The issue for which wheelbase and track really matter isn't pulling in a straight line, or stopping in a straight line: It's when you're making an emergency move within a turn, or recovering from a skid, and the trailer's momentum is offset from the direction your SUV is traveling. The trailer will
REALLY, REALLY want to keep going in the direction it's already moving.
The tail wags the dog, and maybe the dog looses. The three things which a tow vehicle has, to resist being "controlled" by the trailer, are (1) mass; (2) track width; and (3) wheelbase. A long wheelbase and wide track help you to maintain YOUR skid-free contact with the road when the Trailer tries to pull you into a skid. I can't imagine a more sickening sight than seeing the side of my trailer trying to take off in a different direction than my tow vehicle, or even trying to PASS as I get dragged into a skid-- because the next thing I'd probably see would be the world upside down.
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If you STILL think I'm talking about what gear you need to pull up a hill, or how fast you can go, or stopping in a straight line, you simply DON'T GET IT.
Please read again. It has
nothing to do with "engine braking", "higher revs", or up and down. On mountain passes, it's about the switchback turns where some idiot might loose his line and force you to make an emergency maneuver. There's lots of idiots out there, and they do that a lot. If you're coming down, in the turn, and it's been raining....
But yes: some people NEVER get challenged in this way. And also, my 4runner might get dragged off the road too. (It's a question of degree. Yeah, a Tundra would do better in such a situation.) You choose your level of risk. Just please stop pretending that there isn't a possible problem, OK?