I talking about those really big dump trucks on our roads with a secondary load attached by the very long and low to the ground trailer hitch. The one around here often are made with stainless steel buckets (is that what one calls them?) and stainless steel trailer hitches.
So why the long and low trailer hitches? When these trucks make turns the turning radius is never so tight as to require the hitch to be that long so that the buckets could touch. And if these trucks are used off-road on uneven surfaces, why the low hitch which would scrape the ground?
If you are thinking of what I think you are thinking of, they are 14 yard dump trucks with a 14 yard pup.
Simply a dump truck with an extra long tongue, pulling another dump bed.
My take on that is weight limits, per axle. We’re only allowed so much weight per square inch of road surface/tire, meaning that we have to pull ridiculously long loads to conform with the law.
Ever notice how those drawbars have angled and pointed top surfaces, like a roof? I always figured that it was so the driver could pull up over a dump grid, dump the front load over the drawbar into the grid, then pull forward and dump the second load.
bare has part of it, there are minimum distances allowed between sets of axles. But the biggest reason is unloading. The truck will dump the pup box first, then the pup has to be jack-knifed as close to 90 degrees as possible to dump the the truck box. Even when this is done properly when the truck pulls away the pup wheels will be pulled along the edge of the pile from the truck, done improperly, the truck may become stuck trying to pull the pup over the pile (and that’s why the trailer is dumped first).
The hitch has to be that low to give the tail-gate clearance to open, when the truck box is raised the bottom of the gate only clears the hitch by inches.
The steel boxes are ordinary steel, but you are probably seeing aluminum, which is used to give more payload (= more money). Steel boxes are usually painted, aluminum ones are usually polished to a high gloss.
You’re right about the roof-like surface, Sunspace. There are air lines and an electrical cable travelling the length of the hitch. If unprotected they would wear through very quickly. There is also usually a cut piece of conveyor belting or a mud flap bolted to the back of the truck and draped over the hitch. This protects the air lines and cable where they come out of the hitch and keeps gravel etc. out of the pintle. This also sheds the gravel off the hitch, too. The driver should stop as soon as the pup is clear of the truck’s load and clean the hitch before leaving the site, but some are lazy.
I’ve pulled pups, wagons, end-dumps and B-trains in my youth.