[Lil' Lavery]

Just a quick note first. Both 3 wheeled, and 4 wheeled systems are "omni-drives" (which also includes swerve ("crab") drives, mecanum drives, ball drives, and other omni-driections systems). The method of motion is referred to as a "holonomic" drive system, where the motion of the robot is based off of the vectors of the output of the motors. 3-wheeled holonomic systems are also referred to as kilo-drives, and kiwi-drives.

When discussing the 4-wheeled system, let's nickname each of the 4 motors/wheels North, South, East and West (assuming you are placing them in the cardinal directions, as opposed to the corners as shown in the picture, but the same principles still apply in either positioning). To go directly North, you power the East and West motors, but the N and S stay motionless. To go East, you power North and South, and E and W stay motionless. To go along a diagonal, you power all 4 motors equally. To travel anywhere in between, you power all 4 motors varying amounts. To get a curved path instead of a straight path of travel, you power one motor more than that on the other side. Say you want to travel northwards, with a slight curve to the west. You leave N and S idle, power East 100%, and West 90% (so it will curve like a skid steering, aka tank drive, system would). To spin in a 4 wheel system, you move two opposite facing motors in opposite directions, instead of the same (or all 4 if you wish to spin in place).

Likewise in the 3-wheeled system. To travel in the exact direction of one of the wheels, you leave that wheel idle, and power the other two motors equally. To travel in other directions and curves, treat it basically the same way as a 4-wheeled system, just the amount of power to each wheel will be differnt based on direction.

The difference in between systems is in the efficiency. In a 4-wheeled system, you get 50% efficiency from your motors. Think of it when moving along a cardinal axis. You have 2 motors powered, and the other 2 idle, therefore 50% efficiency (the same applies for any other direction, as some of the power is "wasted" as you are not travelling directly into it's output direction). In a 3-wheeled system, you draw 66.7% efficiency. You get 100% efficiency from both systems when spinning in place.
.667x3motors=effective power of 2 motors
.5x4 motors=effective power of 2 motors
Therefore, unless your spinning, you get the same power with 3 motors as you do with 4, but you use 1 less motor.
But with a 4 wheeled system, you have a greater "footprint" or support base, making you more stable. You also get a greater power output when spinning in place, although the value of that tends to be limited. The math to figure out the vectors (and therefore the programming) is also slightly easier though. As well as construction in a kit with all 90 degree angles, such as the vex kit (60 deg angles are obnoxious to bend).


As for control, with the vex controller, i'd recommend the following. Use the left (or right, depends on your preferance) to control direction of travel. Up, the bot moves "north". Left, it travels "west". Right="East", down="south". The other stick would control spin. Left=counter-clockwise, right=clockwise.

I'd leave out the curves for now, it's more difficult to program and to control, and they tend not to be very necessary.