I always go engine first. Then much like Towcat said depending on the trailer setup I adjust accordingly. So i'll pull it up on the trailer and then just creep it forward and watch how the weight shifts. When the tongue weight brings my truck down onto the overloads (about when the truck is level) i usually stop it right there. And please for the sake of others on the road. Don't skimp on the chains and straps. Unfortunatly i've seen trucks strapped to trailers with those chincy 1.5" ratchet straps.
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In the case of the Bumble B tractor i went back axle first. The cast iron axle, liquid filled tires, and cast iron wheel weights weighed a whole lot more than the 4cyl engine.
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And if you aren't around to help load sometimes you get stuck with something like this.
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