ROBOTS are required to use BUMPERS to protect all outside corners of the FRAME PERIMETER.
For adequate protection, at least 8 in. of BUMPER must be placed on each side of each outside
corner (see Figure 4-3). If a side is shorter than 8 in., the entire side must be protected by
BUMPER (see Figure 4-4). A round or circular FRAME PERIMETER, or segment of the frame
perimeter, is considered to have an infinite number of corners.

Does this mean that all corners must have at least 8" on each side, or only that if the side is < 8", then the whole side must be covered in bumpers.

I am specifically asking about the scenario below, where there is a gap in the front of the robot. Can the 2 sides surrounding the gap be less than 8" in length (e.g. 4")?

Q520
Q. Bumper rule R19 states “If a side is shorter than 8 in., the entire side must be protected by BUMPER”. If the side is 27" with a 13" cut-out (14" remaining) does the 8" minimum apply or is the 7" on either side of the cut-out considered a full side less than 8"?

A. Per the Blue Box below R19: the dimension defined in R19 is measured along the FRAME PERIMETER. A lack of physical frame elements between corners of the FRAME PERIMETER does not change the length of the FRAME PERIMETER side.

As far as I understand it, no. The front of the robot in your picture is longer than 8" therefore 8" of it must be covered by bumper.

In addition per R26 Bumpers must be backed by at least 1/2" on each end. For your drawing to be legal the two 4" sides would have to be at least 8". Anything more than that can be an unbumpered opening.

That picture does not have two 4" sides. The “front” side with the gap is the same length as the “back” side with no gap, maybe 18 inches or so extrapolating. The front and back 18" sides each must be protected 8" from each corner. And since the BUMPER must attach firmly to underlying structure, that picture can’t work.

“at least 8 in. of BUMPER must be placed on each side of each outside corner”

This rule is surprisingly clear when read carefully.

The key is understanding what “each side of each outside corner” means. It means that for every extreme convex corner of your frame, the edges that intersect at that corner must have 8 inches of bumper starting from the point of intersection and ending at a minimum of 8 inches in the direction towards the opposing end of each respective edge.

Basically, any given side of a robot will have at least 16 inches of bumper, as each corner of that side will have a minimum of 8 inches.