Originally Posted by Lumatic
But with a circumferential frame a failure of a cross member would be a total failure leaving that crossmember hanging by one end. With the traditional frame a cross member failure is only a partial failure, confined to either the end or the middle with a large percent of the load still supported by the frame.
With a traditional frame the axles are directly supported by the frame. With a frame that goes around the circumference the axles would need to be supported by some other indirect method, unless a new type of axle was designed for the purpose.
I would ad this disclaimer. Please don't get the idea that I am some kind of engineer or in any other way know what I am talking about.
I am an engineer, but don't take my answer as definitive, either. Without actually crunching numbers, any engineer's opinion is only that, opinion.
Frame doesn't support the axles. Axles support the frame. Minor quibble, pay it no mind.
Part of the problem with a perimeter frame is this:
Side-to-side, if your factory-stock trailer is eight feet wide, and the longitudinal frames are four feet apart (for simplest math; I haven't measured a trailer to see the exact spacing), then for ease of analysis, you could draw a line down the center of the trailer, and say that all loads on the left side are transferred to the left frame member, and all loads to the right of the centerline are transferred to the right framing member, and all loads on the centerline are evenly split between the two. In real life, it's not that simple, of course, but again, we're not crunching numbers here. I'm just illustrating a point.
There's an engineering term called "moment arm". A load of 100 pounds applied one foot from your support creates a moment arm of 100 foot-pounds. A load of 100 pounds 2 feet away creates a 200 foot-pound moment. so, in your standard trailer, a 100 pound load right on the centerline provides a moment of 50 pounds (half to each frame) at 2 feet from each frame, for 100 foot-pounds to each frame.
For non-engineers, moment arm is also referred to as torque. A torque wrench is the perfect example of the principle. The force you apply to the torque wrench, times the length of the wrench from your hand to the bolt, equals the torque.
With a perimeter frame, your supports— the longitudingal frames— are eight feet apart, and all loads are somewhere in between the two frames. A 100-pound load right on the centerline provides a (50 pounds × 4 feet =) 200-foot-pound moment to each frame. So, by making the frames twice as far apart, the frames have to support double the torque or moment. Your frames would have to be huge and heavy compared to the frames on a factory-stock trailer, to support the same load.
Now, you could do it a bit different, and put in THREE longitudinal frames, one on each side, one on the centerline, and then your frames could conceivably be the same size as the originals. But you're still adding weight (for the third frame) without actually improving the load-bearing capacity of your trailer.
Now, if your trailer was gutted, and all you had to support was the skin and ribs, you might get away with it. But you've got concentrated (point) loads, including people, appliances, holding tanks, all somewhere between your perimeter frames, and all proiding torque loads on your perimeter frame.
The placement of the frames in your trailer might have been developed by trail and error over the last eighty years. It might have been precisely engineered on a computer in the past five years. Who knows? But it's a safe bet that the location of the frames is probably within a few inches of the ideal spacing for distributing the weight of trailer and contents to the frame.
That's not to say that the metal channels used for the frames are the ideal size and shape, mind you…