The problem is about high frequency shaking that is not continuous, but happens in some point in the video. I have some experience shooting tandems and never had this kind of problem.
My comment to your issues (thanks for them):
-Mounting systems - Fast locking system with lots of hard tape. Maybe the weakest point.
-Helmet burble - Same helmet that I always used
-flying style - Belly (don't think is a problem)
-chincup/no chincup- Same helmet that I always used. No chincup
-parts on camera not secured - No
-parts on helmet not secured - No
-loose element in the wide angle - No
-loose thread on the wide angle- No
-internal lens cover being hit by air - Camera in top of helmet with no protection
-connector door flapping - No
-charger cover flapping - No
-strap fluttering - No
-single screw/no locking screw holding cam in place - Fast locking system with lots of hard tape
-other 'thing' on helmet causing vibration - No
-loose internal lens - The camera is new
-slow shutter due to personalized settings - No
-NLE that throws away a field or combines them improperly (will make stabilization appear to be off when it really isn't).
What can I say. The HC5 is beeing a bit of disapointment for me. Always used a PC7 (sidemounted) and didn't have any problem related to shaking.
Thanks for your help DSE. If you have any other ideas, please don't hesitate.
Could this have any thing related to the capture system?
Regards,
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Filipe Lourenço