The trusty mecanum drivetrain is part of the holonomic drivetrain group allowing the robot to move in any directions ("omni-directional") . Its chassisusually consists of 4 mecanum wheels; each powered separately by one motor.

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Each wheel has special rollers that are mounted at a 45 degree angle to its axis of rotation. This feature allows the robot to strafe, a key advantage of the mecanum drivetrain. In other words, it gives the robot the capability to “move side to side without turning”. 

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However, this "cool" feature has its downfall. Since the rollers are angled at a 45 degree angle when the wheels rotate to move, the forces the wheels exert effectively cancel each other out. As a result, wheel slippage is often resulted making this the primary disadvantage of a mecanum drivetrain. Trading chassis' pushing power with maneuverability makes it very vulnerable to defend against other robot.  For more info.