Update: 2nd Oct 2020. I am going to perform a new test sometime this week. Please help me make that test better.
As I am about to demonstrate, I am not an engineer - I am a degree-holding Materials Scientist by training and an IT professional by career. Try to not judge me too harshly.
Anyway, I convinced myself - from a very quick search on the internet and turning up a few research papers - that filling up tubes would improve stiffness and vibration dampening. Obviously I needed to test this cheaply with materials that I had at hand.
I cut a plastic water pipe in half - giving me 2 nearly identical pipes that could be easily bent. I filled one of them with PU foam, from a spray can. If there was to be any stiffening of the filled tube, it would be easily detectable. The sort of thing that I might be able to measure by eye.
Here’s the thing. There is no detectable difference in stiffness. None. Of course, this is blindingly obvious to a mechanical engineer (stop sniggering in the back, you lot). All of the strength of a pipe is in the skin of the tube (moment of area, I believe, is the relevant property). When I imagined that stiffness would increase I was thinking about I beams.
On the vibration dampening, again measuring by feel, I feel like that the filling helps. Now, I could measure this accurately but is vibration of the supporting pipes actually a problem with any of the current designs?