SRM Zero Offset Drift

gza said:

Hi Andy, what do you mean by a 'multipoint calibration'? I test at 3 o'clock, crank forward obviously, on both sides, with wieghts of 5, 10, 15, 20, 25 30 and 35kg all certified masses. Compared to the PT on my trainer the SRM then reads 11w higher at 200w through to about 15w higher at 450w. When I checked the PT it gave readings 1% low, the Ergomo....well thats work in progress!

Gavin

i'm *guessing* Andy means 12, 3, 6 and 9 o'clock for the multi point calibration, as the SRM can throw up problems at different points.

ric

ric_stern/RST said:

i'm *guessing* Andy means 12, 3, 6 and 9 o'clock for the multi point calibration, as the SRM can throw up problems at different points.

ric

Actually, I meant with multiple masses, as gza has already calibrated his. I have checked the slope of my SRMs at varying positions (i.e., every 45 degress for the road crank, every 90 degrees for the track crank), and while they each have consistent "hot" and "cold" spots, the differences I've seen aren't all that great (CV of ~2% around the circle, IIRC). The same may not be true in the case of Amateur models, however.

Aztec said:

SRM sent me their calibration spreadsheet and instructions. Interestingly, they only suggest you measure with the crank at 3 o'clock/level. No looking for the max frequency as the crank rotates upward.

Well, you wouldn't want to stray too far away from dead-level, since 1) the tangential force vector created by the pull of gravity will eventually diminish, and 2) there can be slight variation in the sensitivity of the crank as a function of position. Still, having done it both ways - i.e., using a spirit level vs. just finding the point at which frequency is maximal - I find my way easier, and just as reproducible.

Re. height off ground and use of pedal blanks: the Velodyne holds the bike high enough even above the built-in mounting steps that a 35 lbs plate can be lifted free of support. Other trainers, though, may have to be raised somehow. Somewhat along the same lines, I use Speedplay pedals, which fit through the hole in the plates I use, but if your pedals don't you'll have to find another way to hang them (e.g., using a chain).

acoggan said:

Actually, I meant with multiple masses, as gza has already calibrated his. I have checked the slope of my SRMs at varying positions (i.e., every 45 degress for the road crank, every 90 degrees for the track crank), and while they each have consistent "hot" and "cold" spots, the differences I've seen aren't all that great (CV of ~2% around the circle, IIRC). The same may not be true in the case of Amateur models, however.

yeah, it was an Amateur i last tested (a while back), and i found differences at different points. I also test with different masses as well.

ric

acoggan said:

Well, you wouldn't want to stray too far away from dead-level, since 1) the tangential force vector created by the pull of gravity will eventually diminish, and 2) there can be slight variation in the sensitivity of the crank as a function of position. Still, having done it both ways - i.e., using a spirit level vs. just finding the point at which frequency is maximal - I find my way easier, and just as reproducible.

Re. height off ground and use of pedal blanks: the Velodyne holds the bike high enough even above the built-in mounting steps that a 35 lbs plate can be lifted free of support. Other trainers, though, may have to be raised somehow. Somewhat along the same lines, I use Speedplay pedals, which fit through the hole in the plates I use, but if your pedals don't you'll have to find another way to hang them (e.g., using a chain).

You've given me a great idea. I have old MTB pedals that will fit through a 25lb plate. To use two plates, I'll have to load them onto an inverted-T pin and hook that onto the pedal. I'll try 25 and 50 lbs, and use your method of finding the highest frequency. Thanks for all the help with this.

acoggan said:

If you just (re)installed your crank, what you observed is normal, but it wasn't due to the temperature increase. Rather, the residual strain in the crank likely increased after you stomped on it a few times. For example, I just got my SRM Pro back from Colorado Springs after a battery change, and the initial zero offset when in the workstand was in the low 300 range. After riding for a short while, though, it went to ~440, which is where it has stayed. It is because of this behavior that SRM recommends setting the zero offset only after you have "bedded in" the crank by applying a high torque several times.

The good news is that your crank will likely quickly settle at a fairly constant value, such that you can correct the data from today's ride using the "Properties" option under the "File" menu in the SRMWin software.

OK, performed the test. Interesting. See what you think. I had a lot of upward zero offset drift during the test. I pulled the pedals and sandwiched a bolt through the hole to hold weights. The results listed below are out of order, as I switched back and forth across sides, but the general inflation of zero offset gives a rough idea of order.

Using 25.3 lbs...
Left:
652, 1069 = 417diff
652, 1070 = 418
655, 1071 = 416
660, 1078 = 418 (PT 54)
660, 1078 = 418 (PT 52)

Right:
658, 1068 = 410
658, 1069 = 411
663, 1074 = 411

This combines to slope = 21.32 if I take the middle values, 21.29 or 21.34 if I take the most extreme values.

Then, with 50.7lbs...
Left:
655, 1483 = 828
660, 1492 = 832 (PT 103)
662, 1497 = 835 (PT 105)

Right:
659, 1481 = 822
662, 1481 = 819
664, 1481 = 817

Using middle values, that slope = 21.21, and then 21.13 and 21.29 if I use the extremes.

SRM's factory slope was 22.6.

I have no clue whether this is a material difference, and whether it explains the ~15-20w diff between this and my PTSL at 220-240ish watts.

I did have my PT on when testing, but it was a pain to try and juggle the PT, SRM meter, and SRM power controller on at all times. I did note a few times when the PT was up that the torque peaked with the crank up pointing upward at a substantially higher angle than when the SRM frequency peaked. I noted the PT's value in parenthesis above for the few times I could catch it.

Using the correct slope of 21.2 (or 21.3, whatever it averages out to be) vs. the factory-stated value of 22.6 will result in a calculated power that is ~6% higher. Your SRM should now read the same or slightly higher than your PowerTap, which is what you'd expect.

As for the changes in zero offset, I would consider that normal behavior (it's likely due to hysteresis in the crank), and wouldn't worry too much about it.

acoggan said:

Using the correct slope of 21.2 (or 21.3, whatever it averages out to be) vs. the factory-stated value of 22.6 will result in a calculated power that is ~6% higher. Your SRM should now read the same or slightly higher than your PowerTap, which is what you'd expect.

As for the changes in zero offset, I would consider that normal behavior (it's likely due to hysteresis in the crank), and wouldn't worry too much about it.

I feel so vindicated. :-)

Thanks for all the help.

acoggan said:

Using the correct slope of 21.2 (or 21.3, whatever it averages out to be) vs. the factory-stated value of 22.6 will result in a calculated power that is ~6% higher. Your SRM should now read the same or slightly higher than your PowerTap, which is what you'd expect.

As for the changes in zero offset, I would consider that normal behavior (it's likely due to hysteresis in the crank), and wouldn't worry too much about it.

Update...

That change to slope put it about 6w higher than my Powertap, both on average and for peak 2 mins-60 mins. Very glad to see that. Now I'll fudge the slope up a bit to match the PT for consistency sake. Although, with me getting away from the PT, I won't be able to easily check vs. the PT in the future. Hmm. Maybe I *should* use your 'most accurate possible' method.