| Last night we tried out the polyurethane foam to see how much extra rigidity it would add to the chassis if we filled it with foam. I've heard claims of 30% stronger which I doubted so we did our own test using some 1" SHS with a sidewall thickness of 1.6mm and measured the vertical deflection using varying weights (water in a bucket). We clamped the length of SHS to the work bench and had 1356mm overhang but placed the bucket that was our varying weight instrument at 1000mm. Here's the setup with my friend measuring the height with zero weight. 
We continued to add 1Kg (1L) lots to the bucket and measure the deflected height. Surprisingly with this setup that looks dodgy actually produced repeatable and sensible results of deflection. 0Kg 0mm
1Kg 2.5mm
2Kg 5.0mm
3Kg 7.5mm
4Kg 10.0mm
5Kg 12.5mm
6Kg 15.0mm
7Kg 17.5mm
8Kg 21.0mm
9Kg 24.0mm Here's a picture of the foam we used - it comes in two gas bottles (part A and part B) that runs down into a spray gun that keeps the 2 liquids separates until they hit the nossle which does the mixing. To fill the SHS steel we added some hose to the end of the nozzle and shoved it down the center. 
After foam filling we redid the deflection test and got the following results: 0Kg 0mm
1Kg 2.4mm
2Kg 4.9mm
3Kg 7.4mm
4Kg 10.0mm
5Kg 12.5mm
6Kg 15.0mm
7Kg 17.5mm
8Kg 21.0mm
9Kg 24.0mm So, initially it helped by 4% and then fizzled back to just the non-foamed-filled SHS steel results at the 4Kg mark. To check the SHS steel did actually get completely filled we chopped it up and everywhere we chopped it looked fully filled as you can see here. 
Now for the good part. While filling up the SHS steel we had a bit of a blockage in the hose and this is what happens when the gun blocks (lucky we had eye goggles and sandblasting gloves on - though my other friend is still getting bits of it peeling off his ears, hair, arms etc ). The conclusion is that foam filling does next to nothing and it's bloody messy. It turned out to be a pretty expensive test ($650 for the foam) but at least it's also useful for insulating and sound deadening so I'll use it around the house in the ceilings trying to block off all the possum holes. 
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