PowerTap watts vs Computrainer watts

"David Wuertele" <[email hidden]> wrote in message
news:[email hidden]...

Quoted message said:

Recently I took a VO2max test at a local business. I mounted my
powertap-laden canondale on their computrainer, strapped on an HRM and
a gas sensor mask, and off we went.

Soon I noticed that my powertap was reading a significantly different
power output than the computrainer - almost 50% different! The
computrainer was reading 300 watts, but my powertap only read 230.
This was surprising and disconcerting.

I had read somewhere on the internet-thingie that the powertap was
very precise, and I had found some user reviews comparing various
power instumentation products and their readings were all within a few
percent of each other.

Is there a simple and reliable method for me to verify whether my
powertap is accurate?

You should put your PT in torque mode and zero it. My money is on the
computrainer being off. What kind of effort were you putting out. If
230w gets you 23mph then 300w will get you 25mph on a flat road.
What are you capable of?

Phil H

David Wuertele wrote :

Quoted message said:

...almost 50% different! The
computrainer was reading 300 watts, but my powertap only read 230.

50%? Really?

(300 - 230) / [(300 + 230) / 2] = ~26.4%

Arithmetic is hard.
(Apologies to Carl Fogel and Barbie)

On 5 Apr 2005 21:42:54 -0700, "Bad Idea" <[email hidden]>

Quoted message said:

David Wuertele wrote :

Quoted message said:

...almost 50% different! The
computrainer was reading 300 watts, but my powertap only read 230.

50%? Really?

(300 - 230) / [(300 + 230) / 2] = ~26.4%

Arithmetic is hard.
(Apologies to Carl Fogel and Barbie)

Dear B. Idea,

The 300 watt computrainer showed about 30% more power than
the 230 watt powertap:

300/230 = 1.304 = ~ 130% of power tap

The 230 powertap showed about 77% of the computrainer's 300
watts:

230/300 = 0.766 = ~ 77%

Either way, the agreement between the two meters is less
than impressive.

I had been to school most all the time and could spell and
read and write just a little, and could say the
multiplication table up to six times seven is thirty-five,
and I don't reckon I could ever get any further than that if
I was to live forever. I don't take no stock in mathematics,
anyway.

H. Finn

"David Wuertele" <[email hidden]> wrote in message
news:[email hidden]...

Quoted message said:

Recently I took a VO2max test at a local business. I mounted my
powertap-laden canondale on their computrainer, strapped on an HRM and
a gas sensor mask, and off we went.

Soon I noticed that my powertap was reading a significantly different
power output than the computrainer - almost 50% different! The
computrainer was reading 300 watts, but my powertap only read 230.
This was surprising and disconcerting.

I had read somewhere on the internet-thingie that the powertap was
very precise, and I had found some user reviews comparing various
power instumentation products and their readings were all within a few
percent of each other.

Is there a simple and reliable method for me to verify whether my
powertap is accurate?

Dave

I find this link to be very helpful with watt/power training questions:

http://lists.topica.com/lists/wattage/read

I highly doubt that this accounts for the delta that you are seeing but the
harder you push a PT the more it will be off. This is linear and due to the
resistance of the drive train. More watts are lost on the drive train a 20
mph than at 10 mph. Since the power meter for the PT is at the other end of
the drive train it is only measuring watts at the wheel. Once again I don
not think that this totally accounts for the differences you are seeing but
may contribute.

-p

Quoted message said:

On 5 Apr 2005 21:42:54 -0700, "Bad Idea" <[email hidden]>

Quoted message said:

David Wuertele wrote :

Quoted message said:

...almost 50% different! The
computrainer was reading 300 watts, but my powertap only read 230.

50%? Really?

(300 - 230) / [(300 + 230) / 2] = ~26.4%

Arithmetic is hard.
(Apologies to Carl Fogel and Barbie)

Dear B. Idea,

The 300 watt computrainer showed about 30% more power than
the 230 watt powertap:

300/230 = 1.304 = ~ 130% of power tap

The 230 powertap showed about 77% of the computrainer's 300
watts:

230/300 = 0.766 = ~ 77%

Either way, the agreement between the two meters is less
than impressive.

I had been to school most all the time and could spell and
read and write just a little, and could say the
multiplication table up to six times seven is thirty-five,
and I don't reckon I could ever get any further than that if
I was to live forever. I don't take no stock in mathematics,
anyway.

H. Finn

FYI:
Nobody here believes that you don't know the formula for percent
difference. And you missed my obvious problem with significance (glad
I indicated approximation).

Did you calibrate the computrainer? Your suppose to warm them up for about
15 minutes and then run it through a calibration procedure each time you
ride it. My guess is the computrainer was off.

"David Wuertele" <[email hidden]> wrote in message
news:[email hidden]...

Quoted message said:

Recently I took a VO2max test at a local business. I mounted my
powertap-laden canondale on their computrainer, strapped on an HRM and
a gas sensor mask, and off we went.

