A brief overview of commonly used methods to get your robots, spinner, lifter, forks or other weapon bits in contact with the other guy's robot.
"why would you use anything else?"
The obvious option and by far the most commonly used method of getting across the arena to wack that other guy. Wheels come in all shapes and sizes and whether driven directly, by gear, or by belt they are the most efficient method of movement. There are 3 commonly used wheel materials in insect weight combat robotics. Each has its own benefits and trade offs.
Foam wheels are the lightest and most commonly used option. Because there is a lot of air inside the structure, they are extremely light weight, even at very large sizes. Even so, they are able to take impacts well. The downside is they are prone to tearing out when stuck by sharp horizontal blades and the traction leaves a bit to be desired on most arena floors.
As with any vehicle the traction that the tire is able to exert on the floor of the arena makes a huge difference in its ability to move itself and by extension other robots. One common fix to this issue is applying a coat of latex body paint or rubber cement over the contact surface of the foam wheel. This provides a rubbery and grippy surface for the wheel to better grab the floor. An excellent tutorial on how to do this can be found here.
Most robot part suppliers (including Fingertech, ItGresa, and Palm Beach) offer a wide variety of foam wheel options. They can also easily be made at home on a budget. All that's needed is a drill, a hole saw, and a sheet of foam.
Molded wheels, are another option. With these the entire tire is molded from a grippy material such as silicone or RTV. This means that wear and light damage on the tire won't remove the ability for it to maintain traction. The primary reason to avoid these is the added weight of the denser tire material. Given the lack of air in the tire structure, every added millimeter of width or diameter significantly increases their weight. As a result these are best used for robots that need smaller grippy tires. Many builders will cast their own tires around a printed hub which allows them to have attachment points for their specific need. However, premade options such as Banebots also exist.
0-ring wheels are simple to make and have the benefit of good traction, but with a host of negatives that are solved by a molded wheel. Essentially an o-ring wheel is a loop of grip material stretched over a rim that holds the o-ring on the robot. A big negative of this style is the tendency for the 0-ring to pop out of place which at best causes the tire to lose effectiveness and at worst can bind the wheel causing complete loss of drive. Gluing the O-ring on can help alleviate this issue but is unlikely to completely prevent it. Pololu is a common supplier for this type of wheel.
Subtypes: Tracks, Omniwheels, Mecanum wheels
Tracks, while technically not wheels, are driven by wheels (cogs). They are essentially o-ring wheels which have been stretched over two or more wheels instead of one. Tracks are used when a builder is looking for maximum surface contact for (in theory) the best possible traction. They tend to be a less common option than a simple 4x4. This is because tracks can easily be damaged or popped off the drive cogs. Still, if properly protected tracks can be a viable method of drive and always have the "cool" factor.
The Omniwheel is a unique type of wheel with certain benefits and significant drawbacks sometimes used in combat robotics. Omniwheels, and their cousin the Mecanum wheel, are constructed with a central tire section that rolls in two directions as with a normal tire but then also have subsections built into the primary tire that roll perpendicular to the first axis. The benefit of these is added mobility. A robot with omniwheels is able to move in any direction, allowing a degree of surprise and the ability to attack quickly from odd angles. The downside to this is the ability for these wheels to push another robot is almost non existent given the unsecured rolling subsections of the wheel. On top of that the wheels are heavier than other options and tend to be delicate given all the added parts and complexity. Never the less, if you simply must build something weird and suboptimal then Omniwheels might be of interest to you.
To understand how the shuffler mechanism works it is possibly easiest to imagine the common internal combustion engine. Inside of which there are pistons and rods connected to a cam shaft in the method seen below.
Most ICE engines harness power by creating small explosions by compressing fuel injected on top of the pistons. This power is then transferred to a rotational energy through means of the rods and cam shaft.
