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Home > Forum > Triathlon Forum > SRM Firmware, and drifting slope

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SRM Firmware, and drifting slope
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motd2k
May 19, 17 1:16
Post #1 of 12 (2060 views)
I've got an older wireless SRM that's drifting slope wildly (+-200Hz in a single ride, inconsistently). I've obviously swapped batteries over, but the problem persists; i've read some mention of trimpot adjustment, does anyone know what kind of resistance I should be expecting between that, or the best way to trim it off to determine if it's the issue?

Also, does anyone know if it's possible to adjust the factory slope (which the unit reports via the 'auto' slope mechanism) and finally, whether it's possible to user update the firmware? I've a decent microelectronics background so should be able to access the diag connector via whatever means it requires. Oscilloscope etc also available.
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Re: SRM Firmware, and drifting slope [motd2k] [ In reply to ]
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Bdaghisallo
May 19, 17 6:06
Post #2 of 12 (2022 views)
Are you sure you're not talking about the zero offset drifting during a ride? Slope is measured in units of Hz/NM and is not what you see when you press 'mode' and 'pro' together. That is the zero offset which is the check you make to zero the PM before use to account for temperature swings and such.

A fluctuating zero offset is usually a sign that the battery is needing replacement.
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Re: SRM Firmware, and drifting slope [Bdaghisallo] [ In reply to ]
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echappist
May 19, 17 8:54
Post #3 of 12 (1996 views)
Bdaghisallo wrote:
Are you sure you're not talking about the zero offset drifting during a ride? Slope is measured in units of Hz/NM and is not what you see when you press 'mode' and 'pro' together. That is the zero offset which is the check you make to zero the PM before use to account for temperature swings and such.

A fluctuating zero offset is usually a sign that the battery is needing replacement.
i think he's talking about zero offset when describing the drift. Slopes are usually btwn 10-50 Hz/Nm; +/-200 would be way out of range.

motd2k wrote:
I've got an older wireless SRM that's drifting slope wildly (+-200Hz in a single ride, inconsistently). I've obviously swapped batteries over, but the problem persists; i've read some mention of trimpot adjustment, does anyone know what kind of resistance I should be expecting between that, or the best way to trim it off to determine if it's the issue?

Also, does anyone know if it's possible to adjust the factory slope (which the unit reports via the 'auto' slope mechanism) and finally, whether it's possible to user update the firmware? I've a decent microelectronics background so should be able to access the diag connector via whatever means it requires. Oscilloscope etc also available.

If you have a Garmin unit (i assume so as you'd otherwise be able to change it easily via PC6-8), go to the screen that either says Details or About, and the slope should be displayed. To change slope, you have to rotate the crank on certain Garmin units (e.g. 500) while on others, the process is completed without needing to rotate crank.
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Re: SRM Firmware, and drifting slope [echappist] [ In reply to ]
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Bdaghisallo
May 19, 17 9:09
Post #4 of 12 (1992 views)
echappist wrote:
Bdaghisallo wrote:
Are you sure you're not talking about the zero offset drifting during a ride? Slope is measured in units of Hz/NM and is not what you see when you press 'mode' and 'pro' together. That is the zero offset which is the check you make to zero the PM before use to account for temperature swings and such.

A fluctuating zero offset is usually a sign that the battery is needing replacement.

i think he's talking about zero offset when describing the drift. Slopes are usually btwn 10-50 Hz/Nm; +/-200 would be way out of range.

motd2k wrote:
I've got an older wireless SRM that's drifting slope wildly (+-200Hz in a single ride, inconsistently). I've obviously swapped batteries over, but the problem persists; i've read some mention of trimpot adjustment, does anyone know what kind of resistance I should be expecting between that, or the best way to trim it off to determine if it's the issue?

Also, does anyone know if it's possible to adjust the factory slope (which the unit reports via the 'auto' slope mechanism) and finally, whether it's possible to user update the firmware? I've a decent microelectronics background so should be able to access the diag connector via whatever means it requires. Oscilloscope etc also available.


