Departure angle

Also called rear departure angle

Departure angle is the steepest slope a vehicle can back down or drive off without dragging its tail, measured between the ground and a line from the rear tire's contact patch to the lowest point behind it.

What is departure angle?

The same geometry as approach angle, taken at the other end. Draw a line back and up from where the rear tire touches the ground until it meets the lowest thing hanging off the back. That angle is what you have when you crest a ledge and the back of the truck has to follow the front over it.

It gets less attention than approach angle and it is usually worse, because the back of a vehicle is where the accessories accumulate. The definitions run in parallel with approach angle: US federal law words it identically but tangent to the rear tire and touching the underside rearward of it, SAE J1100 covers both under dimension A106, and ISO 612:1978 puts it at paragraph 6.11. Both the European and the American rulebooks want at least 20 degrees of it before they will call a light vehicle an off-roader.

What eats it

A tow hitch receiver, almost always. It hangs below the bumper by design and it is the first thing to touch on most trucks. A hitch-mounted bike rack or cargo tray is worse again, and a trailer is a different problem entirely. After that: the rear bumper's lower edge, a spare tire carrier, exhaust tips, and on a long-bed truck the overhang itself.

Rear overhang is the structural version of the problem. The further the body extends behind the rear axle, the worse the angle, and that is fixed at the factory. A short-bed truck has a real advantage here that no modification gives back to a long bed.