Totally understand that! I was concerned about the extra weight that would have been added in filling the tube too... I was even thinking at one stage that expandable filler foam (used in patching walls) might have been a nice solution, as it would have added support inside without adding a lot of weight. I'm unsure how it would stand up to temperatures in the system though and if it would have actually adhered to the aluminium enough to stop flex, or if it would have just separated if there was any bend in the tubing. It's not the most rigid material but it's quite strong for what it is.
Really the main concern was the effect on thin walled aluminium tubing over a long period of time. 0.3xx mm wall thickness just seems a little bit too susceptible to fatigue... but then again I tend to over-engineer everything I touch. Carbon fibre tube, although protruded with unidirectional fibre (I would have preferred wrapped weave!), would actually provide a solution even lighter than aluminium tube with the same OD/ID, but should be a lot more stable over time with a heavy duty cycle. Well, that's my thinking anyway, I don't actually know the properties that could tell me for sure. I'm sure 6mm OD 4mm ID of either aluminium or carbon fibre would both be fairly similar in performance and life expectancy.
Really the main concern was the effect on thin walled aluminium tubing over a long period of time. 0.3xx mm wall thickness just seems a little bit too susceptible to fatigue... but then again I tend to over-engineer everything I touch. Carbon fibre tube, although protruded with unidirectional fibre (I would have preferred wrapped weave!), would actually provide a solution even lighter than aluminium tube with the same OD/ID, but should be a lot more stable over time with a heavy duty cycle. Well, that's my thinking anyway, I don't actually know the properties that could tell me for sure. I'm sure 6mm OD 4mm ID of either aluminium or carbon fibre would both be fairly similar in performance and life expectancy.