMul · 25-04-2013 10:47AM

If you have a PM that is reliable and repeatable why introduce the variable of power from a turbo or use this as your reference point as long as its calibrated to be repeatable. (you have an SRM which is % accurate why look at power from the computrainer?)

that is my question, sorry

hf4z6sqo7vjngi · 25-04-2013 12:41PM

BennyMul wrote: »

stupid question, why would you care (apart from simple knowledge if this is the case stop reading ) if the Turbo power is greater or less than the PM,

should you not be using the PM for monitoring power?

I don't any more, check the date when the thread was originally started:). PM numbers all the way, could not care less what numbers the turbo is saying.

tunney · 25-04-2013 01:01PM

BennyMul wrote: »

If you have a PM that is reliable and repeatable why introduce the variable of power from a turbo or use this as your reference point as long as its calibrated to be repeatable. (you have an SRM which is % accurate why look at power from the computrainer?)

that is my question, sorry

He wasn't talking about that at all.

For some people if they do a T20 on the road they get one number. A T20 on the turbo often gives up to 20% less. With the power figures read from the same power meter.

The difference is not the power is read from the turbo inside and from the power meter outside but that the person does not generate the same on a turbo as they do outdoors.

BennyMul · 25-04-2013 01:22PM

tunney wrote: »

He wasn't talking about that at all.

For some people if they do a T20 on the road they get one number. A T20 on the turbo often gives up to 20% less. With the power figures read from the same power meter.

The difference is not the power is read from the turbo inside and from the power meter outside but that the person does not generate the same on a turbo as they do outdoors.

Sorry read the thread wrong and makes sense now. I knew it was going to be a bad day when I woke up.

Numbfeet · 25-04-2013 07:47PM

It turns out the default is to include zeros, at least thats what the head unit has always been set at.
I bought a power meter just 3 months ago, just as a tool to monitor my progress more accurately and train a bit more specifically, so far the data has been helpfull. Think i need to get a cooling fan to balance things out a bit more. Thanks for all the views and advice.d

Power Meter and Turbo Trainer wattage differences

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