Shufflers work just like this but in reverse using the power of an electric motor to spin the cam shaft which then moves the rods (shuffler legs) in an up and down motion. There is a bit more too it than that, but that's best saved for a tutorial on the subject. So one might ask why anyone would choose such an unconventional way of providing motion for their robot? In almost all Insect weight competitions shuffler robots are allowed a 50% additional weight bonus to account for the added complexity of the cam and foot mechanisms. In many cases it doesn't take all of the additional weight to make a successful mechanism so the additional weight can be put into armor, size, or the more common option "BIG wEapON"
Shuffler mechanisms have become very fast and efficient as brushless motors have improved. As a result, there is talk of nerfing the bonus a little given the dominance of certain designs. For now however, it is an excellent way to get an edge on the competition if not a simple way.
Torque reaction walker, Gyro Walker, Bristle bot
"The best drive is no drive at all!" ~Caleb of Team Kill More Robots... Probably
The last relatively common method of moving a robot is simply using the power of the spinning weapon(s) on your robot. Gyro bots tend to be slow and difficult to maneuver. But, what this class lacks in versatility it makes up for in weapon weight and power. Essentially this class is made of of 3 types of robots that use similar but distinct ways of moving the body.
This ENTIRE class of robots is the least well defined in most common events rule books. Generally they are either given a full walker weight bonus or no bonus at all. MRCA however defines this class as being distinct from a shuffler but not quite a true walker. This difference caused the creation of the "not quite a walker" bonus of 75% additional weight" This class is still new and may potentially change in future MRCA seasons if it proves to be too dominate.
First the Torque reaction walker. This is pretty much exclusively a 2 weapon horizontal spinner robot and was popularized by the 3lb robot "Droopy"
These robots move by varying the speed of their two weapons to "torque pull" forward one side of the robot and then the other causing an odd sort of walking movement.
Gyro walkers are similar to a torque reaction walker in premise. These robots use the Gyroscopic force of a single vertical spinning weapon that has been forced off axis. When off of center the weapon causes a lifting and spinning force that lifts and pulls the side of the robot after which the spinner is brought off axis in the opposite direction causing the other side of the robot to lift. Careful moderation of these lifts and weapon speed can allow a builder to "walk" their robot forward. Control of such a robot is somewhat limited and reverse movement is impossible, but there have been a few successful designs.
Bristle Bots are another class of robot that are similar to the others mentioned. However, they use a different force to achieve movement. Bristle bots use a combination of bristles similar to a toothbrush or hairbrush and vibration to move forward. The bristles are angled at a slight backwards tapper from the front of the robot causing any vibration transferred into them to be pushed against the floor in the opposite direction. While the previous robots of this class are a bit more limited on weapon orientation to allow the force to be properly applied, a bristle walker can in theory use any vibration causing component to exert force into the bristles.
(True walker) "is anyone here?"
A true walker is defined in the MRCA and spark ruleset as: "Robots in which multiple linear or limited-travel rotary actuators are intermittently driven to produce linear travel of the robot. Actuation may be through electric, pneumatic, or hydraulic means. Walkers must have no parts normally in contact with the ground undergoing continuous rotation, and must require some change in timing or sequencing of the driving mechanisms in order to reverse direction. Walkers will typically have control systems significantly more complex than those found on shufflers or rollers, involving multiple actuators, servos, or valves running through a specific sequence to produce motion."
This definition differentiates walkers from the shuffler in which all the cam parts work in unison and cannot be actuated separate from the other parts. To offset the added weight from the walker mechanism builders are allowed a whopping 100% weight bonus. Even still this class is almost never used given the significant increase in complexity and reduced reliability. Very few instances of walkers exist outside of the semi-successful Battlebots participant "CHOMP"
As you can see, there are a wide variety of ways for a robot to transverse the arena. There are numerous types of wheels that can be used on any robot. For someone who wants to build something more unique, shufflers and gyro bots are a great option that'll also give a weight bonus at most competitions. Walker bots are rare and difficult to design, but perfect for anyone looking for a challenge.
Of course, this is not an exhaustive list of ways to move a robot. Feel free to come up with your own unique methods such as flying or hovering. Just make sure to check with your local Event Organizer before building to see if it will be allowed. They might not be fond of your rocket propelled bot.