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PowerTap innacuracy?!? - Page 2

post #16 of 24
1/10/07 at 7:19pm

Re: PowerTap innacuracy?!?

Quote:
Originally Posted by gza
I've been banging on for ages for people to check their PT's but blind faith in the product seems to rule the day - there seem to be more and more cases of people finding out of spec product recently.



The only way to know for sure if your PM is reading correctly is to directly calibrate it using known weights. It seems like a hassle but I calibrate mine with weights from 5-35kg in 5kg incrediments in about 20mins - well worth it for peace of mind!



SRM's seems to get more stable with time, but you sure can't trust the factory calibration - if you are into powermeter data at some point you will fall foul of an calibration drift over the years if you don't periodically check them - I test mine every 3 months now.
would you share the way you do it ?

Gettin PT in the next few days so would like to start off on the right track!
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post #17 of 24
1/10/07 at 7:22pm

Re: PowerTap innacuracy?!?

Quote:
Originally Posted by gza
I've been banging on for ages for people to check their PT's but blind faith in the product seems to rule the day - there seem to be more and more cases of people finding out of spec product recently.



The only way to know for sure if your PM is reading correctly is to directly calibrate it using known weights. It seems like a hassle but I calibrate mine with weights from 5-35kg in 5kg incrediments in about 20mins - well worth it for peace of mind!
With a PT what can be done to correct it? Other than send it back.
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post #18 of 24
1/10/07 at 7:50pm
Thread Starter 

Re: PowerTap innacuracy?!?

Yea, I'm aware that my method is not as accurate as I'd like it to be. The goal was to get an idea if it was within range... and it was. When I had all my weight on the pedal it was hovering in the 810-820 range, but keeping balanced in that awkward position was tough.

Regardless, I'm on vacation right now and don't have all the things required to do the testing accurately. A friend of mine (who's getting his PhD in aerospace engineering) has some calibrated weights that we can use.

Further, it seems to me that you don't even need different weights... just pop it in a different gear to see how torque values change.

Anyways, I'll post my results.
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post #19 of 24
1/11/07 at 12:08am

Re: PowerTap innacuracy?!?

Quote:
Originally Posted by JohnMeyers
Yea, I'm aware that my method is not as accurate as I'd like it to be. The goal was to get an idea if it was within range... and it was. When I had all my weight on the pedal it was hovering in the 810-820 range, but keeping balanced in that awkward position was tough.

Regardless, I'm on vacation right now and don't have all the things required to do the testing accurately. A friend of mine (who's getting his PhD in aerospace engineering) has some calibrated weights that we can use.

Further, it seems to me that you don't even need different weights... just pop it in a different gear to see how torque values change.

Anyways, I'll post my results.
FWIW - I did the same stomp test a month or so back (campag cleats obviously a little easier to stand on) and mine was bang on where it should have been.

As for the Polar - well I have three friends using them, two are operating fine and quite accurately (using an SRM for checking) once all the set up issues were resolved but another, well it just generates a load of data that bears no resemblance to what the rider is producing. I feel for the guy - he's done the tests I've asked so I can help him determine FTP / set some training levels but the data is useless. Been through all the set up (weighing chain, careful positioning of pick up etc etc etc all the stuff Tom talks about) but with this one user it is simply "not fool proof enough" for them (not the shapest knife in the drawer, who knows what settings they inadvertently change or bits they bump). We are gunna need to migrate them to a PT methinks.
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post #20 of 24
1/11/07 at 2:02pm

Re: PowerTap innacuracy?!?

Quote:
Originally Posted by Shawneebiker
With a PT what can be done to correct it? Other than send it back.
Nothing - its just for piece of mind. Like you say if you find its out of spec you have to return it - having no data for a few weeks is less of a problem than many months of incorrect data in my opinion!
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post #21 of 24
1/11/07 at 2:02pm

Re: PowerTap innacuracy?!?

Quote:
Originally Posted by JohnMeyers

Anyways, I'll post my results.
Cool!
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post #22 of 24
1/11/07 at 2:18pm

Re: PowerTap innacuracy?!?

Quote:
Originally Posted by giannip
would you share the way you do it ?

Gettin PT in the next few days so would like to start off on the right track!
I'm in the process of writing up a procedure, which I'll post when I'm done (need pics too) - but very simply:

1. Secure bike in stand ( I also have an old front hub which I clamp in a vise to give more support to the bike, in addition to the workstand supporting the top tube.)

2. Pedal the crank forward until its level with the ground, in the forward position, using a spirit level to check. When it's exactly level, lock the wheel in place by using a zip tie or toe strap to hold the brake lever to the bars. Now the bike is stable, the crank is locked level and the wheel can't move and the powermeter can report the torque applied from the weights applied.

3. Hang 3x10kg weights and a single 5kg plate (each weighed on a verified scale to the gram) from the forward pedal by a chain and hook (also verfied weight)

4. Note the offset value (SRM) or the ft-lb value (PT) in the appropriate CPU screen. insure the weights are as still as possible to give steady readings.

5. Remove weights, recheck zero offset and then reload the weights with 5kg less each time until you get down to only 5kg (ie 35kg, 30kg, 25kg, 20kg etc etc) recording the value each time.

6. Repeat for the other side of the bike!

6. Input values into spreadsheet to calculate either correct slope (SRM) or compare reported ft-lbs to calculated ft-lbs (PT)

For real anal types, you should probably weigh each step 3 times, but I haven't found this adds any real resolution.

Hope this helps?
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post #23 of 24
1/11/07 at 2:38pm

Re: PowerTap innacuracy?!?

Quote:
Originally Posted by acoggan
The fact that your PowerTap read zero when coasting doesn't necessarily mean that the zero offset was set correctly. I'd suggest examining the downloaded file from the ride to see what torque was being recorded when you were coasting.

If the above doesn't explain things, it's quite simple to perform a static calibration of the strain gages: just put your bike in the lowest gear, lock it in a trainer, hang some heavy weights from the pedal, and compared the measured torque to the value you'd expect based on the mass of the weights, crank length, and gear ratio.
Sorry to interrupt but is this the Andy coggan? Or is it a guy that is andy coggan?
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post #24 of 24
1/11/07 at 3:06pm

Re: PowerTap innacuracy?!?

Quote:
Originally Posted by Fastguppy
Sorry to interrupt but is this the Andy coggan? Or is it a guy that is andy coggan?
This is me:

http://www.endurancesportscience.com...ers/coggan.png
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