PowerTap innacuracy?!?

JohnMeyers said:

Anyways, I'll post my results.

Cool!

giannip said:

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?

acoggan said:

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.

[font='Times New Roman'][font=Verdana]Sorry to interrupt but is this the Andy coggan? Or is it a guy that is andy coggan?[/font] [/font]

Fastguppy said:

[font='Times New Roman'][font=Verdana]Sorry to interrupt but is this the Andy coggan? Or is it a guy that is andy coggan?[/font] [/font]

This is me:

http://www.endurancesportscience.com/rw_common/themes/ESS/images/speakers/coggan.png