Rule Number

Does anyone know the rule number that says that the trailer has to be mounted to the short side of the robot is.

I don’t think such a rule exists. If it does exist, then I’d like to know about it too.

I remember seeing this rule being asked about in another thread, I checked everything about the trailer, and I see nothing that refers to where the trailer has to be attached besides the outside perimeter and that the piece it attaches to can’t move or something

There exists no such rule. I don’t know where this misconception came from.

The rule mentions something about the “long dimension of the trailer hitch”, probably where it came from. Here is Rule R18 just for reference.

To attach the TRAILER to the ROBOT, TEAMS must use a Trailer Hitch constructed from materials provided in the 2009 Kit Of Parts. Details on the construction of the Trailer Hitch are provided in Drawing “GE-09040.”
The Trailer Hitch is composed of the “Trailer Spacer” (Part 2 in the referenced drawing) and the “Trailer Mount Bar” (Part 3 in the referenced drawing). The Trailer Spacer is a 7-inch length of square steel tubing provided in the Kit Of Parts. The Trailer Mount Bar is a 7-inch length of robot chassis material (C-channel) to be cut from the provided KOP chassis material.
The Trailer Hitch must be rigidly attached to a fixed location on the ROBOT, with the long dimension of the Trailer Hitch horizontal and the opening of the C-channel facing away from the ROBOT. The horizontal center line of the Trailer Hitch must be 2-13/16 inches above the floor.
The Trailer Hitch must be positioned so that the TRAILER may be locked in place with a standard 1/4– inch diameter hitch pin (McMaster-Carr part number 98416A009). During a competition MATCH, this hitch pin will be provided with the TRAILER as part of the ARENA equipment. See Figure 8-5.
The Trailer Hitch must be located on an outer edge of the ROBOT structure such that it may easily connect with the tongue of the TRAILER (attached to the TRAILER).
The Trailer Hitch must be placed such that, as the TRAILER swings from side to side, the first contact between the TRAILER and ROBOT is BUMPER-to-BUMPER and not TRAILER-tongue-to-BUMPER (to prevent placing excessive stress upon, and possibly damaging, the TRAILER tongue). See Figure 8-6.
The color of the TRAILER (red or blue) will be used to indicate the ALLIANCE of the ROBOT.

Figure 8-6 shows a trailer hitch on a wide side of a robot.