Several have commented about stacks causing soot to dirty up stuff.
With stacks mounted and the tips oriented such as those on my truck, they actually prevent a soot problem.
I NEVER see ( nor find all over my clothes after getting against anything ) any soot on my truck or any of the trailers I pull.
However, when using any of the stackless trucks, the front and exhaust sides of whatever trailer, get covered with soot.
A load of clean Charolais cattle will look like a bunch of mouse-backed Angus-cross after being hauled in an open-air trailer behind a stackless diesel truck; especially one with a "chip".
A long forgotten law dictates the required use of exhaust stacks that angle up and outward on any truck, gas or diesel, big or little, that hauls livestock in a trailer.
Has anyone ever ridden in a trailer behind a gas-burner and been near carbon-monoxided to death ??

: I can sympathise with the livestock.