I don't want to sound rude but if you're getting hung up on that sub-frame, then you're driving over stuff you shouldn't. Just because it's a tractor, doesn't mean it can drive over everything. Do so, will often result on expensive repairs, like broken filter housings, expensive oil spilled everywhere, damaged fuel tanks, etc. The list goes on and on.
Just a couple weeks ago, I had a brain fart and backed over a pile of loose dirt I was moving that was clearly still too tall and managed to pull, twist and bend the steel suction line that lives under the tractor. Luckily it didn't pull the rubber hoses and dumped all the fluid on the ground, but was a way too close call.
Also, the engineers put that sub frame there for a reason.