Date registered: Aug 2009
Vehicle: 1963 Swiss 404.1 w/ M180 (needs love & tranny swap); 1960 German 404.1 w/ M130
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Longer wheel bases are more stable at higher speeds. With a short wheel base truck like a U900, at high speeds you are more likely to be the victim of unpleasant and sometimes difficult to control bounces. Thus the short truck are generally limited to speeds in the high 40s for miles per hour.
I've had my Swiss 404 at speeds in the low 60s per hour, and even with the longer wheel base on the 404s, that speed was more than a little scary --much better to slow down a bit. It's variable, though. Some of the SBU trucks in the long wheel base are quite comfortable at those speeds.
Longer wheel bases also do help with steep uphill or downhill driving. Think of it as a triangle. Your wheels form two vertices of the triangle. The third is at an imaginary point in the air coming out at the plane of level from your uphill wheel. It lies at the point where the truck's center of balance will cause it to roll on over versus coming back down to or staying on the driving surface. Even if the angles are exactly the same, with a short wheel base truck there is less actual distance between that uphill vertex and the imaginary one that is your center of gravity. With a longer wheel base there is more room to bounce and move with the terrain without the balance of the truck either getting very loose or becoming completely out of control.
It's the same thing that happens if you stand with your feet together and have someone push you from the side. You get to the point where you feel like you are going to fall over much more readily than if you stand with your feet far apart and have someone push you.
It's all tradeoffs. As you say, the short wheel base trucks can manuever more tightly, and have a better breakover angle. And the reality is that regardless of wheel base, these trucks will do things that are just plain scary.
And you also have it right that even the long wheel base vehicles aren't necessarily all that big. My Swiss 404 looks big, but it's actually got about the same footprint at the Jeep Grand Wagoneer sitting beside it; it's smaller than my Suburban and most pickup trucks on the road in the US, and isn't even really a whole lot larger, footprint-wise, that the VW Beetle that used to sit next to it.
Now, the SBU series trucks are bigger, but even then, if you look at their width and footprint, and not height, most of them aren't any bigger than a lot of other trucks I see on the road every day around my home.
If you do plan on doing trail driving, though, one thing you need to think about are trees and that height factor. Sometimes that's more important than other footprint considerations. A really tall truck is going to be hitting its head on branches a lot more than someone in a shorter truck.