CT vs PT

[Jeff Anderson]

Hi

I've been learning a lot on this group...some more than I want to know. So
here's my question....and I know some threads talk to this a bit.

I do a CT class 1x per week. Same bike. I also use my PT on the bike so I
have the data from the workout.

We calibrate the CT's at the start - once at start and once after the warmup
as the CT changes as it gets warm. We calibrate them to about 200.

Two *different *CT's

#1 - Did a 3 mi TT - PT said 3.189 miles and power on PT was about 20 watts
higher than CT
#2 - Did a 5 mi TT - PT said 5.25 miles and power on PT was about 25-40
watts lower than CT.

Is this common? How does one align things? Or do I just shutup and do the
workout? 8^)

Regards

Jeff Anderson
***@gmail.com
703.868.8705

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[Greg Steele]

I do not use the CT recordings for loggin purposes. I always use my
srm(s) since that is what I use on the road/dirt.

For the record, the powertap (and SRM) SHOULD be higher than the
computrainer since it is "upstream" of the CT, with tire rolling
resistance (and drivetrain) friction adding to thier numbers.

There is much discussion in the archives about how to tweak your CT to
"match" if you search. But I just leave it as is and use the power tool
I already have. This makes the CT a very expensive computerized
trainer, but if you ride it as much as I do, it is worth it.

g

Post by Jeff Anderson
Hi
I've been learning a lot on this group...some more than I want to know.
So here's my question....and I know some threads talk to this a bit.
I do a CT class 1x per week. Same bike. I also use my PT on the bike so
I have the data from the workout.
We calibrate the CT's at the start - once at start and once after the
warmup as the CT changes as it gets warm. We calibrate them to about 200.
Two *different *CT's
#1 - Did a 3 mi TT - PT said 3.189 miles and power on PT was about 20
watts higher than CT
#2 - Did a 5 mi TT - PT said 5.25 miles and power on PT was about 25-40
watts lower than CT.
Is this common? How does one align things? Or do I just shutup and do
the workout? 8^)
Regards
Jeff Anderson
703.868.8705

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[rmurphy]

well I'd first ask if you were sure you properly zeroed your PT on
both workouts - that has failed to happen in the past I hear ;-).
Otherwise, if you're putting out around 300W or so, that range in CT
vs. PT power is quite high - perhaps 15% across the two different
CTs.

IME that's quite a large variation - not one that I've experienced
here except for an old Nintendo CT that *was* reading about 10-15%
high. If they're all new/newish units. I would honestly be surprised
to see any deviating that much. 1-2% sure, 5% maybe, 15% - no,

Re the difference in distance, have you checked your wheel rollout
lately? In any case, it's very much secondary to proper power
readings.

Hi
I've been learning a lot on this group...some more than I want to know.  So
here's my question....and I know some threads talk to this a bit.
I do a CT class 1x per week.  Same bike. I also use my PT on the bike so I
have the data from the workout.
We calibrate the CT's at the start - once at start and once after the warmup
as the CT changes as it gets warm.  We calibrate them to about 200.
Two *different *CT's
#1 - Did a 3 mi TT - PT said 3.189 miles and power on PT was about 20 watts
higher than CT
#2 - Did a 5 mi TT - PT said 5.25 miles and power on PT was about 25-40
watts lower than CT.
Is this common?  How does one align things?  Or do I just shutup and do the
workout?  8^)
Regards
Jeff Anderson
703.868.8705

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[Jeff Anderson]

I do not and did not zero the PT before each CT session.....I gather folks
zero their PT before every ride. Yes I've seen the emails on this....

3 mi TT was NP of 299 - I can't recall CT avg?
5 mi TT was NP of 270 - I think it said 309 avg.

I have not "rolled out my wheel' either. Tires are standard 700x23 w/ about
110 in them.

Both CT's are older units....

I just go by the PT - we don't use CT to capture data...just to provide 8
riders a chance to train together on same course at same time w/ same
workout

Regards

Jeff Anderson
***@gmail.com
703.868.8705

Post by rmurphy
well I'd first ask if you were sure you properly zeroed your PT on
both workouts - that has failed to happen in the past I hear ;-).
Otherwise, if you're putting out around 300W or so, that range in CT
vs. PT power is quite high - perhaps 15% across the two different
CTs.
IME that's quite a large variation - not one that I've experienced
here except for an old Nintendo CT that *was* reading about 10-15%
high. If they're all new/newish units. I would honestly be surprised
to see any deviating that much. 1-2% sure, 5% maybe, 15% - no,
Re the difference in distance, have you checked your wheel rollout
lately? In any case, it's very much secondary to proper power
readings.

Post by Jeff Anderson
Hi
I've been learning a lot on this group...some more than I want to know.

So

Post by Jeff Anderson
here's my question....and I know some threads talk to this a bit.
I do a CT class 1x per week. Same bike. I also use my PT on the bike so

I

Post by Jeff Anderson
have the data from the workout.
We calibrate the CT's at the start - once at start and once after the

warmup

Post by Jeff Anderson
as the CT changes as it gets warm. We calibrate them to about 200.
Two *different *CT's
#1 - Did a 3 mi TT - PT said 3.189 miles and power on PT was about 20

watts

Post by Jeff Anderson
higher than CT
#2 - Did a 5 mi TT - PT said 5.25 miles and power on PT was about 25-40
watts lower than CT.
Is this common? How does one align things? Or do I just shutup and do

the

Post by Jeff Anderson
workout? 8^)
Regards
Jeff Anderson
703.868.8705

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[Chris Cleeland]

On Fri, Dec 19, 2008 at 10:32 AM, Jeff Anderson

I gather folks zero their PT before every ride.