Soon I noticed that my powertap was reading a significantly different
power output than the computrainer - almost 50% different! The
computrainer was reading 300 watts, but my powertap only read 230.
This was surprising and disconcerting.

I had read somewhere on the internet-thingie that the powertap was
very precise, and I had found some user reviews comparing various
power instumentation products and their readings were all within a few
percent of each other.

Is there a simple and reliable method for me to verify whether my
powertap is accurate?

Dave

"Pippen" <[email hidden]> wrote in message
news:[email hidden]...

Quoted message said:

"David Wuertele" <[email hidden]> wrote in message
news:[email hidden]...

Quoted message said:

Recently I took a VO2max test at a local business. I mounted my
powertap-laden canondale on their computrainer, strapped on an HRM
and
a gas sensor mask, and off we went.

Soon I noticed that my powertap was reading a significantly different
power output than the computrainer - almost 50% different! The
computrainer was reading 300 watts, but my powertap only read 230.
This was surprising and disconcerting.

I had read somewhere on the internet-thingie that the powertap was
very precise, and I had found some user reviews comparing various
power instumentation products and their readings were all within a
few
percent of each other.

Is there a simple and reliable method for me to verify whether my
powertap is accurate?

Dave

I find this link to be very helpful with watt/power training
questions:

http://lists.topica.com/lists/wattage/read

I highly doubt that this accounts for the delta that you are seeing
but the harder you push a PT the more it will be off. This is linear
and due to the resistance of the drive train. More watts are lost on
the drive train a 20 mph than at 10 mph. Since the power meter for the
PT is at the other end of the drive train it is only measuring watts
at the wheel. Once again I don not think that this totally accounts
for the differences you are seeing but may contribute.

So which end of the drive train is the computrainer at? If anything, the
computrainer fails to capture more losses than the PT.

Phil H

"Philip Holman" <[email hidden]> wrote in message
news:[email hidden]...

Quoted message said:

"Pippen" <[email hidden]> wrote in message
news:[email hidden]...

Quoted message said:

"David Wuertele" <[email hidden]> wrote in message
news:[email hidden]...

Quoted message said:

Recently I took a VO2max test at a local business. I mounted my
powertap-laden canondale on their computrainer, strapped on an HRM and
a gas sensor mask, and off we went.

Soon I noticed that my powertap was reading a significantly different
power output than the computrainer - almost 50% different! The
computrainer was reading 300 watts, but my powertap only read 230.
This was surprising and disconcerting.

I had read somewhere on the internet-thingie that the powertap was
very precise, and I had found some user reviews comparing various
power instumentation products and their readings were all within a few
percent of each other.

Is there a simple and reliable method for me to verify whether my
powertap is accurate?

Dave

I find this link to be very helpful with watt/power training questions:

http://lists.topica.com/lists/wattage/read

I highly doubt that this accounts for the delta that you are seeing but
the harder you push a PT the more it will be off. This is linear and due
to the resistance of the drive train. More watts are lost on the drive
train a 20 mph than at 10 mph. Since the power meter for the PT is at the
other end of the drive train it is only measuring watts at the wheel.
Once again I don not think that this totally accounts for the differences
you are seeing but may contribute.

So which end of the drive train is the computrainer at? If anything, the
computrainer fails to capture more losses than the PT.

Phil H

I'm not familiar with the Computrainer, my comments were directed at the PT
(Power Tap). I made the assumption that the Computrainer was accurate.

-p

Quoted message said:

Did you calibrate the computrainer? Your suppose to warm them up
for about 15 minutes and then run it through a calibration procedure
each time you ride it. My guess is the computrainer was off.

The technician doing the VO2max test calibrated the computrainter, and
it was probably 10 to 15 minutes after I started warming up on it. I
don't know what she did to calibrate it and whether she did it right.

Dave

Philip> So which end of the drive train is the computrainer at? If
Philip> anything, the computrainer fails to capture more losses than
Philip> the PT.

The rear wheel of the bike rides on the computrainer, a stationary
resistance trainer. The PT sits in the rear hub. So if drivetrain
losses were at fault, the computrainer should show more losses AKA a
lower power output than the PT. But I saw the reverse, so the
differences can't be accounted for by drivetrain losses.

"David Wuertele" <[email hidden]> wrote in message
news:[email hidden]...

Quoted message said:

Philip> So which end of the drive train is the computrainer at? If
Philip> anything, the computrainer fails to capture more losses than
Philip> the PT.

The rear wheel of the bike rides on the computrainer, a stationary
resistance trainer. The PT sits in the rear hub. So if drivetrain
losses were at fault, the computrainer should show more losses AKA a
lower power output than the PT. But I saw the reverse, so the
differences can't be accounted for by drivetrain losses.

My question was rhetorical. The computrainer probably has to be
calibrated or estimates tire rolling resistance because it cannot
measure it. If tire pressure and force against the roller changes then
the reading will be off. This is directly measured by the PT. My money
is on the PT being more accurate.

Phil H