system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why do semi trucks in the United States have front wheels where the lug nuts protrude past the edge of the tire while each subsequent wheel has recessed lug nuts?

Currently on a road trip from southern to Northern California and all the trucks we've passed so far have this pattern. Is this an industry standard? Or does it relate to safety in some manner?
gpt
It's because the front wheels are single (like on your car), while all other wheels are dual. Because of this, the centers of the wheels extend past the edge of the rim. This means you can put two wheels face to face, and bolt them together to the hub. When you look at a set of dual wheels mounted, you see the concave side of the outer wheel, so you see the lugs as being recessed. Since the front wheels are single, they mount them the same way they do the inner wheels of each dual pair, meaning with the convex side facing out.