Semi-trailer rear axle location

[nickelplate759]

Apologies for not knowing the correct terminology.  In the present, I note that most semi-trailers  have a pair of rear axles , right at the rear of the trailer.   I'm aware that there are all kinds of exceptions, and I've seen flat-bed trailers that seem to be mostly axles, but this question isn't about them...

In looking at pictures of trailers from the 1950s and early 1960s, I note that the rear axles seem to be set in about 5' from the rear of the trailer.   Here are my questions:

1.  Why were they axles set forward in the 1950, and how far forward were they typically?  (there aren't any circa-1955 trailers handy to measure any more).
2.  Why did they move to the rear later on?
3.  When did the change in common practice happen (1960-something I'm pretty sure)?

[Mike C]

   The rear trailer axles can move foreward and backword so the Driver can adjust the amount of weight that is on the cab axle . If you were to move the trailer axles foreward that would take weight off of the truck axles . If they move to the back the trailer then the truck axles take more weight .  This is because by Law there can't be more than a certan amount of weight on the truck axles .      Mike

[samusi01]

This won't be helpful to you as it's well after the date range you are interested in but Sea-Land used these settings in the late 1960s and early 1970s (excerpt from '73 OIER; '69 ORER had same data):



My recommendation would be to start looking at old regulations for your modeling location.