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New SRM Cannondale Hollowgram Power Meter (Resolved)

UPDATE! The magnet has been reoriented and repositioned. Does not solve the issue and both PC7 head units have dropouts. SRM indicates it might be bad firmware, so I am sending the unit in for re-flashing.

SRM took immediate action, sending me an overnight label to ship the unit back, and promised me a one-day turnaround with overnight return shipping. While I would prefer no issue to start with, I have nothing but praise for this level of customer service. If all goes well, the SRM power meter will be back on my Vamoots RSL in time for the Solvang Spring Double.

UPDATE 3/21: SRM turned around the power meter in one day, assuming UPS delivers Friday morning, the power meter goes onto the Moots Vamoots RSL for the Solvang Spring Double on 3/22.

Update 3/23: SRM turned it around in one day, the replacement is here (bad antenna on the original), the crank is on the bike, I am headed for Solvang. Thanks SRM, great service!

RESOLVED 3/24: SRM replaced the power meter for fast turnaound and it worked flawlessly for the Solvang Spring Double Century. It turns out to have been a bad antenna in the original unit.

SRM was very responsive in getting the unit shipped overnight to SRM, and getting me a replacement unit overnight. Great service!!!

Readers might have noticed that I consider a power meter essential for serious training. I have the SRM DA7950 Compact on my LOOK 595 Ultra road racing bike and also on my Moots Psychlo X RSL cyclocross bike. They both have been highly reliable, and a very worthwhile investment.

I finally have my Moots Vamoots RSL road racing bike built up with the SRM Cannondale SI/SL power meter (with the Cannondale Hollowgram SL crankset). I am a little annoyed that the SRM web site does not mention that the Cannondale 'QC850' spindle is required for the SRM spider to fit— one more expense ($100) over and above the standard Hollowgram crankset I bought with the standard spindle, which worked fine without the SRM.

First ride, “dropouts”

Reliabilty of my other SRM power meters has been stellar, so today was a shocker.

Today was the first ride on my Moots Vamoots RSL with the SRM Cannondale power meter, along with a brand-new Power Control 7 (PC7) head unit. I had numerous “dropout” problems on a 32 minute climb (Kings Mtn Road).


  • Especially under hard effort (stand up and 400+ watts), the PC7 would read “0” for a second or so for both power and cadence, then it would read “—” for both power and cadence for 2-15 seconds.
  • At the summit I stopped, and as I watched, the PC7 displayed the same “0” and then “—”.

The “—” in my experience always means complete loss of connectivity to the power meter.Besides, it was a 2.5 hour ride, and 99% of the time the power reading was there and working.

Well, maybe it could be a magnet-placement issue— somehow. I’ll check on that very soon. Reader Eric B writes:

A magnet trips the reed switches as the device rotates, I believe. There's a reed switch per strain gauge, generally speaking. Reed switches can become stuck. anyway, the switches are essentially reset each time they pass the magnet. That clears the way for new deformation values to be read and power to be calculated and sent to the head unit.

Update: Turns out to have been a bad antenna.

All the green downward spikes represent failed power meter functionality. This was a steep and steady grade and I did not stop pedaling. In fact, most of the dropouts seemed to be associated with higher power output especially with the bike turning (stand up and put 400+ watts into the pedals).

Yet once at the top and stopped, I observed the PC7 do the same “0” and “—’ behavior. So it is not strictly a high-power glitch.

Methinks either the PC7 or SRM power meter is defective. Tomorrow (barring rain), I will take both of my head units out and see if the other head unit shows similar issues. If so, then it’s the power meter itself. I can also try the new head unit on either of my other bikes, to see if it functions on those bikes properly— could just be ahead unit problem. But all this takes time, adding to my irritation— I wanted to ride the Vamoots RSL for the Solvang Spring Double, less than a week away. I am hoping that the problem is not the SRM spider itself.

The “dropouts” (downward spikes) are loss of power meter functionality
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