During the winter, definitely. Temp swings are what really affect the
zero point on any strain-gauge-based PM, though some are more affected
than others. If you ride outdoors on a cold day, you should wait for
the hub to stabilize to ambient temps before zero'ing, or at least
zero after it reaches ambient temps. For me, I just start riding and
usually about 10-15 minutes in (which, amazingly, is about the length
of an appropriate warmup in the winter), zero the hub, then set an
interval mark. That way I know where the hub was zero'd.

Note to Saris: it'd be nice to have an option to automatically set an
interval mark in the data stream whenever the hub is manually zero'd.

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[Chris Mayhew]

On Fri, Dec 19, 2008 at 11:40 AM, Chris Cleeland

Post by Chris Cleeland
On Fri, Dec 19, 2008 at 10:32 AM, Jeff Anderson

I gather folks zero their PT before every ride.

During the winter, definitely. Temp swings are what really affect the
zero point on any strain-gauge-based PM, though some are more affected
than others. If you ride outdoors on a cold day, you should wait for
the hub to stabilize to ambient temps before zero'ing, or at least
zero after it reaches ambient temps. For me, I just start riding and
usually about 10-15 minutes in (which, amazingly, is about the length
of an appropriate warmup in the winter), zero the hub, then set an
interval mark. That way I know where the hub was zero'd.

I definitely want to second those points. I've experienced the problem
here quite a bit in the winter. Additionally, on a trainer, you aren't
coasting. Combine that with the temp changes of winter and you can
really see some drift.
--
Chris Mayhew
www.jbvcoaching.com

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[rmurphy]

okay well if you're putting your bike in the car (cold) - then into an
indoor studio ... and not zeroing your PT - that's the most likely
suspect here. How long does it take to ensure good data?

fwiw, when I do use the PT indoots (to check that my CT hasn't drifted
on me), I just check for zero torque whilst the bike is clamped into
the CT but before the roller clamped down.

Post by Jeff Anderson
I do not and did not zero the PT before each CT session.....I gather folks
zero their PT before every ride.  Yes I've seen the emails on this....
3 mi TT was NP of 299 - I can't recall CT avg?
5 mi TT was NP of 270 - I think it said 309 avg.
I have not "rolled out my wheel' either.  Tires are standard 700x23 w/ about
110 in them.
Both CT's are older units....
I just go by the PT - we don't use CT to capture data...just to provide 8
riders a chance to train together on same course at same time w/ same
workout
Regards
Jeff Anderson
703.868.8705

Post by rmurphy
well I'd first ask if you were sure you properly zeroed your PT on
both workouts - that has failed to happen in the past I hear ;-).
Otherwise, if you're putting out around 300W or so, that range in CT
vs. PT power is quite high - perhaps 15% across the two different
CTs.
IME that's quite a large variation - not one that I've experienced
here except for an old Nintendo CT that *was* reading about 10-15%
high. If they're all new/newish units. I would honestly be surprised
to see any deviating that much.   1-2% sure, 5% maybe, 15% - no,
Re the difference in distance, have you checked your wheel rollout
lately? In any case, it's very much secondary to proper power
readings.

Post by Jeff Anderson
Hi
I've been learning a lot on this group...some more than I want to know.

 So

Post by Jeff Anderson
here's my question....and I know some threads talk to this a bit.
I do a CT class 1x per week.  Same bike. I also use my PT on the bike so

I

Post by Jeff Anderson
have the data from the workout.
We calibrate the CT's at the start - once at start and once after the

warmup

Post by Jeff Anderson
as the CT changes as it gets warm.  We calibrate them to about 200.
Two *different *CT's
#1 - Did a 3 mi TT - PT said 3.189 miles and power on PT was about 20

watts

Post by Jeff Anderson
higher than CT
#2 - Did a 5 mi TT - PT said 5.25 miles and power on PT was about 25-40
watts lower than CT.
Is this common?  How does one align things?  Or do I just shutup and do

the

Post by Jeff Anderson
workout?  8^)
Regards
Jeff Anderson
703.868.8705- Hide quoted text -

- Show quoted text -

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[t***@sbcglobal.net]

My CT reads a consistent 10.6% lower across the board than either of
my 2 PT wheels.

Post by Jeff Anderson
Hi
I've been learning a lot on this group...some more than I want to know. So
here's my question....and I know some threads talk to this a bit.
I do a CT class 1x per week. Same bike. I also use my PT on the bike so I
have the data from the workout.
We calibrate the CT's at the start - once at start and once after the warmup
as the CT changes as it gets warm. We calibrate them to about 200.
Two *different *CT's
#1 - Did a 3 mi TT - PT said 3.189 miles and power on PT was about 20 watts
higher than CT
#2 - Did a 5 mi TT - PT said 5.25 miles and power on PT was about 25-40
watts lower than CT.
Is this common? How does one align things? Or do I just shutup and do the
workout? 8^)
Regards
Jeff Anderson
703.868.8705

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