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Most people equate size of the main beam as a determining factor in strength. While this may be true to an extent the biggest limiting factor in todays frames is the substandard quality of the steel. Most lippert frames are Chinese steel. The welds early on were terrible but I have seen to many(not just Rv's but other Lippert frames) tear because of the putrid forging process that is so full of impurities that it tends to act like Iron. It rusts from with in. I have a Newmar with a beam frame. I have cattle trailers, horse trailers, flat bed gooseneck trailers and heavy equipment haulers that all have beam frames. Not a single one has ever failed. They all have U.S.A grade A steel with beam cross members unlike the cheap ladder cross members of most RV frames of today. My Alpenlite had a C-Channel with box underneath that never failed. All the custom RV rigs have beam frames. Box frames do not make it better. None of my Semi trailers have box frames, etc. I just got a Northwood Arctic Fox because they make the frame. It is not Chinese steel. Looking used you can look at Space Craft, New Horizon, Forks, Alpenlite, NUWA, Teton, Citation, Travel Supreme, Newmar, older Sunnybrooks, Alfa, Automate, Excel, Carriage, Lifestyle to name a few. They all made there own frames or had someone down the road make it for them. As far as an engeneering standpoint and grade of steel I think the Lifestyle frames were the best other than the customs. Very stout and well designed. Actual improvements over Carriage. Good luck.