what is a tank drivetrain

A tank drivetrain, also known as a differential drive, consists of two sets of wheels on either side of the robot that are independently driven . It primarily utilizes traction wheels and cannot strafe, or move sideways . To change directions, a tank drivetrain either turns the wheels on the left and right sides in opposite directions to spin the robot, or runs one side faster than the other to move the robot in an arc .

These drivetrains prioritize traction and acceleration over pure maneuverability, giving them the potential to play defense and traverse obstacles . They are relatively simple to build . Variations of this drivetrain include:

Tread Drive: Uses tank treads or wide belts to power movement .
2-Wheel Drive (Pushbot): An introductory rookie drivetrain that often features direct-driven traction wheels paired with unpowered omni wheels .

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Tank (Skid-Steer) Drivetrains > Tread Drive > Advantages

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Very good at traversing terrain
Fantastic traction and pushing power

Suffers in maneuverability and top speed
Very complex to implement
Treads are prone to breakage and can fall off easily

(Figure: 5975 Cybots, Res-Q)

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Drivetrains > Drivetrain types

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Tank Drivetrain — A tank drivetrain primarily utilizes traction wheels and cannot strafe (move sideways). To change directions, a tank drivetrain relies on either turning the wheels on the left and right side in the opposite direction (thus spinning the robot) or running one side faster than the other side (thus making the robot take the path of an arc). Tank drivetrains prioritize traction and acceleration over pure maneuverability, giving these drivetrains the potential to traverse obstacles and play defense. Tank drivetrains are relatively simple to build, yet are still competitive at the highest levels.

Holonomic Drivetrain — A holonomic drivetrain, in contrast to a tank drivetrain, can move sideways, due to using either mecanum or omni wheels. These kinds of wheels have special rollers that allow strafing movements. Thus, holonomic drivetrains prioritize movement over traction. Holonomic drivetrains eliminate the time it takes to turn for a tank drivetrain. However, holonomic is susceptible to defense and can suffer with a heavy robot. Holonomic has been proven to be competitive at the highest level for many years, and is common among world-class robots.

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Tank (Skid-Steer) Drivetrains

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Recommended only for first drivetrain, not for competitions

This rookie drivetrain is considered one of the inferior drivetrains, though it is usable at low competition levels. This is the introductory drivetrain type for many rookies, as it is promoted in official guides published by FIRST® (giving it the name pushbot). It often has direct driven traction wheels with unpowered omni wheels.

This type of drivetrain has poor turning as the center of turning is at the back of the robot between the two powered wheels. In comparison to other drivetrains, it has poor acceleration due to only using two motors.

Even though it may not be an optimal drivetrain, it is still possible to be competitive as long as the drivetrain is reliable. As a consensus, we would advise every new team to build the pushbot primarily to learn how to build with a kit. The pushbot is a good starting point and helps the team get familiarized with using kit parts, attaching wheels, mounting motors, etc. However, it is subpar to every other drivetrain in a competitive context.

Attention: While the pushbot is a good first drivetrain for new teams to get acquainted with the kit, it is recommended that teams move away from this drivetrain when building their competition robot.

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Tank (Skid-Steer) Drivetrains > Tread Drive

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Tread drive is the use of tank treads or wide belts to power movement, much like a real life tank. Unfortunately, in FTC, it is not a competitive drivetrain for a number of reasons.

Tread is complex, and has many points of failure. Treads are also very prone to defense, and a slight hit from another robot is enough to misalign the treads. Commercial Off-The-Shelf (COTS) tread options aren't great either - TETRIX tracks have a tendency to snap and derail when used on robot drivetrains, making them not suitable for competition use.

While it is possible to implement tread drive successfully, such as in the example below, most inexperienced teams do not have the capability and know-how to do so. Tread drive has negligible traction improvements at the cost of maneuverability. There are better options to traverse terrain, such as a 4WD.

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Tank (Skid-Steer) Drivetrains > 2 Wheel Drive (Pushbot Drive) > Advantages

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Most simple drivetrain to build
No need to worry about powering all four wheels

Slower than other options
Underpowered (all other drivetrain options typically use 4 motors)
Uses the Tetrix MAX motor, which is prone to burning out easily, is underpowered, and has a fragile gearbox.
Lacks agility and maneuverability due to 2 motor turning
Poor acceleration due to 2 motors
Often direct driven,

which is highly discouraged for drivetrains

(Figure: Example pushbot drivetrain included in the starter FTC® guide)

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Kinematics > Tank (Differential Drive)

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A tank, or differential drive, is a drivetrain consisting of two sets of wheels on either side of the robot that are independently driven. These are described under further detail in the /docs/common-mechanisms/drivetrains/tank section.

The following variables are used in this section.

vr denotes the linear velocity of the right wheel(s)
vl denotes the linear velocity of the left wheel(s)
vf denotes the forward velocity of the robot, relative to itself
omega denotes the rotational velocity of the robot in radians/second
rb denotes the base track radius, or the distance between the wheel and center of the robot (half of the distance between wheels)

Warning: These variables, with the exception of omega, represent linear velocities NOT rotational velocities. Wheel rotational velocity in radians/second can be converted to linear velocity by multiplying by the wheel's radius.

Positive rotational velocity (omega) will spin the robot COUNTERCLOCKWISE when viewed from above.

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