Rob
The Chaser trailer weighs in at 700 lbs, http://www.adventuretrailers.com/chaser.html
It's problematic making the chassis out of aluminum due to brittle welds and resultant weld failures. There are ways to design around this, but it makes the construction a little more difficult.
This design seems to use the structural benefits of a steel chassis with the weight savings of an aluminum body.
Absolutely correct. There is 120 pounds of weight saved with the aluminum body and that is comparing .125 aluminum to .0625 steel. The chassis is rigid and so is the tub, there is a very real fatigue issue when aluminum is not used correctly. The rack transfers all the rack weight to the top structural edge of the tub through the mounting feet, the lid is really just a lid.
Proper chassis constuction when using aluminum is not to place welds in high stress areas and most problems are design related rather than weld related, the broken welds almost always point to design error. We successfully build with aluminum and steel all the time as well as repair other peoples problems. Check out this little machine we build, it's 4' wide 9' long, weighs just under 2000 pounds, has .79 psi ground pressure, and 4400 foot pounds of tractive effort so it has to be tough while being light. There is a very delicate balance of steel and aluminum in the design.