SRM vs Computrainer wattage discrepancy?

[acoggan]

AC, did you get the 'characterization' step procedure I sent a few days ago?

I did - thanks!

I'll take that and append a description of how to find, prepare the test, adjust the pot and re-test. It's sensitive but seemingly pretty robust once it's literally tweaked into position. I'm talking perhaps 1/8 or less of a turn here to correct perhaps a 5% difference. The only tools required are an Allen key to loosen the flywheel on the shaft and a very small flathead screwdriver with at least 1" of shaft.

That's the additional information I was hoping to obtain. Again, thanks!

 

[PhilH]

Seems I saved the emails from the CT forum. This is from Nov 23, 2004

Here is the relevant section from Roger's post.

DISCLAIMER: If you make this adjustment and then subsequently need me
to put it on the dyno to recalibrate it -- it'll cost you $75.
Proceed at your own risk.


To adjust the CompuTrainer remove the sticker on the inside edge of
the Load Generator (on the side closest to the tire). This is the
one indicating the power supply to use, etc. Behind this sticker is
a series of rectangular holes. In the uppermost left hole is a
potentiometer. Turning this pot to the left lowers the load at a
given speed. Turning this pot to the right increases the load. If
the CompuTrainer reads low on the watts reading, then chances are the
load at the Load Generator is too high, which is limiting your speed,
which results in a lower wattage display

and here is part of Rick's post from back then

To second Paul's cautions: only attempt this if you really NEED a
close match between the CT and another PM. Be sure that the other
device isn't miscalibrated. Proceed with caution and when you have
time for repeated trials.

My procedure was to warmup for 15-20min. Calibrate and set rrc where
you'd normally set it. Use ergo mode to set a resistance that you
can repeatedly handle for 5 minute trials. Take an interval average
over 5 minutes and note the baseline watts delta. Then tweak the pot
1/16th or 1/8th turn. Get back on the erg - ride a couple of minutes
at moderate power then start the 'benchmark' interval again. Repeat
until you're satisfied and confirm that rrc hasn't drifted
significantly during the procedure!!

 

[rmur17]

Seems I saved the emails from the CT forum. This is from Nov 23, 2004

Here is the relevant section from Roger's post.

and here is part of Rick's post from back then

I have a much more detailed procedure (Excel) prepared now and can email it to anyone interested. PM or email if that's an option..

rmur

 

[Max358]

Original post reporting back: I did the zero offset check on the SRM, which revealed quite a different number. I set it at the new number and checked that the slope that was written on the inwardly facing cover (26.0) was correct in the software and in the power control. Good so far.
Then connected to the CT and did normal warmup of 20 mins at 150w. Watts on the CT and SRM were about 5-7 watts different with the SRM reading higher. Perfect, I thought.
Then, did the calibration on the CT, which moved the set point from 2.07 to 2.40. When I started it back up, the SRM was reading a steady 24 watts lower than the CT. Rats. Back to square one.
I'm not sure which one is correct. Obviously I'd like to believe the higher wattage reading, but not sure what to do next. What now?
Thanks,
MAX

 

[acoggan]

Original post reporting back: I did the zero offset check on the SRM, which revealed quite a different number. I set it at the new number and checked that the slope that was written on the inwardly facing cover (26.0) was correct in the software and in the power control. Good so far.
Then connected to the CT and did normal warmup of 20 mins at 150w. Watts on the CT and SRM were about 5-7 watts different with the SRM reading higher. Perfect, I thought.
Then, did the calibration on the CT, which moved the set point from 2.07 to 2.40. When I started it back up, the SRM was reading a steady 24 watts lower than the CT. Rats. Back to square one.
I'm not sure which one is correct. Obviously I'd like to believe the higher wattage reading, but not sure what to do next. What now?
Thanks,
MAX

You have only one option: calibrate your SRM (and henceforth remember to set the zero offset before every ride).

 

[Max358]

You have only one option: calibrate your SRM (and henceforth remember to set the zero offset before every ride).

Sorry Andy, I am new to all this, so please bear with me. I did calibrate the SRM with the zero offset calibration procedure, and set the zero offset based on what the Power Control said, and it still read low (or, perhaps, it read correctly, and the CT is reading too high). Could I warm up and calibrate the CT, then add pressure to the rear wheel until the numbers equalized?

 

[Woofer]

Sorry Andy, I am new to all this, so please bear with me. I did calibrate the SRM with the zero offset calibration procedure, and set the zero offset based on what the Power Control said, and it still read low (or, perhaps, it read correctly, and the CT is reading too high). Could I warm up and calibrate the CT, then add pressure to the rear wheel until the numbers equalized?

The thing I think you are missing is that the SRM is accurate to with around 2 percent, assuming you did a static calibration and the crank has settled in on the new bike( new to the SRM).

The only way to test the accuracy of the CT is to put a powermeter on a bike and compare results to the CT. The CT is not the basis of accuracy.

If you are going for accuracy whatever the CT reads should be slightly less than the SRM due to drivetrain losses. If you wish to make them equal for whatever reason, follow rmur's procedure on adjusting the CT pot.

 

[acoggan]

Sorry Andy, I am new to all this, so please bear with me. I did calibrate the SRM with the zero offset calibration procedure, and set the zero offset based on what the Power Control said

Sorry, I suppose that was too cryptic. By "calibrate", I simply meant determine the slope of the frequency-torque relationship by hanging known masses from the crank. Until you do this, you don't really know that your SRM is accurate, as experience demonstrates that the slope as determined at the factory is often incorrect (and/or it changes in the field). To get good data you'll also need to set the zero offset every time you ride/every time you might expect it to change (e.g., you climb from a warm valley to a cold mountain top), but setting the zero offset isn't the same as determining the slope.

Once you verify/assure the accuracy of your SRM, you can then use it to determine whether or not your Computrainer is accurate.

 

[Max358]

Ah, now I follow. Hanging weights and such is above me, but I agree with the prior post that the CT is probably not the standard here. Bummer that my threshold power just went down 24 watts!
As a check, though, I'm going down to a local training center with 8 CT's to see where their machines stand compared to my SRM. Thanks all!