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Defensive Mechanisms

With VRC drives becoming more powerful, Defensive Mechanisms can help give teams an edge when it comes to defensive play.
With robots competing in the VEX Robotics Competition becoming more and more effective on average, defensive play is becoming more common as a strategy used to slow down the scoring of opposing teams. In order to become more effective at defensive play, and stifle the effects of defensive play used against your robot, various mechanisms outlined below can be implemented.

Wedges

One of the key tasks in taking control of a match with the use of defensive mechanisms is to get the upper hand on mechanical advantage. Wedges are a common way to do this with two key benefits. The first being that when a robot is partially lifted off of the ground, it has less traction and therefore can be pushed easier. The other key benefit is the addition of the normal force from the lifted robot to the robot with wedges. This gives the instigator mechanical advantage with added traction and stability.
There are two main variations of wedges, the first of which takes its shape in the form of long, narrow pieces of material designed in a way that there are multiple spaced across the length of a robot. The advantage of having separate, narrow "forks" is that the smaller surface area may be able to exploit smaller infrequent vulnerabilities in an opponent's ground game, or uneven surfaces that are small enough for a wedge to slip in and lift up the robot.
1961Z Change Up, Riptide 4.0
The second main variety of wedge is a much longer, unibody wedge that spans an entire side of the robot. The main advantage of this variation is ease of use - with one long sloped surface, it is much easier for the driver to get the wedge under an opposing robot. In addition, this variation of wedge is more easily able to function as another mechanism of the robot. For instance, the wedge pictured below doubled as a method to raise mobile goals off of the ground and into a pneumatic clamp.
BLRS 15" Robot, Tipping Point
One similarity across all variations of wedges is that they are all relatively low to the ground. By achieving this low ground clearance, forks stand a better chance of getting underneath side skirts or other low areas of an opposing robot. One thing to consider when designing wedges of any type is to make sure that the wedge does not dig into the field tiles, as that would violate the rules of the game. This can be accomplished by wedges having enough clearance to be clearly elevated above the ground, or by adjusting the shape of the wedges to be rounded, so that they ride over the field tiles while still being very low to the ground.
It should be noted that wedges have been met with controversy. With the legality originally discussed in this Vexforum post. They have repeatedly been ruled legal but all teams using a wedge should be ready and willing to justify the legality if questioned by a head referee or EP.

Skirts

In order to prevent wedges from getting under a robot, skirts have become one prominent tool in securing the ground game. Rather than acting in an offensive fashion, such as Wedges, skirts defend a robot by extending its ground clearance to almost floor-level.
1961Z Change Up, Riptide 4.0
Skirts are typically mounted to the robot in a sloped fashion, such as in the picture mounted above. When a robot is engaging in defensive play, pushing a robot from the side with a sloped surface can cause the offensive robot to ride up the skirt slightly, losing traction and mechanical advantage - the main exception to this being if the offensive robot can get wedges under the defensive robot, lifting them instead. Keeping that exception in mind, it is important to design skirts to be as low to the ground as possible or alternatively, fold out of the way if lifted from the bottom.
1961Z Change Up, Riptide 4.0

Defensive Wheel Setups

In addition to the ground game mechanisms defined above, defensive wheel setups are a popular way to prevent defensive play from being as effective on a robot. Both mechanisms defined below, Traction Wheels and Drop-Centered Wheels, revolve around preventing a robot from being pushed around as easily and are often used in conjunction with each other.

Traction Wheels

Traction Wheels are, as the name implies, wheels added to the drivetrain of a robot to increase the amount of traction a robot has with the ground.
BLRS 15" Robot, Tipping Point
Traction wheels can be constructed in numerous ways, the most common of which is by using a flex wheel as the middle wheel of a chassis, though other ways could include using the stock traction wheels sold by VEX or stretching a traction wheel tire over a sprocket, as pictured above, to increase the outer diameter and function as a traction wheel.
BLRS2 15" Robot, Tipping Point
When adding traction wheels to a robot, it is important to consider where on a drivetrain the traction wheels are located. This is because the farther the traction wheel is from the center of the drivetrain, the more the center of rotation will be offset. This would be due to the traction differential between the two sides (front to back) of the drivetrain. In addition, it would be inadvisable to use traction wheels on a holonomic chassis, due to the increase in traction.

Drop-Centered Wheels

Drop-Centered Wheels are another method of increasing traction on the field. With drop-centered wheels, one wheel on each side of the chassis is mounted or stretched to be slightly larger than the other wheels. This enables the drop-centered wheel to be pressed slightly more into the chassis by the weight of the robot, increasing the traction of the wheel. When drop-centering a wheel, it is important to make sure that the difference between wheel locations is not extreme, as too much of a drop-center can make a robot considerably easier to tip.
BLRS2 15" Robot, Tipping Point

Teams Contributed to this Article:

  • BLRS (Purdue SIGBots)