If you have a Garmin unit (i assume so as you'd otherwise be able to change it easily via PC6-8), go to the screen that either says Details or About, and the slope should be displayed. To change slope, you have to rotate the crank on certain Garmin units (e.g. 500) while on others, the process is completed without needing to rotate crank.


You can see and change the slope but in order to establish a new slope figure you must go through the calibration procedure using a known mass.
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Re: SRM Firmware, and drifting slope [Bdaghisallo] [ In reply to ]
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echappist
May 19, 17 9:45
Post #5 of 12 (1983 views)
indeed. but one assumes that's a given in the context of changing slopes

as for the OP, post office scales are pretty accurate (calibrated monthly) and can be a less expensive alternative to getting certified weights
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Re: SRM Firmware, and drifting slope [Bdaghisallo] [ In reply to ]
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motd2k
May 19, 17 10:48
Post #6 of 12 (1966 views)
Indeed, sorry, I was of course talking about drifting zero offset! I've changed the batteries, it's not those. I note that the trimpot inside the PM adjusts the frequency of the signal (and thus, adjusts the zero offset). I'll need to double check with a calibrated weight (I usually just weigh individual scoops of wet sand into a bucket, which I weighed empty, and use the combined weight) to ensure that the trimpot isn't changing the slope too.

The powermeter does seem to report a factory 'default' slope, which appears to be unique to each powermeter, and it's this that I was pondering changing in the firmware.
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Re: SRM Firmware, and drifting slope [motd2k] [ In reply to ]
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motd2k
May 19, 17 10:51
Post #7 of 12 (1965 views)
Oh, I've also noticed that there's a hairline fracture down the inside of the spider, around one of the crank arm attachment bolts. I suspect that this may be the root cause of the offset drifting as perhaps the crack absorbs varying strain as it naturally shifts during a ride.

Anyone have any bright ideas on how to repair the aluminium spider? SRM want 1500EUR for a swap out, and refuse to replace the spider individually...
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Re: SRM Firmware, and drifting slope [motd2k] [ In reply to ]
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Pantelones
May 19, 17 15:59
Post #8 of 12 (1929 views)
motd2k wrote:
Oh, I've also noticed that there's a hairline fracture down the inside of the spider, around one of the crank arm attachment bolts. I suspect that this may be the root cause of the offset drifting as perhaps the crack absorbs varying strain as it naturally shifts during a ride.

Anyone have any bright ideas on how to repair the aluminium spider? SRM want 1500EUR for a swap out, and refuse to replace the spider individually...

Probably not repairable. It would have to be brazed or welded. Both very risky to do without removing the electronics or damaging the strain gauge installations. I don't think brazing would be strong enough anyways. You are probably SOL.
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Re: SRM Firmware, and drifting slope [motd2k] [ In reply to ]
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gregf83
May 19, 17 16:19
Post #9 of 12 (1922 views)
Have you tried disabling auto-zero? I used to use a Garmin 800 head unit and it wouldn't allow me to disable auto-zero. There was a particular corner I would go around and half the time the auto zero would attempt to zero at some point in the corner with ridiculous values. The newer Garmins can disable the auto-zero function.
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Re: SRM Firmware, and drifting slope [motd2k] [ In reply to ]
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goodboyr
May 19, 17 16:27
Post #10 of 12 (1919 views)
Lol. You could get two quarqs for that price.
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Re: SRM Firmware, and drifting slope [motd2k] [ In reply to ]
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motd2k
May 26, 17 8:58
Post #11 of 12 (1822 views)
I desoldered and replaced the 10ohm trimable potentiometer - problem solved!
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Re: SRM Firmware, and drifting slope [goodboyr] [ In reply to ]
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Sausagetail
May 26, 17 9:14
Post #12 of 12 (1818 views)
You'd need at least two.
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